diff options
| author | Andy Bonventre <andybons@chromium.org> | 2015-09-22 17:29:52 -0400 |
|---|---|---|
| committer | Mark Mentovai <mark@chromium.org> | 2015-09-22 17:29:52 -0400 |
| commit | 0ff15b41edf1dfd50776877edce7cae6e757574f (patch) | |
| tree | c670617291b3476ab2622d7ea2509d8369db2f0d /docs | |
| parent | Linux ExceptionHandler: don't allocate the CrashContext on the stack (diff) | |
| download | breakpad-0ff15b41edf1dfd50776877edce7cae6e757574f.tar.xz | |
[Docs] add markdown docs (converted from Wiki)
BUG=none
R=mark
CC=google-breakpad-dev@googlegroups.com
Review URL: https://codereview.chromium.org/1357773004 .
Patch from Andy Bonventre <andybons@chromium.org>.
Diffstat (limited to 'docs')
| -rw-r--r-- | docs/OWNERS | 1 | ||||
| -rw-r--r-- | docs/breakpad.png | bin | 0 -> 84153 bytes | |||
| -rw-r--r-- | docs/breakpad.svg | 1023 | ||||
| -rw-r--r-- | docs/client_design.md | 224 | ||||
| -rw-r--r-- | docs/contributing_to_breakpad.md | 35 | ||||
| -rw-r--r-- | docs/exception_handling.md | 128 | ||||
| -rw-r--r-- | docs/getting_started_with_breakpad.md | 121 | ||||
| -rw-r--r-- | docs/linux_starter_guide.md | 97 | ||||
| -rw-r--r-- | docs/linux_system_calls.md | 47 | ||||
| -rw-r--r-- | docs/mac_breakpad_starter_guide.md | 184 | ||||
| -rw-r--r-- | docs/mozilla_brown_bag_talk.md | 84 | ||||
| -rw-r--r-- | docs/processor_design.md | 230 | ||||
| -rw-r--r-- | docs/stack_walking.md | 160 | ||||
| -rw-r--r-- | docs/symbol_files.md | 497 | ||||
| -rw-r--r-- | docs/windows_client_integration.md | 70 |
15 files changed, 2901 insertions, 0 deletions
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x="508.10672" + y="377.58029" /></text> + <text + xml:space="preserve" + style="font-size:18px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;fill:#000000;fill-opacity:1;stroke:none;font-family:Electron;-inkscape-font-specification:Electron" + x="464.03134" + y="379.15485" + id="text4128-2-2"><tspan + sodipodi:role="line" + x="464.03134" + y="379.15485" + id="tspan4132-6-0">copy symbol file</tspan></text> + <path + style="fill:none;stroke:#000000;stroke-width:2;stroke-linecap:butt;stroke-linejoin:miter;stroke-opacity:1;stroke-miterlimit:4;stroke-dasharray:none;marker-end:url(#Arrow1Mend-9)" + d="m 86.873118,525.06256 c -30.304576,41.41625 -8.08122,61.6193 2.020305,73.74113" + id="path4488" /> + <text + xml:space="preserve" + style="font-size:18px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;fill:#000000;fill-opacity:1;stroke:none;font-family:Electron;-inkscape-font-specification:Electron" + x="82.19368" + y="553.91125" + id="text4128-2-5"><tspan + sodipodi:role="line" + x="82.19368" + y="553.91125" + id="tspan4705">Crash! Breakpad client</tspan><tspan + sodipodi:role="line" + x="82.19368" + y="576.41125" + id="tspan4709">writes minidump...</tspan></text> + <path + style="fill:none;stroke:#000000;stroke-width:2;stroke-linecap:butt;stroke-linejoin:miter;stroke-opacity:1;stroke-miterlimit:4;stroke-dasharray:none;marker-end:url(#Arrow1Mend-9)" + d="m 175.76654,651.33163 c 73.74114,32.32488 199.00005,11.11167 226.27417,4.04061" + id="path4711" + sodipodi:nodetypes="cc" /> + <text + xml:space="preserve" + style="font-size:18px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;fill:#000000;fill-opacity:1;stroke:none;font-family:Electron;-inkscape-font-specification:Electron" + x="195.66714" + y="632.6698" + id="text4128-2-5-0"><tspan + sodipodi:role="line" + x="195.66714" + y="632.6698" + id="tspan4709-9">...and submits it</tspan><tspan + sodipodi:role="line" + x="195.66714" + y="655.1698" + id="tspan4933">to crash collector</tspan></text> + <text + xml:space="preserve" + style="font-size:18px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;fill:#000000;fill-opacity:1;stroke:none;font-family:Electron;-inkscape-font-specification:Electron" + x="566.60748" + y="534.8764" + id="text4128-2-2-1"><tspan + sodipodi:role="line" + x="566.60748" + y="534.8764" + id="tspan6038">Breakpad</tspan><tspan + sodipodi:role="line" + x="566.60748" + y="557.3764" + id="tspan6042">minidump</tspan><tspan + sodipodi:role="line" + x="566.60748" + y="579.8764" + id="tspan6044">processor</tspan></text> + <path + style="fill:none;stroke:#000000;stroke-width:2;stroke-linecap:butt;stroke-linejoin:miter;stroke-opacity:1;stroke-miterlimit:4;stroke-dasharray:none;marker-end:none" + d="m 478.8123,528.09302 c 0,46.46701 10.10153,64.64976 33.33504,64.64976" + id="path5378" /> + <path + style="fill:#7389ff;fill-opacity:1;stroke:#000000;stroke-width:2;stroke-miterlimit:4;stroke-opacity:1;stroke-dasharray:none" + d="m 359.69771,149.79174 -2.15995,5.52276 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--git a/docs/client_design.md b/docs/client_design.md new file mode 100644 index 00000000..30ffb2fb --- /dev/null +++ b/docs/client_design.md @@ -0,0 +1,224 @@ +# Breakpad Client Libraries + +## Objective + +The Breakpad client libraries are responsible for monitoring an application for +crashes (exceptions), handling them when they occur by generating a dump, and +providing a means to upload dumps to a crash reporting server. These tasks are +divided between the “handler” (short for “exception handler”) library linked in +to an application being monitored for crashes, and the “sender” library, +intended to be linked in to a separate external program. + +## Background + +As one of the chief tasks of the client handler is to generate a dump, an +understanding of [dump files](processor_design.md) will aid in understanding the +handler. + +## Overview + +Breakpad provides client libraries for each of its target platforms. Currently, +these exist for Windows on x86 and Mac OS X on both x86 and PowerPC. A Linux +implementation has been written and is currently under review. + +Because the mechanisms for catching exceptions and the methods for obtaining the +information that a dump contains vary between operating systems, each target +operating system requires a completely different handler implementation. Where +multiple CPUs are supported for a single operating system, the handler +implementation will likely also require separate code for each processor type to +extract CPU-specific information. One of the goals of the Breakpad handler is to +provide a prepackaged cross-platform system that masks many of these +system-level differences and quirks from the application developer. Although the +underlying implementations differ, the handler library for each system follows +the same set of principles and exposes a similar interface. + +Code that wishes to take advantage of Breakpad should be linked against the +handler library, and should, at an appropriate time, install a Breakpad handler. +For applications, it is generally desirable to install the handler as early in +the start-up process as possible. Developers of library code using Breakpad to +monitor itself may wish to install a Breakpad handler when the library is +loaded, or may only want to install a handler when calls are made in to the +library. + +The handler can be triggered to generate a dump either by catching an exception +or at the request of the application itself. The latter case may be useful in +debugging assertions or other conditions where developers want to know how a +program got in to a specific non-crash state. After generating a dump, the +handler calls a user-specified callback function. The callback function may +collect additional data about the program’s state, quit the program, launch a +crash reporter application, or perform other tasks. Allowing for this +functionality to be dictated by a callback function preserves flexibility. + +The sender library is also has a separate implementation for each supported +platform, because of the varying interfaces for accessing network resources on +different operating systems. The sender transmits a dump along with other +application-defined information to a crash report server via HTTP. Because dumps +may contain sensitive data, the sender allows for the use of HTTPS. + +The canonical example of the entire client system would be for a monitored +application to link against the handler library, install a Breakpad handler from +its main function, and provide a callback to launch a small crash reporter +program. The crash reporter program would be linked against the sender library, +and would send the crash dump when launched. A separate process is recommended +for this function because of the unreliability inherent in doing any significant +amount of work from a crashed process. + +## Detailed Design + +### Exception Handler Installation + +The mechanisms for installing an exception handler vary between operating +systems. On Windows, it’s a relatively simple matter of making one call to +register a [top-level exception filter] +(http://msdn.microsoft.com/library/en-us/debug/base/setunhandledexceptionfilter.asp) +callback function. On most Unix-like systems such as Linux, processes are +informed of exceptions by the delivery of a signal, so an exception handler +takes the form of a signal handler. The native mechanism to catch exceptions on +Mac OS X requires a large amount of code to set up a Mach port, identify it as +the exception port, and assign a thread to listen for an exception on that port. +Just as the preparation of exception handlers differ, the manner in which they +are called differs as well. On Windows and most Unix-like systems, the handler +is called on the thread that caused the exception. On Mac OS X, the thread +listening to the exception port is notified that an exception has occurred. The +different implementations of the Breakpad handler libraries perform these tasks +in the appropriate ways on each platform, while exposing a similar interface on +each. + +A Breakpad handler is embodied in an `ExceptionHandler` object. Because it’s a +C++ object, `ExceptionHandler`s may be created as local variables, allowing them +to be installed and removed as functions are called and return. This provides +one possible way for a developer to monitor only a portion of an application for +crashes. + +### Exception Basics + +Once an application encounters an exception, it is in an indeterminate and +possibly hazardous state. Consequently, any code that runs after an exception +occurs must take extreme care to avoid performing operations that might fail, +hang, or cause additional exceptions. This task is not at all straightforward, +and the Breakpad handler library seeks to do it properly, accounting for all of +the minute details while allowing other application developers, even those with +little systems programming experience, to reap the benefits. All of the Breakpad +handler code that executes after an exception occurs has been written according +to the following guidelines for safety at exception time: + +* Use of the application heap is forbidden. The heap may be corrupt or + otherwise unusable, and allocators may not function. +* Resource allocation must be severely limited. The handler may create a new + file to contain the dump, and it may attempt to launch a process to continue + handling the crash. +* Execution on the thread that caused the exception is significantly limited. + The only code permitted to execute on this thread is the code necessary to + transition handling to a dedicated preallocated handler thread, and the code + to return from the exception handler. +* Handlers shouldn’t handle crashes by attempting to walk stacks themselves, + as stacks may be in inconsistent states. Dump generation should be performed + by interfacing with the operating system’s memory manager and code module + manager. +* Library code, including runtime library code, must be avoided unless it + provably meets the above guidelines. For example, this means that the STL + string class may not be used, because it performs operations that attempt to + allocate and use heap memory. It also means that many C runtime functions + must be avoided, particularly on Windows, because of heap operations that + they may perform. + +A dedicated handler thread is used to preserve the state of the exception thread +when an exception occurs: during dump generation, it is difficult if not +impossible for a thread to accurately capture its own state. Performing all +exception-handling functions on a separate thread is also critical when handling +stack-limit-exceeded exceptions. It would be hazardous to run out of stack space +while attempting to handle an exception. Because of the rule against allocating +resources at exception time, the Breakpad handler library creates its handler +thread when it installs its exception handler. On Mac OS X, this handler thread +is created during the normal setup of the exception handler, and the handler +thread will be signaled directly in the event of an exception. On Windows and +Linux, the handler thread is signaled by a small amount of code that executes on +the exception thread. Because the code that executes on the exception thread in +this case is small and safe, this does not pose a problem. Even when an +exception is caused by exceeding stack size limits, this code is sufficiently +compact to execute entirely within the stack’s guard page without causing an +exception. + +The handler thread may also be triggered directly by a user call, even when no +exception occurs, to allow dumps to be generated at any point deemed +interesting. + +### Filter Callback + +When the handler thread begins handling an exception, it calls an optional +user-defined filter callback function, which is responsible for judging whether +Breakpad’s handler should continue handling the exception or not. This mechanism +is provided for the benefit of library or plug-in code, whose developers may not +be interested in reports of crashes that occur outside of their modules but +within processes hosting their code. If the filter callback indicates that it is +not interested in the exception, the Breakpad handler arranges for it to be +delivered to any previously-installed handler. + +### Dump Generation + +Assuming that the filter callback approves (or does not exist), the handler +writes a dump in a directory specified by the application developer when the +handler was installed, using a previously generated unique identifier to avoid +name collisions. The mechanics of dump generation also vary between platforms, +but in general, the process involves enumerating each thread of execution, and +capturing its state, including processor context and the active portion of its +stack area. The dump also includes a list of the code modules loaded in to the +application, and an indicator of which thread generated the exception or +requested the dump. In order to avoid allocating memory during this process, the +dump is written in place on disk. + +### Post-Dump Behavior + +Upon completion of writing the dump, a second callback function is called. This +callback may be used to launch a separate crash reporting program or to collect +additional data from the application. The callback may also be used to influence +whether Breakpad will treat the exception as handled or unhandled. Even after a +dump is successfully generated, Breakpad can be made to behave as though it +didn’t actually handle an exception. This function may be useful for developers +who want to test their applications with Breakpad enabled but still retain the +ability to use traditional debugging techniques. It also allows a +Breakpad-enabled application to coexist with a platform’s native crash reporting +system, such as Mac OS X’ [CrashReporter] +(http://developer.apple.com/technotes/tn2004/tn2123.html) and [Windows Error +Reporting](http://msdn.microsoft.com/isv/resources/wer/). + +Typically, when Breakpad handles an exception fully and no debuggers are +involved, the crashed process will terminate. + +Authors of both callback functions that execute within a Breakpad handler are +cautioned that their code will be run at exception time, and that as a result, +they should observe the same programming practices that the Breakpad handler +itself adheres to. Notably, if a callback is to be used to collect additional +data from an application, it should take care to read only “safe” data. This +might involve accessing only static memory locations that are updated +periodically during the course of normal program execution. + +### Sender Library + +The Breakpad sender library provides a single function to send a crash report to +a crash server. It accepts a crash server’s URL, a map of key-value parameters +that will accompany the dump, and the path to a dump file itself. Each of the +key-value parameters and the dump file are sent as distinct parts of a multipart +HTTP POST request to the specified URL using the platform’s native HTTP +facilities. On Linux, [libcurl](http://curl.haxx.se/) is used for this function, +as it is the closest thing to a standard HTTP library available on that +platform. + +## Future Plans + +Although we’ve had great success with in-process dump generation by following +our guidelines for safe code at exception time, we are exploring options for +allowing dumps to be generated in a separate process, to further enhance the +handler library’s robustness. + +On Windows, we intend to offer tools to make it easier for Breakpad’s settings +to be managed by the native group policy management system. + +We also plan to offer tools that many developers would find desirable in the +context of handling crashes, such as a mechanism to determine at launch if the +program last terminated in a crash, and a way to calculate “crashiness” in terms +of crashes over time or the number of application launches between crashes. + +We are also investigating methods to capture crashes that occur early in an +application’s launch sequence, including crashes that occur before a program’s +main function begins executing. diff --git a/docs/contributing_to_breakpad.md b/docs/contributing_to_breakpad.md new file mode 100644 index 00000000..b8d261e9 --- /dev/null +++ b/docs/contributing_to_breakpad.md @@ -0,0 +1,35 @@ +# Introduction + +Thanks for thinking of contributing to Breakpad! Unfortunately there are some +pesky legal issues to get out of the way, but they're quick and painless. + +## Legal + +If you're doing work individually, not as part of any employment, you'll need to +sign the <a +href='http://code.google.com/legal/individual-cla-v1.0.html'>Individual +Contributor License Agreement</a>. This agreement can be completed +electronically. + +If you're contributing to Breakpad as part of your employment with another +organization, you'll need to sign a <a +href='http://code.google.com/legal/corporate-cla-v1.0.html'> Corporate +Contributor License Agreement</a>. Once completed this document will need to be +faxed. + +**_IMPORTANT_**: The authors(you!) of the contributions will maintain all +copyrights; the agreements you sign will grant rights to Google to use your +work. + +Thanks, and if you have any questions let me know and I'll loop in the legal guy +here to get you an answer. + +## Technical + +Once you have signed the agreement you can be added to our contributors list and +have write access to code. For full details on getting started see our trunk +`README`. + +## List of people who have signed contributor agreements + +None so far. diff --git a/docs/exception_handling.md b/docs/exception_handling.md new file mode 100644 index 00000000..e48a52ae --- /dev/null +++ b/docs/exception_handling.md @@ -0,0 +1,128 @@ +The goal of this document is to give an overview of the exception handling +options in breakpad. + +# Basics + +Exception handling is a mechanism designed to handle the occurrence of +exceptions, special conditions that change the normal flow of program execution. + +`SetUnhandledExceptionFilter` replaces all unhandled exceptions when Breakpad is +enabled. TODO: More on first and second change and vectored v. try/catch. + +There are two main types of exceptions across all platforms: in-process and +out-of-process. + +# In-Process + +In process exception handling is relatively simple since the crashing process +handles crash reporting. It is generally considered unsafe to write a minidump +from a crashed process. For example, key data structures could be corrupted or +the stack on which the exception handler runs could have been overwritten. For +this reason all platforms also support some level of out-of-process exception +handling. + +## Windows + +In-process exception handling Breakpad creates a 'handler head' that waits +infinitely on a semaphore at start up. When this thread is woken it writes the +minidump and signals to the excepting thread that it may continue. A filter will +tell the OS to kill the process if the minidump is written successfully. +Otherwise it continues. + +# Out-of-Process + +Out-of-process exception handling is more complicated than in-process exception +handling because of the need to set up a separate process that can read the +state of the crashing process. + +## Windows + +Breakpad uses two abstractions around the exception handler to make things work: +`CrashGenerationServer` and `CrashGenerationClient`. The constructor for these +takes a named pipe name. + +During server start up a named pipe and registers callbacks for client +connections are created. The named pipe is used for registration and all IO on +the pipe is done asynchronously. `OnPipeConnected` is called when a client +attempts to connect (call `CreateFile` on the pipe). `OnPipeConnected` does the +state machine transition from `Initial` to `Connecting` and on through +`Reading`, `Reading_Done`, `Writing`, `Writing_Done`, `Reading_ACK`, and +`Disconnecting`. + +When registering callbacks, the client passes in two pointers to pointers: 1. A +pointer to the `EXCEPTION_INFO` pointer 1. A pointer to the `MDRawAssertionInfo` +which handles various non-exception failures like assertions + +The essence of registration is adding a "`ClientInfo`" object that contains +handles used for synchronization with the crashing process to an array +maintained by the server. This is how we can keep track of all the clients on +the system that have registered for minidumps. These handles are: * +`server_died(mutex)` * `dump_requested(Event)` * `dump_generated(Event)` + +The server registers asynchronous waits on these events with the `ClientInfo` +object as the callback context. When the `dump_requested` event is set by the +client, the `OnDumpRequested()` callback is called. The server uses the handles +inside `ClientInfo` to communicate with the child process. Once the child sets +the event, it waits for two objects: 1. the `dump_generated` event 1. the +`server_died` mutex + +In the end handles are "duped" into the client process, and the clients use +`SetEvent` to request events, wait on the other event, or the `server_died` +mutex. + +## Linux + +### Current Status + +As of July 2011, Linux had a minidump generator that is not entirely +out-of-process. The minidump was generated from a separate process, but one that +shared an address space, file descriptors, signal handles and much else with the +crashing process. It worked by using the `clone()` system call to duplicate the +crashing process, and then uses `ptrace()` and the `/proc` file system to +retrieve the information required to write the minidump. Since then Breakpad has +updated Linux exception handling to provide more benefits of out-of-process +report generation. + +### Proposed Design + +#### Overview + +Breakpad would use a per-user daemon to write out a minidump that does not have, +interact with or depend on the crashing process. We don't want to start a new +separate process every time a user launches a Breakpad-enabled process. Doing +one daemon per machine is unacceptable for security concerns around one user +being able to initiate a minidump generation for another user's process. + +#### Client/Server Communication + +On Breakpad initialization in a process, the initializer would check if the +daemon is running and, if not, start it. The race condition between the check +and the initialization is not a problem because multiple daemons can check if +the IPC endpoint already exists and if a server is listening. Even if multiple +copies of the daemon try to `bind()` the filesystem to name the socket, all but +one will fail and can terminate. + +This point is relevant for error handling conditions. Linux does not clean the +file system representation of a UNIX domain socket even if both endpoints +terminate, so checking for existence is not strong enough. However checking the +process list or sending a ping on the socket can handle this. + +Breakpad uses UNIX domain sockets since they support full duplex communication +(unlike Windows, named pipes on Linux are half) and the kernal automatically +creates a private channel between the client and server once the client calls +`connect()`. + +#### Minidump Generation + +Breakpad could use the current system with `ptrace()` and `/proc` within the +daemon executable. + +Overall the operations look like: 1. Signal from OS indicating crash 1. Signal +Handler suspends all threads except itself 1. Signal Handler sends +`CRASH_DUMP_REQUEST` message to server and waits for response 1. Server inspects +1. Minidump is asynchronously written to disk by the server 1. Server responds +indicating inspection is done + +## Mac OSX + +Out-of-process exception handling is fully supported on Mac. diff --git a/docs/getting_started_with_breakpad.md b/docs/getting_started_with_breakpad.md new file mode 100644 index 00000000..a41b5406 --- /dev/null +++ b/docs/getting_started_with_breakpad.md @@ -0,0 +1,121 @@ +# Introduction + +Breakpad is a library and tool suite that allows you to distribute an +application to users with compiler-provided debugging information removed, +record crashes in compact "minidump" files, send them back to your server, and +produce C and C++ stack traces from these minidumps. Breakpad can also write +minidumps on request for programs that have not crashed. + +Breakpad is currently used by Google Chrome, Firefox, Google Picasa, Camino, +Google Earth, and other projects. + +![http://google-breakpad.googlecode.com/svn/wiki/breakpad.png] +(http://google-breakpad.googlecode.com/svn/wiki/breakpad.png) + +Breakpad has three main components: + +* The **client** is a library that you include in your application. It can + write minidump files capturing the current threads' state and the identities + of the currently loaded executable and shared libraries. You can configure + the client to write a minidump when a crash occurs, or when explicitly + requested. + +* The **symbol dumper** is a program that reads the debugging information + produced by the compiler and produces a **symbol file**, in [Breakpad's own + format](symbol_files.md). + +* The **processor** is a program that reads a minidump file, finds the + appropriate symbol files for the versions of the executables and shared + libraries the minidump mentions, and produces a human-readable C/C++ stack + trace. + +# The minidump file format + +The minidump file format is similar to core files but was developed by Microsoft +for its crash-uploading facility. A minidump file contains: + +* A list of the executable and shared libraries that were loaded in the + process at the time the dump was created. This list includes both file names + and identifiers for the particular versions of those files that were loaded. + +* A list of threads present in the process. For each thread, the minidump + includes the state of the processor registers, and the contents of the + threads' stack memory. These data are uninterpreted byte streams, as the + Breakpad client generally has no debugging information available to produce + function names or line numbers, or even identify stack frame boundaries. + +* Other information about the system on which the dump was collected: + processor and operating system versions, the reason for the dump, and so on. + +Breakpad uses Windows minidump files on all platforms, instead of the +traditional core files, for several reasons: + +* Core files can be very large, making them impractical to send across a + network to the collector for processing. Minidumps are smaller, as they were + designed to be used this way. + +* The core file format is poorly documented. For example, the Linux Standards + Base does not describe how registers are stored in `PT_NOTE` segments. + +* It is harder to persuade a Windows machine to produce a core dump file than + it is to persuade other machines to write a minidump file. + +* It simplifies the Breakpad processor to support only one file format. + +# Overview/Life of a minidump + +A minidump is generated via calls into the Breakpad library. By default, +initializing Breakpad installs an exception/signal handler that writes a +minidump to disk at exception time. On Windows, this is done via +`SetUnhandledExceptionFilter()`; on OS X, this is done by creating a thread that +waits on the Mach exception port; and on Linux, this is done by installing a +signal handler for various exceptions like `SIGILL, SIGSEGV` etc. + +Once the minidump is generated, each platform has a slightly different way of +uploading the crash dump. On Windows & Linux, a separate library of functions is +provided that can be called into to do the upload. On OS X, a separate process +is spawned that prompts the user for permission, if configured to do so, and +sends the file. + +# Terminology + +**In-process vs. out-of-process exception handling** - it's generally considered +that writing the minidump from within the crashed process is unsafe - key +process data structures could be corrupted, or the stack on which the exception +handler runs could have been overwritten, etc. All 3 platforms support what's +known as "out-of-process" exception handling. + +# Integration overview + +## Breakpad Code Overview + +All the client-side code is found by visiting the Google Project at +http://code.google.com/p/google-breakpad. The following directory structure is +present in the `src` directory: + +* `processor` Contains minidump-processing code that is used on the server + side and isn't of use on the client side +* `client` Contains client minidump-generation libraries for all platforms +* `tools` Contains source code & projects for building various tools on each + platform. + +(Among other directories) + +* <a + href='http://code.google.com/p/google-breakpad/wiki/WindowsClientIntegration'>Windows + Integration Guide</a> +* <a + href='http://code.google.com/p/google-breakpad/wiki/MacBreakpadStarterGuide'>Mac + Integration Guide</a> +* <a href='http://code.google.com/p/google-breakpad/wiki/LinuxStarterGuide'> + Linux Integration Guide</a> + +## Build process specifics(symbol generation) + +This applies to all platforms. Inside `src/tools/{platform}/dump_syms` is a tool +that can read debugging information for each platform (e.g. for OS X/Linux, +DWARF and STABS, and for Windows, PDB files) and generate a Breakpad symbol +file. This tool should be run on your binary before it's stripped(in the case of +OS X/Linux) and the symbol files need to be stored somewhere that the minidump +processor can find. There is another tool, `symupload`, that can be used to +upload symbol files if you have written a server that can accept them. diff --git a/docs/linux_starter_guide.md b/docs/linux_starter_guide.md new file mode 100644 index 00000000..9ab48624 --- /dev/null +++ b/docs/linux_starter_guide.md @@ -0,0 +1,97 @@ +# How To Add Breakpad To Your Linux Application + +This document is an overview of using the Breakpad client libraries on Linux. + +## Building the Breakpad libraries + +Breakpad provides an Autotools build system that will build both the Linux +client libraries and the processor libraries. Running `./configure && make` in +the Breakpad source directory will produce +**src/client/linux/libbreakpad\_client.a**, which contains all the code +necessary to produce minidumps from an application. + +## Integrating Breakpad into your Application + +First, configure your build process to link **libbreakpad\_client.a** into your +binary, and set your include paths to include the **src** directory in the +**google-breakpad** source tree. Next, include the exception handler header: ``` + +# include "client/linux/handler/exception_handler.h" + +``` + +Now you can instantiate an `ExceptionHandler` object. Exception handling is active for the lifetime of the `ExceptionHandler` object, so you should instantiate it as early as possible in your application's startup process, and keep it alive for as close to shutdown as possible. To do anything useful, the `ExceptionHandler` constructor requires a path where it can write minidumps, as well as a callback function to receive information about minidumps that were written: +``` + +static bool dumpCallback(const google_breakpad::MinidumpDescriptor& descriptor, +void* context, bool succeeded) { printf("Dump path: %s\n", descriptor.path()); +return succeeded; } + +void crash() { volatile int* a = (int*)(NULL); *a = 1; } + +int main(int argc, char* argv[]) { google_breakpad::MinidumpDescriptor +descriptor("/tmp"); google_breakpad::ExceptionHandler eh(descriptor, NULL, +dumpCallback, NULL, true, -1); crash(); return 0; } ``` + +Compiling and running this example should produce a minidump file in /tmp, and +it should print the minidump filename before exiting. You can read more about +the other parameters to the `ExceptionHandler` constructor <a +href='http://code.google.com/p/google-breakpad/source/browse/trunk/src/client/linux/handler/exception_handler.h'>in +the exception_handler.h source file</a>. + +**Note**: You should do as little work as possible in the callback function. +Your application is in an unsafe state. It may not be safe to allocate memory or +call functions from other shared libraries. The safest thing to do is `fork` and +`exec` a new process to do any work you need to do. If you must do some work in +the callback, the Breakpad source contains <a +href='http://code.google.com/p/google-breakpad/source/browse/trunk/src/common/linux/linux_libc_support.h'>some +simple reimplementations of libc functions</a>, to avoid calling directly into +libc, as well as <a href='http://code.google.com/p/linux-syscall-support/'>a +header file for making Linux system calls</a> (in **src/third\_party/lss**) to +avoid calling into other shared libraries. + +## Sending the minidump file + +In a real application, you would want to handle the minidump in some way, likely +by sending it to a server for analysis. The Breakpad source tree contains <a +href='http://code.google.com/p/google-breakpad/source/browse/#svn/trunk/src/common/linux'>some +HTTP upload source</a> that you might find useful, as well as <a +href='http://code.google.com/p/google-breakpad/source/browse/#svn/trunk/src/tools/linux/symupload'>a +minidump upload tool</a>. + +## Producing symbols for your application + +To produce useful stack traces, Breakpad requires you to convert the debugging +symbols in your binaries to <a +href='http://code.google.com/p/google-breakpad/wiki/SymbolFiles'>text-format +symbol files</a>. First, ensure that you've compiled your binaries with `-g` to +include debugging symbols. Next, compile the `dump_syms` tool by running +`configure && make` in the Breakpad source directory. Next, run `dump_syms` on +your binaries to produce the text-format symbols. For example, if your main +binary was named `test`: `$ google-breakpad/src/tools/linux/dump_syms/dump_syms +./test > test.sym +` + +In order to use these symbols with the `minidump_stackwalk` tool, you will need +to place them in a specific directory structure. The first line of the symbol +file contains the information you need to produce this directory structure, for +example (your output will vary): `$ head -n1 test.sym MODULE Linux x86_64 +6EDC6ACDB282125843FD59DA9C81BD830 test $ mkdir -p +./symbols/test/6EDC6ACDB282125843FD59DA9C81BD830 $ mv test.sym +./symbols/test/6EDC6ACDB282125843FD59DA9C81BD830 +` + +You may also find the <a +href='http://mxr.mozilla.org/mozilla-central/source/toolkit/crashreporter/tools/symbolstore.py'>symbolstore.py</a> +script in the Mozilla repository useful, as it encapsulates these steps. + +## Processing the minidump to produce a stack trace + +Breakpad includes a tool called `minidump_stackwalk` which can take a minidump +plus its corresponding text-format symbols and produce a symbolized stacktrace. +It should be in the **google-breakpad/src/processor** directory if you compiled +the Breakpad source using the directions above. Simply pass it the minidump and +the symbol path as commandline parameters: +`google-breakpad/src/processor/minidump_stackwalk minidump.dmp ./symbols +` It produces verbose output on stderr, and the stacktrace on stdout, so you may +want to redirect stderr. diff --git a/docs/linux_system_calls.md b/docs/linux_system_calls.md new file mode 100644 index 00000000..17ada7e0 --- /dev/null +++ b/docs/linux_system_calls.md @@ -0,0 +1,47 @@ +# Introduction + +Linux implements its userland-to-kernel transition using a special library +called linux-gate.so that is mapped by the kernel into every process. For more +information, see + +http://www.trilithium.com/johan/2005/08/linux-gate/ + +In a nutshell, the problem is that the system call gate function, +kernel\_vsyscall does not use EBP to point to the frame pointer. + +However, the Breakpad processor supports special frames like this via STACK +lines in the symbol file. If you look in src/client/linux/data you will see +symbol files for linux-gate.so for both Intel & AMD(the implementation of +kernel\_vsyscall changes depending on the CPU manufacturer). When processing +minidumps from Linux 2.6, having these symbol files is necessary for walking the +stack for crashes that happen while a thread is in a system call. + +If you're just interested in processing minidumps, those two symbol files should +be all you need! + +# Details + +The particular details of understanding the linux-gate.so symbol files can be +found by reading about STACK lines inside +src/common/windows/pdb\_source\_line\_writer.cc, and the above link. To +summarize briefly, we just have to inform the processor how to get to the +previous frame when the EIP is inside kernel\_vsyscall, and we do that by +telling the processor how many bytes kernel\_vsyscall has pushed onto the stack +in it's prologue. For example, one of the symbol files looks somewhat like the +following: + +MODULE Linux x86 random\_debug\_id linux-gate.so PUBLIC 400 0 kernel\_vsyscall +STACK WIN 4 100 1 1 0 0 0 0 0 1 + +The PUBLIC line indicates that kernel\_vsyscall is at offset 400 (in bytes) from +the beginning of linux-gate.so. The STACK line indicates the size of the +function(100), how many bytes it pushes(1), and how many bytes it pops(1). The +last 1 indicates that EBP is pushed onto the stack before being used by the +function. + +# Warnings + +These functions might change significantly depending on kernel version. In my +opinion, the actual function stack information is unlikely to change frequently, +but the Linux kernel might change the address of kernel\_vsyscall w.r.t the +beginning of linux-gate.so, which would cause these symbol files to be invalid. diff --git a/docs/mac_breakpad_starter_guide.md b/docs/mac_breakpad_starter_guide.md new file mode 100644 index 00000000..6e0bdb0e --- /dev/null +++ b/docs/mac_breakpad_starter_guide.md @@ -0,0 +1,184 @@ +# How To Add Breakpad To Your Mac Client Application + +This document is a step-by-step recipe to get your Mac client app to build with +Breakpad. + +## Preparing a binary build of Breakpad for use in your tree + +You can either check in a binary build of the Breakpad framework & tools or +build it as a dependency of your project. The former is recommended, and +detailed here, since building dependencies through other projects is +problematic(matching up configuration names), and the Breakpad code doesn't +change nearly often enough as your application's will. + +## Building the requisite targets + +All directories are relative to the `src` directory of the Breakpad checkout. + +* Build the 'All' target of `client/mac/Breakpad.xcodeproj` in Release mode. +* Execute `cp -R client/mac/build/Release/Breakpad.framework <location in your + source tree>` +* Inside `tools/mac/dump_syms` directory, build dump\_syms.xcodeproj, and copy + tools/mac/dump\_syms/build/Release/dump\_syms to a safe location where it + can be run during the build process. + +## Adding Breakpad.framework + +Inside your application's framework, add the Breakpad.Framework to your +project's framework settings. When you select it from the file chooser, it will +let you pick a target to add it to; go ahead and check the one that's relevant +to your application. + +## Copy Breakpad into your Application Package + +Copy Breakpad into your Application Package, so it will be around at run time. + +Go to the Targets section of your Xcode Project window. Hit the disclosure +triangle to reveal the build phases of your application. Add a new Copy Files +phase using the Contextual menu (Control Click). On the General panel of the new +'Get Info' of this new phase, set the destination to 'Frameworks' Close the +'Info' panel. Use the Contextual Menu to Rename your new phase 'Copy Frameworks' +Now drag Breakpad again into this Copy Frameworks phase. Drag it from whereever +it appears in the project file tree. + +## Add a New Run Script build phase + +Near the end of the build phases, add a new Run Script build phase. This will be +run before Xcode calls /usr/bin/strip on your project. This is where you'll be +calling dump\_sym to output the symbols for each architecture of your build. In +my case, the relevant lines read: + +``` +#!/bin/sh +$TOOL_DIR=<location of dump_syms from step 3 above> + +"$TOOL_DIR/dump_syms" -a ppc "$PROD" > "$TARGET_NAME ppc.breakpad" + +"$TOOL_DIR/dump_syms" -a i386 "$PROD" > "$TARGET_NAME i386.breakpad" +``` + +## Adjust the Project Settings + +* Turn on Separate Strip, +* Set the Strip Style to Non-Global Symbols. + +## Write Code! + +You'll need to have an object that acts as the delegate for NSApplication. +Inside this object's header, you'll need to add + +1. add an ivar for Breakpad and +2. a declaration for the applicationShouldTerminate:(NSApplication`*` sender) + message. + +``` +#import <Breakpad/Breakpad.h> + +@interface BreakpadTest : NSObject { + . + . + . + BreakpadRef breakpad; + . + . + . +} +. +. +- (NSApplicationTerminateReply)applicationShouldTerminate:(NSApplication *)sender; +. +. +@end +``` + +Inside your object's implementation file, + +1. add the following method InitBreakpad +2. modify your awakeFromNib method to look like the one below, +3. modify/add your application's delegate method to look like the one below + +``` +static BreakpadRef InitBreakpad(void) { + NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init]; + BreakpadRef breakpad = 0; + NSDictionary *plist = [[NSBundle mainBundle] infoDictionary]; + if (plist) { + // Note: version 1.0.0.4 of the framework changed the type of the argument + // from CFDictionaryRef to NSDictionary * on the next line: + breakpad = BreakpadCreate(plist); + } + [pool release]; + return breakpad; +} + +- (void)awakeFromNib { + breakpad = InitBreakpad(); +} + +- (NSApplicationTerminateReply)applicationShouldTerminate:(NSApplication *)sender { + BreakpadRelease(breakpad); + return NSTerminateNow; +} +``` + +## Configure Breakpad + +Configure Breakpad for your application. + +1. Take a look inside the Breakpad.framework at the Breakpad.h file for the + keys, default values, and descriptions to be passed to BreakpadCreate(). +2. Add/Edit the Breakpad specific entries in the dictionary passed to + BreakpadCreate() -- typically your application's info plist. + +Example from the Notifier Info.plist: +`<key>BreakpadProduct</key><string>Google_Notifier_Mac</string> +<key>BreakpadProductDisplay</key><string>${PRODUCT_NAME}</string> +` + +## Build Your Application + +Almost done! + +## Verify + +Double-check: + +Your app should have in its package contents: +myApp.app/Contents/Frameworks/Breakpad.framework. + +The symbol files have reasonable contents (you can look at them with a text +editor.) + +Look again at the Copy Frameworks phase of your project. Are you leaking .h +files? Select them and delete them. (If you drag a bunch of files into your +project, Xcode often wants to copy your .h files into the build, revealing +Google secrets. Be vigilant!) + +## Upload the symbol file + +You'll need to configure your build process to store symbols in a location that +is accessible by the minidump processor. There is a tool in tools/mac/symupload +that can be used to send the symbol file via HTTP post. + +1. Test + +Configure breakpad to send reports to a URL by adding to your app's Info.plist: + +``` +<key>BreakpadURL</key> +<string>upload URL</string> +<key>BreakpadReportInterval</key> +<string>30</string> +``` + +## Final Notes + +Breakpad checks whether it is being run under a debugger, and if so, normally +does nothing. But, you can force Breakpad to function under a debugger by +setting the Unix shell variable BREAKPAD\_IGNORE\_DEBUGGER to a non-zero value. +You can bracket the source code in the above Write The Code step with #if DEBUG +to completely eliminate it from Debug builds. See +//depot/googlemac/GoogleNotifier/main.m for an example. FYI, when your process +forks(), exception handlers are reset to the default for child processes. So +they must reinitialize Breakpad, otherwise exceptions will be handled by Apple's +Crash Reporter. diff --git a/docs/mozilla_brown_bag_talk.md b/docs/mozilla_brown_bag_talk.md new file mode 100644 index 00000000..8322d244 --- /dev/null +++ b/docs/mozilla_brown_bag_talk.md @@ -0,0 +1,84 @@ +# Breakpad Crash Reporting for Mozilla + +* January 24, 2007 + * Links updated February 14, 2007 +* Mozilla HQ +* Mark Mentovai +* Brian Ryner + +## What is a crash reporter? + +* Enables developers to analyze crashes that occur in the wild +* Produces stack backtraces that help identify how a program failed +* Offers higher-level data aggregation (topcrashes, MTBF statistics) + +## Motivation + +* Talkback is proprietary and unmaintained +* Smaller open-source projects have few options +* Larger projects need flexibility and scalability + +## Design Options + +* Stackwalking done on client + * Apple CrashReporter + * GNOME BugBuddy +* Client sends memory dump + * Talkback + * Windows Error Reporting + * Breakpad + +## Goals + +* Provide libraries around which systems can be based +* Open-source +* Cross-platform + * Mac OS X x86, PowerPC + * Linux x86 + * Windows x86 +* No requirement to distribute symbols + +## Client Libraries + +* Exception handler installed at application startup + * Spawns a separate thread +* Minidump file written at crash time + * Format used by Windows debuggers +* Separate application invoked to send + * HTTP[S](S.md) POST, can include additional parameters + +## Symbols + +* Cross-platform symbol file format +* Contents + * Function names + * Source file names and line numbers + * Windows: Frame pointer omission data + * Future: parameters and local variables +* Symbol conversion methods + +## Processor + +* Examines minidump file and invokes stackwalker +* Symbol files requested from a SymbolSupplier +* Produces stack trace +* Output may be placed where convenient + +## Intergation + +* Breakpad client present in Gran Paradiso Alpha 1 for Windows + * Disabled by default + * Enable with `MOZ_AIRBAG` +* Proof-of-concept collector + * http://mavra.perilith.com/~luser/airbag-collector/list.pl +* Other platforms coming soon + +## More Information + +* Project home: http://code.google.com/p/google-breakpad/ +* Mailing lists + * [google-breakpad-dev@googlegroups.com] + (http://groups.google.com/group/google-breakpad-dev/) + * [google-breakpad-discuss@googlegroups.com] + (http://groups.google.com/group/google-breakpad-discuss/) +* Ask me (irc.mozilla.org: mento) diff --git a/docs/processor_design.md b/docs/processor_design.md new file mode 100644 index 00000000..c2af41a1 --- /dev/null +++ b/docs/processor_design.md @@ -0,0 +1,230 @@ +# Breakpad Processor Library + +## Objective + +The Breakpad processor library is an open-source framework to access the the +information contained within crash dumps for multiple platforms, and to use that +information to produce stack traces showing the call chain of each thread in a +process. After processing, this data is made available to users of the library. + +## Background + +The Breakpad processor is intended to sit at the core of a comprehensive +crash-reporting system that does not require debugging information to be +provided to those running applications being monitored. Some existing +crash-reporting systems, such as [GNOME](http://www.gnome.org/)’s Bug-Buddy and +[Apple](http://www.apple.com/)’s [CrashReporter] +(http://developer.apple.com/technotes/tn2004/tn2123.html), require symbolic +information to be present on the end user’s computer; in the case of +CrashReporter, the reports are transmitted only to Apple, not to third-party +developers. Other systems, such as [Microsoft](http://www.microsoft.com/)’s +[Windows Error Reporting](http://msdn.microsoft.com/isv/resources/wer/) and +SupportSoft’s Talkback, transmit only a snapshot of a crashed process’ state, +which can later be combined with symbolic debugging information without the need +for it to be present on end users’ computers. Because symbolic debugging +information consumes a large amount of space and is otherwise not needed during +the normal operation of software, and because some developers are reluctant to +release debugging symbols to their customers, Breakpad follows the latter +approach. + +We know of no currently-maintained crash-reporting systems that meet our +requirements, which are to: * allow for symbols to be separate from the +application, * handle crash reports from multiple platforms, * allow developers +to operate their own crash-reporting platform, and to * be open-source. Windows +Error Reporting only functions for Microsoft products, and requires the +involvement of Microsoft’s servers. Talkback, while cross-platform, has not been +maintained and at this point does not support Mac OS X on x86, which we consider +to be a significant platform. Talkback is also closed-source commercial +software, and has very specific requirements for its server platform. + +We are aware of Windows-only crash-reporting systems that leverage Microsoft’s +debugging interfaces. Such systems, even if extended to support dumps from other +platforms, are tied to using Windows for at least a portion of the processor +platform. + +## Overview + +The Breakpad processor itself is written in standard C++ and will work on a +variety of platforms. The dumps it accepts may also have been created on a +variety of systems. The library is able to combine dumps with symbolic debugging +information to create stack traces that include function signatures. The +processor library includes simple command-line tools to examine dumps and +process them, producing stack traces. It also exposes several layers of APIs +enabling crash-reporting systems to be built around the Breakpad processor. + +## Detailed Design + +### Dump Files + +In the processor, the dump data is of primary significance. Dumps typically +contain: + +* CPU context (register data) as it was at the time the crash occurred, and an + indication of which thread caused the crash. General-purpose registers are + included, as are special-purpose registers such as the instruction pointer + (program counter). +* Information about each thread of execution within a crashed process, + including: + * The memory region used for each thread’s stack. + * CPU context for each thread, which for various reasons is not the same + as the crash context in the case of the crashed thread. +* A list of loaded code segments (or modules), including: + * The name of the file (`.so`, `.exe`, `.dll`, etc.) which provides the + code. + * The boundaries of the memory region in which the code segment is visible + to the process. + * A reference to the debugging information for the code module, when such + information is available. + +Ordinarily, dumps are produced as a result of a crash, but other triggers may be +set to produce dumps at any time a developer deems appropriate. The Breakpad +processor can handle dumps in the minidump format, either generated by an +[Breakpad client “handler”](client_design.md) implementation, or by another +implementation that produces dumps in this format. The +[DbgHelp.dll!MiniDumpWriteDump] +(http://msdn2.microsoft.com/en-us/library/ms680360.aspx) function on Windows +produces dumps in this format, and is the basis for the Breakpad handler +implementation on that platform. + +The [minidump format] +(http://msdn.microsoft.com/en-us/library/ms679293%28VS.85%29.aspx) is +essentially a simple container format, organized as a series of streams. Each +stream contains some type of data relevant to the crash. A typical “normal” +minidump contains streams for the thread list, the module list, the CPU context +at the time of the crash, and various bits of additional system information. +Other types of minidump can be generated, such as a full-memory minidump, which +in addition to stack memory contains snapshots of all of a process’ mapped +memory regions. + +The minidump format was chosen as Breakpad’s dump format because it has an +established track record on Windows, and it can be adapted to meet the needs of +the other platforms that Breakpad supports. Most other operating systems use +“core” files as their native dump formats, but the capabilities of core files +vary across platforms, and because core files are usually presented in a +platform’s native executable format, there are complications involved in +accessing the data contained therein without the benefit of the header files +that define an executable format’s entire structure. Because minidumps are +leaner than a typical executable format, a redefinition of the format in a +cross-platform header file, `minidump_format.h`, was a straightforward task. +Similarly, the capabilities of the minidump format are understood, and because +it provides an extensible container, any of Breakpad’s needs that could not be +met directly by the standard minidump format could likely be met by extending it +as needed. Finally, using this format means that the dump file is compatible +with native debugging tools at least on Windows. A possible future avenue for +exploration is the conversion of minidumps to core files, to enable this same +benefit on other platforms. + +We have already provided an extension to the minidump format that allows it to +carry dumps generated on systems with PowerPC processors. The format already +allows for variable CPUs, so our work in this area was limited to defining a +context structure sufficient to represent the execution state of a PowerPC. We +have also defined an extension that allows minidumps to indicate which thread of +execution requested a dump be produced for non-crash dumps. + +Often, the information contained within a dump alone is sufficient to produce a +full stack backtrace for each thread. Certain optimizations that compilers +employ in producing code frustrate this process. Specifically, the “frame +pointer omission” optimization of x86 compilers can make it impossible to +produce useful stack traces given only a stack snapshot and CPU context. In +these cases, however, compiler-emitted debugging information can aid in +producing useful stack traces. The Breakpad processor is able to take advantage +of this debugging information as supplied by Microsoft’s C/C++ compiler, the +only compiler to apply such optimizations by default. As a result, the Breakpad +processor can produce useful stack traces even from code with frame pointer +omission optimizations as produced by this compiler. + +### Symbol Files + +The [symbol files](symbol_files.md) that the Breakpad processor accepts allow +for frame pointer omission data, but this is only one of their capabilities. +Each symbol file also includes information about the functions, source files, +and source code line numbers for a single module of code. A module is an +individually-loadble chunk of code: these can be executables containing a main +program (`exe` files on Windows) or shared libraries (`.so` files on Linux, +`.dylib` files, frameworks, and bundles on Mac OS X, and `.dll` files on +Windows). Dumps contain information about which of these modules were loaded at +the time the dump was produced, and given this information, the Breakpad +processor attempts to locate debugging symbols for the module through a +user-supplied function embodied in a “symbol supplier.” Breakpad includes a +sample symbol supplier, called `SimpleSymbolSupplier`, that is used by its +command-line tools; this supplier locates symbol files by pathname. +`SimpleSymbolSupplier` is also available to other users of the Breakpad +processor library. This allows for the use of a simple reference implementation, +but preserves flexibility for users who may have more demanding symbol file +storage needs. + +Breakpad’s symbol file format is text-based, and was defined to be fairly +human-readable and to encompass the needs of multiple platforms. The Breakpad +processor itself does not operate directly with native symbol formats ([DWARF] +(http://dwarf.freestandards.org/) and [STABS] +(http://sourceware.org/gdb/current/onlinedocs/stabs.html) on most Unix-like +systems, [.pdb files] +(http://msdn2.microsoft.com/en-us/library/yd4f8bd1(VS.80).aspx) on Windows), +because of the complications in accessing potentially complex symbol formats +with slight variations between platforms, stored within different types of +binary formats. In the case of `.pdb` files, the debugging format is not even +documented. Instead, Breakpad’s symbol files are produced on each platform, +using specific debugging APIs where available, to convert native symbols to +Breakpad’s cross-platform format. + +### Processing + +Most commonly, a developer will enable an application to use Breakpad by +building it with a platform-specific [client “handler”](client_design.md) +library. After building the application, the developer will create symbol files +for Breakpad’s use using the included `dump_syms` or `symupload` tools, or +another suitable tool, and place the symbol files where the processor’s symbol +supplier will be able to locate them. + +When a dump file is given to the processor’s `MinidumpProcessor` class, it will +read it using its included minidump reader, contained in the `Minidump` family +of classes. It will collect information about the operating system and CPU that +produced the dump, and determine whether the dump was produced as a result of a +crash or at the direct request of the application itself. It then loops over all +of the threads in a process, attempting to walk the stack associated with each +thread. This process is achieved by the processor’s `Stackwalker` components, of +which there are a slightly different implementations for each CPU type that the +processor is able to handle dumps from. Beginning with a thread’s context, and +possibly using debugging data, the stackwalker produces a list of stack frames, +containing each instruction executed in the chain. These instructions are +matched up with the modules that contributed them to a process, and the +`SymbolSupplier` is invoked to locate a symbol file. The symbol file is given to +a `SourceLineResolver`, which matches the instruction up with a specific +function name, source file, and line number, resulting in a representation of a +stack frame that can easily be used to identify which code was executing. + +The results of processing are made available in a `ProcessState` object, which +contains a vector of threads, each containing a vector of stack frames. + +For small-scale use of the Breakpad processor, and for testing and debugging, +the `minidump_stackwalk` tool is provided. It invokes the processor and displays +the full results of processing, optionally allowing symbols to be provided to +the processor by a pathname-based symbol supplier, `SimpleSymbolSupplier`. + +For lower-level testing and debugging, the processor library also includes a +`minidump_dump` tool, which walks through an entire minidump file and displays +its contents in somewhat readable form. + +### Platform Support + +The Breakpad processor library is able to process dumps produced on Mac OS X +systems running on x86, x86-64, and PowerPC processors, on Windows and Linux +systems running on x86 or x86-64 processors, and on Android systems running ARM +or x86 processors. The processor library itself is written in standard C++, and +should function properly in most Unix-like environments. It has been tested on +Linux and Mac OS X. + +## Future Plans + +There are currently no firm plans or timetables to implement any of these +features, although they are possible avenues for future exploration. + +The symbol file format can be extended to carry information about the locations +of parameters and local variables as stored in stack frames and registers, and +the processor can use this information to provide enhanced stack traces showing +function arguments and variable values. + +On Mac OS X and Linux, we can provide tools to convert files from the minidump +format into the native core format. This will enable developers to open dump +files in a native debugger, just as they are presently able to do with minidumps +on Windows. diff --git a/docs/stack_walking.md b/docs/stack_walking.md new file mode 100644 index 00000000..c74f22d4 --- /dev/null +++ b/docs/stack_walking.md @@ -0,0 +1,160 @@ +# Introduction + +This page aims to provide a detailed description of how Breakpad produces stack +traces from the information contained within a minidump file. + +# Details + +## Starting the Process + +Typically the stack walking process is initiated by instantiating the +[MinidumpProcessor] +(http://code.google.com/p/google-breakpad/source/browse/trunk/src/processor/minidump_processor.cc) +class and calling the [MinidumpProcessor::Process] +(http://code.google.com/p/google-breakpad/source/browse/trunk/src/processor/minidump_processor.cc#61) +method, providing it a minidump file to process. To produce a useful stack +trace, the MinidumpProcessor requires two other objects which are passed in its +constructor: a [SymbolSupplier] +(http://code.google.com/p/google-breakpad/source/browse/trunk/src/google_breakpad/processor/symbol_supplier.h) +and a [SourceLineResolverInterface] +(http://code.google.com/p/google-breakpad/source/browse/trunk/src/google_breakpad/processor/source_line_resolver_interface.h). +The SymbolSupplier object is responsible for locating and providing SymbolFiles +that match modules from the minidump. The SourceLineResolverInterface is +responsible for loading the symbol files and using the information contained +within to provide function and source information for stack frames, as well as +information on how to unwind from a stack frame to its caller. More detail will +be provided on these interactions later. + +A number of data streams are extracted from the minidump to begin stack walking: +the list of threads from the process ([MinidumpThreadList] +(http://code.google.com/p/google-breakpad/source/browse/trunk/src/google_breakpad/processor/minidump.h#335)), +the list of modules loaded in the process ([MinidumpModuleList] +(http://code.google.com/p/google-breakpad/source/browse/trunk/src/google_breakpad/processor/minidump.h#501)), +and information about the exception that caused the process to crash +([MinidumpException] +(http://code.google.com/p/google-breakpad/source/browse/trunk/src/google_breakpad/processor/minidump.h#615)). + +## Enumerating Threads + +For each thread in the thread list ([MinidumpThread] +(http://code.google.com/p/google-breakpad/source/browse/trunk/src/google_breakpad/processor/minidump.h#299)), +the thread memory containing the stack for the thread ([MinidumpMemoryRegion] +(http://code.google.com/p/google-breakpad/source/browse/trunk/src/google_breakpad/processor/minidump.h#236)) +and the CPU context representing the CPU state of the thread at the time the +dump was written ([MinidumpContext] +(http://code.google.com/p/google-breakpad/source/browse/trunk/src/google_breakpad/processor/minidump.h#171)) +are extracted from the minidump. If the thread being processed is the thread +that produced the exception then a CPU context is obtained from the +MinidumpException object instead, which represents the CPU state of the thread +at the point of the exception. A stack walker is then instantiated by calling +the [Stackwalker::StackwalkerForCPU] +(http://code.google.com/p/google-breakpad/source/browse/trunk/src/google_breakpad/processor/stackwalker.h#77) +method and passing it the CPU context, the thread memory, the module list, as +well as the SymbolSupplier and SourceLineResolverInterface. This method selects +the specific !Stackwalker subclass based on the CPU architecture of the provided +CPU context and returns an instance of that subclass. + +## Walking a thread's stack + +Once a !Stackwalker instance has been obtained, the processor calls the +[Stackwalker::Walk] +(http://code.google.com/p/google-breakpad/source/browse/trunk/src/google_breakpad/processor/source_line_resolver_interface.h) +method to obtain a list of frames representing the stack of this thread. The +!Stackwalker starts by calling the GetContextFrame method which returns a +StackFrame representing the top of the stack, with CPU state provided by the +initial CPU context. From there, the stack walker repeats the following steps +for each frame in turn: + +### Finding the Module + +The address of the instruction pointer of the current frame is used to determine +which module contains the current frame by calling the module list's +[GetModuleForAddress] +(http://code.google.com/p/google-breakpad/source/browse/trunk/src/google_breakpad/processor/code_modules.h#56) +method. + +### Locating Symbols + +If a module is located, the SymbolSupplier is asked to locate symbols +corresponding to the module by calling its [GetCStringSymbolData] +(http://code.google.com/p/google-breakpad/source/browse/trunk/src/google_breakpad/processor/symbol_supplier.h#87) +method. Typically this is implemented by using the module's debug filename (the +PDB filename for Windows dumps) and debug identifier (a GUID plus one extra +digit) as a lookup key. The [SimpleSymbolSupplier] +(http://code.google.com/p/google-breakpad/source/browse/trunk/src/processor/simple_symbol_supplier.cc) +class simply uses these as parts of a file path to locate a flat file on disk. + +### Loading Symbols + +If a symbol file is located, the SourceLineResolverInterface is then asked to +load the symbol file by calling its [LoadModuleUsingMemoryBuffer] +(http://code.google.com/p/google-breakpad/source/browse/trunk/src/google_breakpad/processor/source_line_resolver_interface.h#71) +method. The [BasicSourceLineResolver] +(http://code.google.com/p/google-breakpad/source/browse/trunk/src/processor/basic_source_line_resolver.cc) +implementation parses the text-format [symbol file](symbol_files.md) into +in-memory data structures to make lookups by address of function names, source +line information, and unwind information easy. + +### Getting source line information + +If a symbol file has been successfully loaded, the SourceLineResolverInterface's +[FillSourceLineInfo] +(http://code.google.com/p/google-breakpad/source/browse/trunk/src/google_breakpad/processor/source_line_resolver_interface.h#89) +method is called to provide a function name and source line information for the +current frame. This is done by subtracting the base address of the module +containing the current frame from the instruction pointer of the current frame +to obtain a relative virtual address (RVA), which is a code offset relative to +the start of the module. This RVA is then used as a lookup into a table of +functions ([FUNC lines](SymbolFiles#FUNC_records.md) from the symbol file), each +of which has an associated address range (function start address, function +size). If a function is found whose address range contains the RVA, then its +name is used. The RVA is then used as a lookup into a table of source lines +([line records](SymbolFiles#Line_records.md) from the symbol file), each of +which also has an associated address range. If a match is found it will provide +the file name and source line associated with the current frame. If no match was +found in the function table, another table of publicly exported symbols may be +consulted ([PUBLIC lines](SymbolFiles#PUBLIC_records.md) from the symbol file). +Public symbols contain only a start address, so the lookup simply looks for the +nearest symbol that is less than the provided RVA. + +### Finding the caller frame + +To find the next frame in the stack, the !Stackwalker calls its [GetCallerFrame] +(http://code.google.com/p/google-breakpad/source/browse/trunk/src/google_breakpad/processor/stackwalker.h#186) +method, passing in the current frame. Each !Stackwalker subclass implements +GetCallerFrame differently, but there are common patterns. + +Typically the first step is to query the SourceLineResolverInterface for the +presence of detailed unwind information. This is done using its +[FindWindowsFrameInfo] +(http://code.google.com/p/google-breakpad/source/browse/trunk/src/google_breakpad/processor/source_line_resolver_interface.h#96) +and [FindCFIFrameInfo] +(http://code.google.com/p/google-breakpad/source/browse/trunk/src/google_breakpad/processor/source_line_resolver_interface.h#102) +methods. These methods look for Windows unwind info extracted from a PDB file +([STACK WIN](SymbolFiles#STACK_WIN_records.md) lines from the symbol file), or +DWARF CFI extracted from a binary ([STACK CFI](SymbolFiles#STACK_CFI_records.md) +lines from the symbol file) respectively. The information covers address ranges, +so the RVA of the current frame is used for lookup as with function and source +line information. + +If unwind info is found it provides a set of rules to recover the register state +of the caller frame given the current register state as well as the thread's +stack memory. The rules are evaluated to produce the caller frame. + +If unwind info is not found then the !Stackwalker may resort to other methods. +Typically on architectures which specify a frame pointer unwinding by +dereferencing the frame pointer is tried next. If that is successful it is used +to produce the caller frame. + +If no caller frame was found by any other method most !Stackwalker +implementations resort to stack scanning by looking at each word on the stack +down to a fixed depth (implemented in the [Stackwalker::ScanForReturnAddress] +(http://code.google.com/p/google-breakpad/source/browse/trunk/src/google_breakpad/processor/stackwalker.h#131) +method) and using a heuristic to attempt to find a reasonable return address +(implemented in the [Stackwalker::InstructionAddressSeemsValid] +(http://code.google.com/p/google-breakpad/source/browse/trunk/src/google_breakpad/processor/stackwalker.h#111) +method). + +If no caller frame is found or the caller frame seems invalid, stack walking +stops. If a caller frame was found then these steps repeat using the new frame +as the current frame. diff --git a/docs/symbol_files.md b/docs/symbol_files.md new file mode 100644 index 00000000..2fc157fc --- /dev/null +++ b/docs/symbol_files.md @@ -0,0 +1,497 @@ +# Introduction + +Given a minidump file, the Breakpad processor produces stack traces that include +function names and source locations. However, minidump files contain only the +byte-by-byte contents of threads' registers and stacks, without function names +or machine-code-to-source mapping data. The processor consults Breakpad symbol +files for the information it needs to produce human-readable stack traces from +the binary-only minidump file. + +The platform-specific symbol dumping tools parse the debugging information the +compiler provides (whether as DWARF or STABS sections in an ELF file or as +stand-alone PDB files), and write that information back out in the Breakpad +symbol file format. This format is much simpler and less detailed than compiler +debugging information, and values legibility over compactness. + +# Overview + +Breakpad symbol files are ASCII text files, with lines delimited as appropriate +for the host platform. Each line is a _record_, divided into fields by single +spaces; in some cases, the last field of the record can contain spaces. The +first field is a string indicating what sort of record the line represents +(except for line records; these are very common, making them the default saves +space). Some fields hold decimal or hexadecimal numbers; hexadecimal numbers +have no "0x" prefix, and use lower-case letters. + +Breakpad symbol files contain the following record types. With some +restrictions, these may appear in any order. + +* A `MODULE` record describes the executable file or shared library from which + this data was derived, for use by symbol suppliers. A `MODULE' record should + be the first record in the file. + +* A `FILE` record gives a source file name, and assigns it a number by which + other records can refer to it. + +* A `FUNC` record describes a function present in the source code. + +* A line record indicates to which source file and line a given range of + machine code should be attributed. The line is attributed to the function + defined by the most recent `FUNC` record. + +* A `PUBLIC` record gives the address of a linker symbol. + +* A `STACK` record provides information necessary to produce stack traces. + +# `MODULE` records + +A `MODULE` record provides meta-information about the module the symbol file +describes. It has the form: + +> `MODULE` _operatingsystem_ _architecture_ _id_ _name_ + +For example: `MODULE Linux x86 D3096ED481217FD4C16B29CD9BC208BA0 firefox-bin +` These records provide meta-information about the executable or shared library +from which this symbol file was generated. A symbol supplier might use this +information to find the correct symbol files to use to interpret a given +minidump, or to perform other sorts of validation. If present, a `MODULE` record +should be the first line in the file. + +The fields are separated by spaces, and cannot contain spaces themselves, except +for _name_. + +* The _operatingsystem_ field names the operating system on which the + executable or shared library was intended to run. This field should have one + of the following values: | **Value** | **Meaning** | + |:----------|:--------------------| | Linux | Linux | | mac | Macintosh OSX + | | windows | Microsoft Windows | + +* The _architecture_ field indicates what processor architecture the + executable or shared library contains machine code for. This field should + have one of the following values: | **Value** | **Instruction Set + Architecture** | |:----------|:---------------------------------| | x86 | + Intel IA-32 | | x86\_64 | AMD64/Intel 64 | | ppc | 32-bit PowerPC | | ppc64 + | 64-bit PowerPC | | unknown | unknown | + +* The _id_ field is a sequence of hexadecimal digits that identifies the exact + executable or library whose contents the symbol file describes. The way in + which it is computed varies from platform to platform. + +* The _name_ field contains the base name (the final component of the + directory path) of the executable or library. It may contain spaces, and + extends to the end of the line. + +# `FILE` records + +A `FILE` record holds a source file name for other records to refer to. It has +the form: + +> `FILE` _number_ _name_ + +For example: `FILE 2 /home/jimb/mc/in/browser/app/nsBrowserApp.cpp +` + +A `FILE` record provides the name of a source file, and assigns it a number +which other records (line records, in particular) can use to refer to that file +name. The _number_ field is a decimal number. The _name_ field is the name of +the file; it may contain spaces. + +# `FUNC` records + +A `FUNC` record describes a source-language function. It has the form: + +> `FUNC` _address_ _size_ _parameter\_size_ _name_ + +For example: `FUNC c184 30 0 nsQueryInterfaceWithError::operator()(nsID const&, +void**) const +` + +The _address_ and _size_ fields are hexadecimal numbers indicating the start +address and length in bytes of the machine code instructions the function +occupies. (Breakpad symbol files cannot accurately describe functions whose code +is not contiguous.) The start address is relative to the module's load address. + +The _parameter\_size_ field is a hexadecimal number indicating the size, in +bytes, of the arguments pushed on the stack for this function. Some calling +conventions, like the Microsoft Windows `stdcall` convention, require the called +function to pop parameters passed to it on the stack from its caller before +returning. The stack walker uses this value, along with data from `STACK` +records, to step from the called function's frame to the caller's frame. + +The _name_ field is the name of the function. In languages that use linker +symbol name mangling like C++, this should be the source language name (the +"unmangled" form). This field may contain spaces. + +# Line records + +A line record describes the source file and line number to which a given range +of machine code should be attributed. It has the form: + +> _address_ _size_ _line_ _filenum_ + +For example: `c184 7 59 4 +` + +Because they are so common, line records do not begin with a string indicating +the record type. All other record types' names use upper-case letters; +hexadecimal numbers, like a line record's _address_, use lower-case letters. + +The _address_ and _size_ fields are hexadecimal numbers indicating the start +address and length in bytes of the machine code. The address is relative to the +module's load address. + +The _line_ field is the line number to which the machine code should be +attributed, in decimal; the first line of the source file is line number 1. The +_filenum_ field is a decimal number appearing in a prior `FILE` record; the name +given in that record is the source file name for the machine code. + +The line is assumed to belong to the function described by the last preceding +`FUNC` record. Line records may not appear before the first `FUNC' record. + +No two line records in a symbol file cover the same range of addresses. However, +there may be many line records with identical line and file numbers, as a given +source line may contribute many non-contiguous blocks of machine code. + +# `PUBLIC` records + +A `PUBLIC` record describes a publicly visible linker symbol, such as that used +to identify an assembly language entry point or region of memory. It has the +form: + +> PUBLIC _address_ _parameter\_size_ _name_ + +For example: `PUBLIC 2160 0 Public2_1 +` + +The Breakpad processor essentially treats a `PUBLIC` record as defining a +function with no line number data and an indeterminate size: the code extends to +the next address mentioned. If a given address is covered by both a `PUBLIC` +record and a `FUNC` record, the processor uses the `FUNC` data. + +The _address_ field is a hexadecimal number indicating the symbol's address, +relative to the module's load address. + +The _parameter\_size_ field is a hexadecimal number indicating the size of the +parameters passed to the code whose entry point the symbol marks, if known. This +field has the same meaning as the _parameter\_size_ field of a `FUNC` record; +see that description for more details. + +The _name_ field is the name of the symbol. In languages that use linker symbol +name mangling like C++, this should be the source language name (the "unmangled" +form). This field may contain spaces. + +# `STACK WIN` records + +Given a stack frame, a `STACK WIN` record indicates how to find the frame that +called it. It has the form: + +> STACK WIN _type_ _rva_ _code\_size_ _prologue\_size_ _epilogue\_size_ +> _parameter\_size_ _saved\_register\_size_ _local\_size_ _max\_stack\_size_ +> _has\_program\_string_ _program\_string\_OR\_allocates\_base\_pointer_ + +For example: `STACK WIN 4 2170 14 1 0 0 0 0 0 1 $eip 4 + ^ = $esp $ebp 8 + = +$ebp $ebp ^ = +` + +All fields of a `STACK WIN` record, except for the last, are hexadecimal +numbers. + +The _type_ field indicates what sort of stack frame data this record holds. Its +value should be one of the values of the [StackFrameTypeEnum] +(http://msdn.microsoft.com/en-us/library/bc5207xw%28VS.100%29.aspx) type in +Microsoft's [Debug Interface Access (DIA)] +(http://msdn.microsoft.com/en-us/library/x93ctkx8%28VS.100%29.aspx) API. +Breakpad uses only records of type 4 (`FrameTypeFrameData`) and 0 +(`FrameTypeFPO`); it ignores others. These types differ only in whether the last +field is an _allocates\_base\_pointer_ flag (`FrameTypeFPO`) or a program string +(`FrameTypeFrameData`). If more than one record covers a given address, Breakpad +prefers `FrameTypeFrameData` records over `FrameTypeFPO` records. + +The _rva_ and _code\_size_ fields give the starting address and length in bytes +of the machine code covered by this record. The starting address is relative to +the module's load address. + +The _prologue\_size_ and _epilogue\_size_ fields give the length, in bytes, of +the prologue and epilogue machine code within the record's range. Breakpad does +not use these values. + +The _parameter\_size_ field gives the number of argument bytes this function +expects to have been passed. This field has the same meaning as the +_parameter\_size_ field of a `FUNC` record; see that description for more +details. + +The _saved\_register\_size_ field gives the number of bytes in the stack frame +dedicated to preserving the values of any callee-saves registers used by this +function. + +The _local\_size_ field gives the number of bytes in the stack frame dedicated +to holding the function's local variables and temporary values. + +The _max\_stack\_size_ field gives the maximum number of bytes pushed on the +stack in the frame. Breakpad does not use this value. + +If the _has\_program\_string_ field is zero, then the `STACK WIN` record's final +field is an _allocates\_base\_pointer_ flag, as a hexadecimal number; this is +expected for records whose _type_ is 0. Otherwise, the final field is a program +string. + +## Interpreting a `STACK WIN` record + +Given the register values for a frame F, we can find the calling frame as +follows: + +* If the _has\_program\_string_ field of a `STACK WIN` record is zero, then + the final field is _allocates\_base\_pointer_, a flag indicating whether the + frame uses the frame pointer register, `%ebp`, as a general-purpose + register. + * If _allocates\_base\_pointer_ is true, then `%ebp` does not point to the + frame's base address. Instead, + * Let _next\_parameter\_size_ be the parameter size of the function + frame F called (**not** this record's _parameter\_size_ field), or + zero if F is the youngest frame on the stack. You must find this + value in F's callee's `FUNC`, `STACK WIN`, or `PUBLIC` records. + * Let _frame\_size_ be the sum of the _local\_size_ field, the + _saved\_register\_size_ field, and _next\_parameter\_size_. > > With + those definitions in place, we can recover the calling frame as + follows: + * F's return address is at `%esp +`_frame\_size_, + * the caller's value of `%ebp` is saved at `%esp + +`_next\_parameter\_size_`+`_saved\_register\_size_`- 8`, and + * the caller's value of `%esp` just before the call instruction was + `%esp +`_frame\_size_`+ 4`. > > (Why do we include + _next\_parameter\_size_ in the sum when computing _frame\_size_ and + the address of the saved `%ebp`? When a function A has called a + function B, the arguments that A pushed for B are considered part of + A's stack frame: A's value for `%esp` points at the last argument + pushed for B. Thus, we must include the size of those arguments + (given by the debugging info for B) along with the size of A's + register save area and local variable area (given by the debugging + info for A) when computing the overall size of A's frame.) + * If _allocates\_base\_pointer_ is false, then F's function doesn't use + `%ebp` at all. You may recover the calling frame as above, except that + the caller's value of `%ebp` is the same as F's value for `%ebp`, so no + steps are necessary to recover it. +* If the _has\_program\_string_ field of a `STACK WIN` record is not zero, + then the record's final field is a string containing a program to be + interpreted to recover the caller's frame. The comments in the + [postfix\_evaluator.h] + (http://code.google.com/p/google-breakpad/source/browse/trunk/src/processor/postfix_evaluator.h#40) + header file explain the language in which the program is written. You should + place the following variables in the dictionary before interpreting the + program: + * `$ebp` and `$esp` should be the values of the `%ebp` and `%esp` + registers in F. + * `.cbParams`, `.cbSavedRegs`, and `.cbLocals`, should be the values of + the `STACK WIN` record's _parameter\_size_, _saved\_register\_size_, and + _local\_size_ fields. + * `.raSearchStart` should be set to the address on the stack to begin + scanning for a return address, if necessary. The Breakpad processor sets + this to the value of `%esp` in F, plus the _frame\_size_ value mentioned + above. + +> If the program stores values for `$eip`, `$esp`, `$ebp`, `$ebx`, `$esi`, or +> `$edi`, then those are the values of the given registers in the caller. If the +> value of `$eip` is zero, that indicates that the end of the stack has been +> reached. + +The Breakpad processor checks that the value yielded by the above for the +calling frame's instruction address refers to known code; if the address seems +to be bogus, then it uses a heuristic search to find F's return address and +stack base. + +# `STACK CFI` records + +`STACK CFI` ("Call Frame Information") records describe how to walk the stack +when execution is at a given machine instruction. These records take one of two +forms: + +> `STACK CFI INIT` _address_ _size_ _register<sub>1</sub>_: +> _expression<sub>1</sub>_ _register<sub>2</sub>_: _expression<sub>2</sub>_ ... +> +> `STACK CFI` _address_ _register<sub>1</sub>_: _expression<sub>1</sub>_ +> _register<sub>2</sub>_: _expression<sub>2</sub>_ ... + +For example: + +``` +STACK CFI INIT 804c4b0 40 .cfa: $esp 4 + $eip: .cfa 4 - ^ +STACK CFI 804c4b1 .cfa: $esp 8 + $ebp: .cfa 8 - ^ +``` + +The _address_ and _size_ fields are hexadecimal numbers. Each +_register_<sub>i</sub> is the name of a register or pseudoregister. Each +_expression_ is a Breakpad postfix expression, which may contain spaces, but +never ends with a colon. (The appropriate register names for a given +architecture are determined when `STACK CFI` records are first enabled for that +architecture, and should be documented in the appropriate +`stackwalker_`_architecture_`.cc` source file.) + +STACK CFI records describe, at each machine instruction in a given function, how +to recover the values the machine registers had in the function's caller. +Naturally, some registers' values are simply lost, but there are three cases in +which they can be recovered: + +* You can always recover the program counter, because that's the function's + return address. If the function is ever going to return, the PC must be + saved somewhere. + +* You can always recover the stack pointer. The function is responsible for + popping its stack frame before it returns to the caller, so it must be able + to restore this, as well. + +* You should be able to recover the values of callee-saves registers. These + are registers whose values the callee must preserve, either by saving them + in its own stack frame before using them and re-loading them before + returning, or by not using them at all. + +(As an exception, note that functions which never return may not save any of +this data. It may not be possible to walk the stack past such functions' stack +frames.) + +Given rules for recovering the values of a function's caller's registers, we can +walk up the stack. Starting with the current set of registers --- the PC of the +instruction we're currently executing, the current stack pointer, etc. --- we +use CFI to recover the values those registers had in the caller of the current +frame. This gives us a PC in the caller whose CFI we can look up; we apply the +process again to find that function's caller; and so on. + +Concretely, CFI records represent a table with a row for each machine +instruction address and a column for each register. The table entry for a given +address and register contains a rule describing how, when the PC is at that +address, to restore the value that register had in the caller. + +There are some special columns: + +* A column named `.cfa`, for "Canonical Frame Address", tells how to compute + the base address of the frame; other entries can refer to the CFA in their + rules. + +* A column named `.ra` represents the return address. + +For example, suppose we have a machine with 32-bit registers, one-byte +instructions, a stack that grows downwards, and an assembly language that +resembles C. Suppose further that we have a function whose machine code looks +like this: + +``` +func: ; entry point; return address at sp +func+0: sp -= 16 ; allocate space for stack frame +func+1: sp[12] = r0 ; save 4-byte r0 at sp+12 + ... ; stuff that doesn't affect stack +func+10: sp -= 4; *sp = x ; push some 4-byte x on the stack + ... ; stuff that doesn't affect stack +func+20: r0 = sp[16] ; restore saved r0 +func+21: sp += 20 ; pop whole stack frame +func+22: pc = *sp; sp += 4 ; pop return address and jump to it +``` + +The following table would describe the function above: + +**code address** | **.cfa** | **r0 (on Google Code)** | **r1 (on Google Code)** | ... | **.ra** +:--------------- | :------- | :---------------------- | :---------------------- | :-- | :------- +func+0 | sp | | | | `cfa[0]` +func+1 | sp+16 | | | | `cfa[0]` +func+2 | sp+16 | `cfa[-4]` | | | `cfa[0]` +func+11 | sp+20 | `cfa[-4]` | | | `cfa[0]` +func+21 | sp+20 | | | | `cfa[0]` +func+22 | sp | | | | `cfa[0]` + +Some things to note here: + +* Each row describes the state of affairs **before** executing the instruction + at the given address. Thus, the row for func+0 describes the state before we + execute the first instruction, which allocates the stack frame. In the next + row, the formula for computing the CFA has changed, reflecting the + allocation. + +* The other entries are written in terms of the CFA; this allows them to + remain unchanged as the stack pointer gets bumped around. For example, to + find the caller's value for r0 (on Google Code) at func+2, we would first + compute the CFA by adding 16 to the sp, and then subtract four from that to + find the address at which r0 (on Google Code) was saved. + +* Although the example doesn't show this, most calling conventions designate + "callee-saves" and "caller-saves" registers. The callee must restore the + values of "callee-saves" registers before returning (if it uses them at + all), whereas the callee is free to use "caller-saves" registers without + restoring their values. A function that uses caller-saves registers + typically does not save their original values at all; in this case, the CFI + marks such registers' values as "unrecoverable". + +* Exactly where the CFA points in the frame --- at the return address? below + it? At some fixed point within the frame? --- is a question of definition + that depends on the architecture and ABI in use. But by definition, the CFA + remains constant throughout the lifetime of the frame. It's up to + architecture- specific code to know what significance to assign the CFA, if + any. + +To save space, the most common type of CFI record only mentions the table +entries at which changes take place. So for the above, the CFI data would only +actually mention the non-blank entries here: + +**insn** | **cfa** | **r0 (on Google Code)** | **r1 (on Google Code)** | ... | **ra** +:------- | :------ | :---------------------- | :---------------------- | :-- | :------- +func+0 | sp | | | | `cfa[0]` +func+1 | sp+16 | | | | +func+2 | | `cfa[-4]` | | | +func+11 | sp+20 | | | | +func+21 | | r0 (on Google Code) | | | +func+22 | sp | | | | + +A `STACK CFI INIT` record indicates that, at the machine instruction at +_address_, belonging to some function, the value that _register<sub>n</sub>_ had +in that function's caller can be recovered by evaluating +_expression<sub>n</sub>_. The values of any callee-saves registers not mentioned +are assumed to be unchanged. (`STACK CFI` records never mention caller-saves +registers.) These rules apply starting at _address_ and continue up to, but not +including, the address given in the next `STACK CFI` record. The _size_ field is +the total number of bytes of machine code covered by this record and any +subsequent `STACK CFI` records (until the next `STACK CFI INIT` record). The +_address_ field is relative to the module's load address. + +A `STACK CFI` record (no `INIT`) is the same, except that it mentions only those +registers whose recovery rules have changed from the previous CFI record. There +must be a prior `STACK CFI INIT` or `STACK CFI` record in the symbol file. The +_address_ field of this record must be greater than that of the previous record, +and it must not be at or beyond the end of the range given by the most recent +`STACK CFI INIT` record. The address is relative to the module's load address. + +Each expression is a breakpad-style postfix expression. Expressions may contain +spaces, but their tokens may not end with colons. When an expression mentions a +register, it refers to the value of that register in the callee, even if a prior +name/expression pair gives that register's value in the caller. The exception is +`.cfa`, which refers to the canonical frame address computed by the .cfa rule in +force at the current instruction. + +The special expression `.undef` indicates that the given register's value cannot +be recovered. + +The register names preceding the expressions are always followed by colons. The +expressions themselves never contain tokens ending with colons. + +There are two special register names: + +* `.cfa` ("Canonical Frame Address") is the base address of the stack frame. + Other registers' rules may refer to this. If no rule is provided for the + stack pointer, the value of `.cfa` is the caller's stack pointer. + +* `.ra` is the return address. This is the value of the restored program + counter. We use `.ra` instead of the architecture-specific name for the + program counter. + +The Breakpad stack walker requires that there be rules in force for `.cfa` and +`.ra` at every code address from which it unwinds. If those rules are not +present, the stack walker will ignore the `STACK CFI` data, and try to use a +different strategy. + +So the CFI for the example function above would be as follows, if `func` were at +address 0x1000 (relative to the module's load address): + +``` +STACK CFI INIT 1000 .cfa: $sp .ra: .cfa ^ +STACK CFI 1001 .cfa: $sp 16 + +STACK CFI 1002 $r0: .cfa 4 - ^ +STACK CFI 100b .cfa: $sp 20 + +STACK CFI 1015 $r0: $r0 +STACK CFI 1016 .cfa: $sp +``` diff --git a/docs/windows_client_integration.md b/docs/windows_client_integration.md new file mode 100644 index 00000000..99a84926 --- /dev/null +++ b/docs/windows_client_integration.md @@ -0,0 +1,70 @@ +# Windows Integration overview + +## Windows Client Code + +The Windows client code is in the `src/client/windows` directory of the tree. +Since the header files are fairly well commented some specifics are purposely +omitted from this document. + +## Integration of minidump-generation + +Once you build the solution inside `src/client/windows`, an output file of +`exception_handler.lib` will be generated. You can either check this into your +project's directory or build directly from the source, as the project itself +does. + +Enabling Breakpad in your application requires you to `#include +"exception_handler.h"` and instantiate the `ExceptionHandler` object like so: + +``` + handler = new ExceptionHandler(const wstring& dump_path, + FilterCallback filter, + MinidumpCallback callback, + void* callback_context, + int handler_types, + MINIDUMP_TYPE dump_type, + const wchar_t* pipe_name, + const CustomClientInfo* custom_info); +``` + +The parameters, in order, are: + +* pathname for minidumps to be written to - this is ignored if OOP dump + generation is used +* A callback that is called when the exception is first handled - you can + return true/false here to continue/stop exception processing +* A callback that is called after minidumps have been written +* Context for the callbacks +* Which exceptions to handle - see `HandlerType` enumeration in + exception\_handler.h +* The type of minidump to generate, using the `MINIDUMP_TYPE` definitions in + `DbgHelp.h` +* A pipe name that can be used to communicate with a crash generation server +* A pointer to a CustomClientInfo class that can be used to send custom data + along with the minidump when using OOP generation + +You can also see `src/client/windows/tests/crash_generation_app/*` for a sample +app that uses OOP generation. + +## OOP Minidump Generation + +For out of process minidump generation, more work is needed. If you look inside +`src/client/windows/crash_generation`, you will see a file called +`crash_generation_server.h`. This file is the interface for a crash generation +server, which must be instantiated with the same pipe name that is passed to the +client above. The logistics of running a separate process that instantiates the +crash generation server is left up to you, however. + +## Build process specifics(symbol generation, upload) + +The symbol creation step is talked about in the general overview doc, since it +doesn't vary much by platform. You'll need to make sure that the symbols are +available wherever minidumps are uploaded to for processing. + +## Out in the field - uploading the minidump + +Inside `src/client/windows/sender` is a class implementation called +`CrashReportSender`. This class can be compiled into a separate standalone CLI +or in the crash generation server and used to upload the report; it can know +when to do so via one of the callbacks provided by the `CrashGenerationServer` +or the `ExceptionHandler` object for in-process generation. |
