Handle very large stack traces

The main motivation for this change is to handle very large stack
traces, normally the result of infinite recursion. This part is
actually fairly simple, relaxing a few self-imposed limits on how
many frames we can unwind and the max size for stack memory.

Relaxing these limits requires stricter and more consistent checks for
stack unwinding. There are a number of unwinding invariants that apply
to all the platforms:

1. stack pointer (and frame pointer) must be within the stack memory
   (frame pointer, if preset, must point to the right frame too)
2. unwinding must monotonically increase SP
   (except for the first frame unwind, this must be a strict increase)
3. Instruction pointer (return address) must point to a valid location
4. stack pointer (and frame pointer) must be appropriately aligned

This change is focused on 2), which is enough to guarantee that the
unwinding doesn't get stuck in an infinite loop.

1) is implicitly validated part of accessing the stack memory
   (explicit checks might be nice though).
4) is ABI specific and while it may be valuable in catching suspicious
   frames is not in the scope of this change.
3) is also an interesting check but thanks to just-in-time compilation
   it's more complex than just calling 
   StackWalker::InstructionAddressSeemsValid() 
   and we don't want to drop parts of the callstack due to an overly
   conservative check.

Bug: chromium:735989

Change-Id: I9aaba77c7fd028942d77c87d51b5e6f94e136ddd
Reviewed-on: https://chromium-review.googlesource.com/563771
Reviewed-by: Mark Mentovai <mark@chromium.org>
Reviewed-by: Ivan Penkov <ivanpe@chromium.org>

2 files changed, 17 insertions, 3 deletions

diff --git a/src/google_breakpad/processor/minidump.h b/src/google_breakpad/processor/minidump.h
index 2651a323..34a2a44d 100644
--- a/src/google_breakpad/processor/minidump.h
+++ b/src/google_breakpad/processor/minidump.h
@@ -257,8 +257,7 @@ class MinidumpMemoryRegion : public MinidumpObject,
   bool hexdump_;
   unsigned int hexdump_width_;
 
-  // The largest memory region that will be read from a minidump.  The
-  // default is 1MB.
+  // The largest memory region that will be read from a minidump.
   static uint32_t max_bytes_;
 
   // Base address and size of the memory region, and its position in the
diff --git a/src/google_breakpad/processor/stackwalker.h b/src/google_breakpad/processor/stackwalker.h
index 4378f75b..0c458d50 100644
--- a/src/google_breakpad/processor/stackwalker.h
+++ b/src/google_breakpad/processor/stackwalker.h
@@ -126,7 +126,15 @@ class Stackwalker {
   // * This address is within a loaded module for which we have symbols,
   //   and falls inside a function in that module.
   // Returns false otherwise.
-  bool InstructionAddressSeemsValid(uint64_t address);
+  bool InstructionAddressSeemsValid(uint64_t address) const;
+
+  // Checks whether we should stop the stack trace.
+  // (either we reached the end-of-stack or we detected a
+  //  broken callstack invariant)
+  bool TerminateWalk(uint64_t caller_ip,
+                     uint64_t caller_sp,
+                     uint64_t callee_sp,
+                     bool first_unwind) const;
 
   // The default number of words to search through on the stack
   // for a return address.
@@ -217,6 +225,13 @@ class Stackwalker {
   // the caller.  |stack_scan_allowed| controls whether stack scanning is
   // an allowable frame-recovery method, since it is desirable to be able to
   // disable stack scanning in performance-critical use cases.
+  //
+  // CONSIDER: a way to differentiate between:
+  //  - full stack traces
+  //  - explicitly truncated traces (max_frames_)
+  //  - stopping after max scanned frames
+  //  - failed stack walk (breaking one of the stack walk invariants)
+  //
   virtual StackFrame* GetCallerFrame(const CallStack* stack,
                                      bool stack_scan_allowed) = 0;