path: root/src/processor/stackwalker_x86.cc

// Copyright (c) 2006, Google Inc.
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// modification, are permitted provided that the following conditions are
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//
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// stackwalker_x86.cc: x86-specific stackwalker.
//
// See stackwalker_x86.h for documentation.
//
// Author: Mark Mentovai


#include "processor/postfix_evaluator-inl.h"

#include "processor/stackwalker_x86.h"
#include "google_airbag/processor/call_stack.h"
#include "google_airbag/processor/memory_region.h"
#include "google_airbag/processor/stack_frame_cpu.h"
#include "processor/linked_ptr.h"
#include "processor/stack_frame_info.h"

namespace google_airbag {


StackwalkerX86::StackwalkerX86(const MDRawContextX86 *context,
                               MemoryRegion *memory,
                               const CodeModules *modules,
                               SymbolSupplier *supplier,
                               SourceLineResolverInterface *resolver)
    : Stackwalker(memory, modules, supplier, resolver),
      context_(context) {
  if (memory_->GetBase() + memory_->GetSize() - 1 > 0xffffffff) {
    // The x86 is a 32-bit CPU, the limits of the supplied stack are invalid.
    // Mark memory_ = NULL, which will cause stackwalking to fail.
    memory_ = NULL;
  }
}


StackFrame* StackwalkerX86::GetContextFrame() {
  if (!context_ || !memory_)
    return NULL;

  StackFrameX86 *frame = new StackFrameX86();

  // The instruction pointer is stored directly in a register, so pull it
  // straight out of the CPU context structure.
  frame->context = *context_;
  frame->context_validity = StackFrameX86::CONTEXT_VALID_ALL;
  frame->instruction = frame->context.eip;

  return frame;
}


StackFrame* StackwalkerX86::GetCallerFrame(
    const CallStack *stack,
    const vector< linked_ptr<StackFrameInfo> > &stack_frame_info) {
  if (!memory_ || !stack)
    return NULL;

  StackFrameX86 *last_frame = static_cast<StackFrameX86*>(
      stack->frames()->back());
  StackFrameInfo *last_frame_info = stack_frame_info.back().get();

  // This stackwalker sets each frame's %esp to its value immediately prior
  // to the CALL into the callee.  This means that %esp points to the last
  // callee argument pushed onto the stack, which may not be where %esp points
  // after the callee returns.  Specifically, the value is correct for the
  // cdecl calling convention, but not other conventions.  The cdecl
  // convention requires a caller to pop its callee's arguments from the
  // stack after the callee returns.  This is usually accomplished by adding
  // the known size of the arguments to %esp.  Other calling conventions,
  // including stdcall, thiscall, and fastcall, require the callee to pop any
  // parameters stored on the stack before returning.  This is usually
  // accomplished by using the RET n instruction, which pops n bytes off
  // the stack after popping the return address.
  //
  // Because each frame's %esp will point to a location on the stack after
  // callee arguments have been PUSHed, when locating things in a stack frame
  // relative to %esp, the size of the arguments to the callee need to be
  // taken into account.  This seems a little bit unclean, but it's better
  // than the alternative, which would need to take these same things into
  // account, but only for cdecl functions.  With this implementation, we get
  // to be agnostic about each function's calling convention.  Furthermore,
  // this is how Windows debugging tools work, so it means that the %esp
  // values produced by this stackwalker directly correspond to the %esp
  // values you'll see there.
  //
  // If the last frame has no callee (because it's the context frame), just
  // set the callee parameter size to 0: the stack pointer can't point to
  // callee arguments because there's no callee.  This is correct as long
  // as the context wasn't captured while arguments were being pushed for
  // a function call.  Note that there may be functions whose parameter sizes
  // are unknown, 0 is also used in that case.  When that happens, it should
  // be possible to walk to the next frame without reference to %esp.

  int frames_already_walked = stack_frame_info.size();
  u_int32_t last_frame_callee_parameter_size = 0;
  if (frames_already_walked >= 2) {
    StackFrameInfo *last_frame_callee_info =
        stack_frame_info[frames_already_walked - 2].get();
    if (last_frame_callee_info &&
        last_frame_callee_info->valid & StackFrameInfo::VALID_PARAMETER_SIZE) {
      last_frame_callee_parameter_size =
          last_frame_callee_info->parameter_size;
    }
  }

  // Set up the dictionary for the PostfixEvaluator.  %ebp and %esp are used
  // in each program string, and their previous values are known, so set them
  // here.  .cbCalleeParams is an Airbag extension that allows us to use
  // the PostfixEvaluator engine when certain types of debugging information
  // are present without having to write the constants into the program string
  // as literals.
  PostfixEvaluator<u_int32_t>::DictionaryType dictionary;
  dictionary["$ebp"] = last_frame->context.ebp;
  dictionary["$esp"] = last_frame->context.esp;
  dictionary[".cbCalleeParams"] = last_frame_callee_parameter_size;

  if (last_frame_info && last_frame_info->valid == StackFrameInfo::VALID_ALL) {
    // FPO debugging data is available.  Initialize constants.
    dictionary[".cbSavedRegs"] = last_frame_info->saved_register_size;
    dictionary[".cbLocals"] = last_frame_info->local_size;
    dictionary[".raSearchStart"] = last_frame->context.esp +
                                   last_frame_callee_parameter_size +
                                   last_frame_info->local_size +
                                   last_frame_info->saved_register_size;
  }
  if (last_frame_info &&
      last_frame_info->valid & StackFrameInfo::VALID_PARAMETER_SIZE) {
    // This is treated separately because it can either come from FPO data or
    // from other debugging data.
    dictionary[".cbParams"] = last_frame_info->parameter_size;
  }

  // Decide what type of program string to use.  The program string is in
  // postfix notation and will be passed to PostfixEvaluator::Evaluate.
  // Given the dictionary and the program string, it is possible to compute
  // the return address and the values of other registers in the calling
  // function.
  string program_string;
  if (last_frame_info && last_frame_info->valid == StackFrameInfo::VALID_ALL) {
    // FPO data available.
    if (!last_frame_info->program_string.empty()) {
      // The FPO data has its own program string, which will tell us how to
      // get to the caller frame, and may even fill in the values of
      // nonvolatile registers and provide pointers to local variables and
      // parameters.
      program_string = last_frame_info->program_string;
    } else if (last_frame_info->allocates_base_pointer) {
      // The function corresponding to the last frame doesn't use the frame
      // pointer for conventional purposes, but it does allocate a new
      // frame pointer and use it for its own purposes.  Its callee's
      // information is still accessed relative to %esp, and the previous
      // value of %ebp can be recovered from a location in its stack frame,
      // within the saved-register area.
      //
      // Functions that fall into this category use the %ebp register for
      // a purpose other than the frame pointer.  They restore the caller's
      // %ebp before returning.  These functions create their stack frame
      // after a CALL by decrementing the stack pointer in an amount
      // sufficient to store local variables, and then PUSHing saved
      // registers onto the stack.  Arguments to a callee function, if any,
      // are PUSHed after that.  Walking up to the caller, therefore,
      // can be done solely with calculations relative to the stack pointer
      // (%esp).  The return address is recovered from the memory location
      // above the known sizes of the callee's parameters, saved registers,
      // and locals.  The caller's stack pointer (the value of %esp when
      // the caller executed CALL) is the location immediately above the
      // saved return address.  The saved value of %ebp to be restored for
      // the caller is at a known location in the saved-register area of
      // the stack frame.
      //
      // %eip_new = *(%esp_old + callee_params + saved_regs + locals)
      // %ebp_new = *(%esp_old + callee_params + saved_regs - 8)
      // %esp_new = %esp_old + callee_params + saved_regs + locals + 4
      program_string = "$eip .raSearchStart ^ = "
                       "$ebp $esp .cbCalleeParams + .cbSavedRegs + 8 - ^ = "
                       "$esp .raSearchStart 4 + =";
    } else {
      // The function corresponding to the last frame doesn't use %ebp at
      // all.  The callee frame is located relative to %esp.  %ebp is reset
      // to itself only to cause it to appear to have been set in
      // dictionary_validity.
      //
      // The called procedure's instruction pointer and stack pointer are
      // recovered in the same way as the case above, except that no
      // frame pointer (%ebp) is used at all, so it is not saved anywhere
      // in the callee's stack frame and does not need to be recovered.
      // Because %ebp wasn't used in the callee, whatever value it has
      // is the value that it had in the caller, so it can be carried
      // straight through without bringing its validity into question.
      //
      // %eip_new = *(%esp_old + callee_params + saved_regs + locals)
      // %esp_new = %esp_old + callee_params + saved_regs + locals + 4
      // %ebp_new = %ebp_old
      program_string = "$eip .raSearchStart ^ = "
                       "$esp .raSearchStart 4 + = "
                       "$ebp $ebp =";
    }
  } else {
    // No FPO information is available for the last frame.  Assume that the
    // standard %ebp-using x86 calling convention is in use.
    //
    // The typical x86 calling convention, when frame pointers are present,
    // is for the calling procedure to use CALL, which pushes the return
    // address onto the stack and sets the instruction pointer (%eip) to
    // the entry point of the called routine.  The called routine then
    // PUSHes the calling routine's frame pointer (%ebp) onto the stack
    // before copying the stack pointer (%esp) to the frame pointer (%ebp).
    // Therefore, the calling procedure's frame pointer is always available
    // by dereferencing the called procedure's frame pointer, and the return
    // address is always available at the memory location immediately above
    // the address pointed to by the called procedure's frame pointer.  The
    // calling procedure's stack pointer (%esp) is 8 higher than the value
    // of the called procedure's frame pointer at the time the calling
    // procedure made the CALL: 4 bytes for the return address pushed by the
    // CALL itself, and 4 bytes for the callee's PUSH of the caller's frame
    // pointer.
    //
    // %eip_new = *(%ebp_old + 4)
    // %esp_new = %ebp_old + 8
    // %ebp_new = *(%ebp_old)
    program_string = "$eip $ebp 4 + ^ = "
                     "$esp $ebp 8 + = "
                     "$ebp $ebp ^ =";
  }

  // Now crank it out, making sure that the program string set the three
  // required variables.
  PostfixEvaluator<u_int32_t> evaluator =
      PostfixEvaluator<u_int32_t>(&dictionary, memory_);
  PostfixEvaluator<u_int32_t>::DictionaryValidityType dictionary_validity;
  if (!evaluator.Evaluate(program_string, &dictionary_validity) ||
      dictionary_validity.find("$eip") == dictionary_validity.end() ||
      dictionary_validity.find("$esp") == dictionary_validity.end() ||
      dictionary_validity.find("$ebp") == dictionary_validity.end()) {
    return NULL;
  }

  // Treat an instruction address of 0 as end-of-stack.  Treat incorrect stack
  // direction as end-of-stack to enforce progress and avoid infinite loops.
  if (dictionary["$eip"] == 0 ||
      dictionary["$esp"] <= last_frame->context.esp) {
    return NULL;
  }

  // Create a new stack frame (ownership will be transferred to the caller)
  // and fill it in.
  StackFrameX86 *frame = new StackFrameX86();

  frame->context = last_frame->context;
  frame->context.eip = dictionary["$eip"];
  frame->context.esp = dictionary["$esp"];
  frame->context.ebp = dictionary["$ebp"];
  frame->context_validity = StackFrameX86::CONTEXT_VALID_EIP |
                                StackFrameX86::CONTEXT_VALID_ESP |
                                StackFrameX86::CONTEXT_VALID_EBP;

  // These are nonvolatile (callee-save) registers, and the program string
  // may have filled them in.
  if (dictionary_validity.find("$ebx") != dictionary_validity.end()) {
    frame->context.ebx = dictionary["$ebx"];
    frame->context_validity |= StackFrameX86::CONTEXT_VALID_EBX;
  }
  if (dictionary_validity.find("$esi") != dictionary_validity.end()) {
    frame->context.esi = dictionary["$esi"];
    frame->context_validity |= StackFrameX86::CONTEXT_VALID_ESI;
  }
  if (dictionary_validity.find("$edi") != dictionary_validity.end()) {
    frame->context.edi = dictionary["$edi"];
    frame->context_validity |= StackFrameX86::CONTEXT_VALID_EDI;
  }

  // frame->context.eip is the return address, which is one instruction
  // past the CALL that caused us to arrive at the callee.  Set
  // frame->instruction to one less than that.  This won't reference the
  // beginning of the CALL instruction, but it's guaranteed to be within the
  // CALL, which is sufficient to get the source line information to match up
  // with the line that contains a function call.  Callers that require the
  // exact return address value may access the context.eip field of
  // StackFrameX86.
  frame->instruction = frame->context.eip - 1;

  return frame;
}


}  // namespace google_airbag