src/processor/stackwalker_arm64.cc - breakpad - Git at Google

 // Copyright (c) 2013 Google Inc.
 // All rights reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 // notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 // copyright notice, this list of conditions and the following disclaimer
 // in the documentation and/or other materials provided with the
 // distribution.
 //     * Neither the name of Google Inc. nor the names of its
 // contributors may be used to endorse or promote products derived from
 // this software without specific prior written permission.
 //
 // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

 // stackwalker_arm64.cc: arm64-specific stackwalker.
 //
 // See stackwalker_arm64.h for documentation.
 //
 // Author: Mark Mentovai, Ted Mielczarek, Jim Blandy, Colin Blundell

 #include <vector>

 #include "common/scoped_ptr.h"
 #include "google_breakpad/processor/call_stack.h"
 #include "google_breakpad/processor/memory_region.h"
 #include "google_breakpad/processor/source_line_resolver_interface.h"
 #include "google_breakpad/processor/stack_frame_cpu.h"
 #include "processor/cfi_frame_info.h"
 #include "processor/logging.h"
 #include "processor/stackwalker_arm64.h"

 namespace google_breakpad {


 StackwalkerARM64::StackwalkerARM64(const SystemInfo* system_info,
                                    const MDRawContextARM64* context,
                                    MemoryRegion* memory,
                                    const CodeModules* modules,
                                    StackFrameSymbolizer* resolver_helper)
     : Stackwalker(system_info, memory, modules, resolver_helper),
       context_(context),
       context_frame_validity_(StackFrameARM64::CONTEXT_VALID_ALL) { }


 StackFrame* StackwalkerARM64::GetContextFrame() {
   if (!context_) {
     BPLOG(ERROR) << "Can't get context frame without context";
     return NULL;
   }

   StackFrameARM64* frame = new StackFrameARM64();

   // The instruction pointer is stored directly in a register (x32), so pull it
   // straight out of the CPU context structure.
   frame->context = *context_;
   frame->context_validity = context_frame_validity_;
   frame->trust = StackFrame::FRAME_TRUST_CONTEXT;
   frame->instruction = frame->context.iregs[MD_CONTEXT_ARM64_REG_PC];

   return frame;
 }

 StackFrameARM64* StackwalkerARM64::GetCallerByCFIFrameInfo(
     const vector<StackFrame*> &frames,
     CFIFrameInfo* cfi_frame_info) {
   StackFrameARM64* last_frame = static_cast<StackFrameARM64*>(frames.back());

   static const char* register_names[] = {
     "x0",  "x1",  "x2",  "x3",  "x4",  "x5",  "x6",  "x7",
     "x8",  "x9",  "x10", "x11", "x12", "x13", "x14", "x15",
     "x16", "x17", "x18", "x19", "x20", "x21", "x22", "x23",
     "x24", "x25", "x26", "x27", "x28", "x29", "x30", "sp",
     "pc",  NULL
   };

   // Populate a dictionary with the valid register values in last_frame.
   CFIFrameInfo::RegisterValueMap<uint64_t> callee_registers;
   for (int i = 0; register_names[i]; i++) {
     if (last_frame->context_validity & StackFrameARM64::RegisterValidFlag(i))
       callee_registers[register_names[i]] = last_frame->context.iregs[i];
   }

   // Use the STACK CFI data to recover the caller's register values.
   CFIFrameInfo::RegisterValueMap<uint64_t> caller_registers;
   if (!cfi_frame_info->FindCallerRegs(callee_registers, *memory_,
                                       &caller_registers)) {
     return NULL;
   }
   // Construct a new stack frame given the values the CFI recovered.
   scoped_ptr<StackFrameARM64> frame(new StackFrameARM64());
   for (int i = 0; register_names[i]; i++) {
     CFIFrameInfo::RegisterValueMap<uint64_t>::iterator entry =
       caller_registers.find(register_names[i]);
     if (entry != caller_registers.end()) {
       // We recovered the value of this register; fill the context with the
       // value from caller_registers.
       frame->context_validity |= StackFrameARM64::RegisterValidFlag(i);
       frame->context.iregs[i] = entry->second;
     } else if (19 <= i && i <= 29 && (last_frame->context_validity &
                                       StackFrameARM64::RegisterValidFlag(i))) {
       // If the STACK CFI data doesn't mention some callee-saves register, and
       // it is valid in the callee, assume the callee has not yet changed it.
       // Registers r19 through r29 are callee-saves, according to the Procedure
       // Call Standard for the ARM AARCH64 Architecture, which the Linux ABI
       // follows.
       frame->context_validity |= StackFrameARM64::RegisterValidFlag(i);
       frame->context.iregs[i] = last_frame->context.iregs[i];
     }
   }
   // If the CFI doesn't recover the PC explicitly, then use .ra.
   if (!(frame->context_validity & StackFrameARM64::CONTEXT_VALID_PC)) {
     CFIFrameInfo::RegisterValueMap<uint64_t>::iterator entry =
       caller_registers.find(".ra");
     if (entry != caller_registers.end()) {
       frame->context_validity |= StackFrameARM64::CONTEXT_VALID_PC;
       frame->context.iregs[MD_CONTEXT_ARM64_REG_PC] = entry->second;
     }
   }
   // If the CFI doesn't recover the SP explicitly, then use .cfa.
   if (!(frame->context_validity & StackFrameARM64::CONTEXT_VALID_SP)) {
     CFIFrameInfo::RegisterValueMap<uint64_t>::iterator entry =
       caller_registers.find(".cfa");
     if (entry != caller_registers.end()) {
       frame->context_validity |= StackFrameARM64::CONTEXT_VALID_SP;
       frame->context.iregs[MD_CONTEXT_ARM64_REG_SP] = entry->second;
     }
   }

   // If we didn't recover the PC and the SP, then the frame isn't very useful.
   static const uint64_t essentials = (StackFrameARM64::CONTEXT_VALID_SP
                                      | StackFrameARM64::CONTEXT_VALID_PC);
   if ((frame->context_validity & essentials) != essentials)
     return NULL;

   frame->trust = StackFrame::FRAME_TRUST_CFI;
   return frame.release();
 }

 StackFrameARM64* StackwalkerARM64::GetCallerByStackScan(
     const vector<StackFrame*> &frames) {
   StackFrameARM64* last_frame = static_cast<StackFrameARM64*>(frames.back());
   uint64_t last_sp = last_frame->context.iregs[MD_CONTEXT_ARM64_REG_SP];
   uint64_t caller_sp, caller_pc;

   if (!ScanForReturnAddress(last_sp, &caller_sp, &caller_pc,
                             frames.size() == 1 /* is_context_frame */)) {
     // No plausible return address was found.
     return NULL;
   }

   // ScanForReturnAddress found a reasonable return address. Advance
   // %sp to the location above the one where the return address was
   // found.
   caller_sp += 8;

   // Create a new stack frame (ownership will be transferred to the caller)
   // and fill it in.
   StackFrameARM64* frame = new StackFrameARM64();

   frame->trust = StackFrame::FRAME_TRUST_SCAN;
   frame->context = last_frame->context;
   frame->context.iregs[MD_CONTEXT_ARM64_REG_PC] = caller_pc;
   frame->context.iregs[MD_CONTEXT_ARM64_REG_SP] = caller_sp;
   frame->context_validity = StackFrameARM64::CONTEXT_VALID_PC |
                             StackFrameARM64::CONTEXT_VALID_SP;

   return frame;
 }

 StackFrameARM64* StackwalkerARM64::GetCallerByFramePointer(
     const vector<StackFrame*> &frames) {
   StackFrameARM64* last_frame = static_cast<StackFrameARM64*>(frames.back());

   uint64_t last_fp = last_frame->context.iregs[MD_CONTEXT_ARM64_REG_FP];

   uint64_t caller_fp = 0;
   if (last_fp && !memory_->GetMemoryAtAddress(last_fp, &caller_fp)) {
     BPLOG(ERROR) << "Unable to read caller_fp from last_fp: 0x"
                  << std::hex << last_fp;
     return NULL;
   }

   uint64_t caller_lr = 0;
   if (last_fp && !memory_->GetMemoryAtAddress(last_fp + 8, &caller_lr)) {
     BPLOG(ERROR) << "Unable to read caller_lr from last_fp + 8: 0x"
                  << std::hex << (last_fp + 8);
     return NULL;
   }

   uint64_t caller_sp = last_fp ? last_fp + 16 :
       last_frame->context.iregs[MD_CONTEXT_ARM64_REG_SP];

   // Create a new stack frame (ownership will be transferred to the caller)
   // and fill it in.
   StackFrameARM64* frame = new StackFrameARM64();

   frame->trust = StackFrame::FRAME_TRUST_FP;
   frame->context = last_frame->context;
   frame->context.iregs[MD_CONTEXT_ARM64_REG_FP] = caller_fp;
   frame->context.iregs[MD_CONTEXT_ARM64_REG_SP] = caller_sp;
   frame->context.iregs[MD_CONTEXT_ARM64_REG_PC] =
       last_frame->context.iregs[MD_CONTEXT_ARM64_REG_LR];
   frame->context.iregs[MD_CONTEXT_ARM64_REG_LR] = caller_lr;
   frame->context_validity = StackFrameARM64::CONTEXT_VALID_PC |
                             StackFrameARM64::CONTEXT_VALID_LR |
                             StackFrameARM64::CONTEXT_VALID_FP |
                             StackFrameARM64::CONTEXT_VALID_SP;
   return frame;
 }

 StackFrame* StackwalkerARM64::GetCallerFrame(const CallStack* stack,
                                              bool stack_scan_allowed) {
   if (!memory_ || !stack) {
     BPLOG(ERROR) << "Can't get caller frame without memory or stack";
     return NULL;
   }

   const vector<StackFrame*> &frames = *stack->frames();
   StackFrameARM64* last_frame = static_cast<StackFrameARM64*>(frames.back());
   scoped_ptr<StackFrameARM64> frame;

   // See if there is DWARF call frame information covering this address.
   scoped_ptr<CFIFrameInfo> cfi_frame_info(
       frame_symbolizer_->FindCFIFrameInfo(last_frame));
   if (cfi_frame_info.get())
     frame.reset(GetCallerByCFIFrameInfo(frames, cfi_frame_info.get()));

   // If CFI failed, or there wasn't CFI available, fall back to frame pointer.
   if (!frame.get())
     frame.reset(GetCallerByFramePointer(frames));

   // If everything failed, fall back to stack scanning.
   if (stack_scan_allowed && !frame.get())
     frame.reset(GetCallerByStackScan(frames));

   // If nothing worked, tell the caller.
   if (!frame.get())
     return NULL;

   // Should we terminate the stack walk? (end-of-stack or broken invariant)
   if (TerminateWalk(frame->context.iregs[MD_CONTEXT_ARM64_REG_PC],
                     frame->context.iregs[MD_CONTEXT_ARM64_REG_SP],
                     last_frame->context.iregs[MD_CONTEXT_ARM64_REG_SP],
                     frames.size() == 1)) {
     return NULL;
   }

   // The new frame's context's PC is the return address, which is one
   // instruction past the instruction that caused us to arrive at the callee.
   // ARM64 instructions have a uniform 4-byte encoding, so subtracting 4 off
   // the return address gets back to the beginning of the call instruction.
   // Callers that require the exact return address value may access
   // frame->context.iregs[MD_CONTEXT_ARM64_REG_PC].
   frame->instruction = frame->context.iregs[MD_CONTEXT_ARM64_REG_PC] - 4;

   return frame.release();
 }


 }  // namespace google_breakpad
	// Copyright (c) 2013 Google Inc.
	// All rights reserved.
	//
	// Redistribution and use in source and binary forms, with or without
	// modification, are permitted provided that the following conditions are
	// met:
	//
	// * Redistributions of source code must retain the above copyright
	// notice, this list of conditions and the following disclaimer.
	// * Redistributions in binary form must reproduce the above
	// copyright notice, this list of conditions and the following disclaimer
	// in the documentation and/or other materials provided with the
	// distribution.
	// * Neither the name of Google Inc. nor the names of its
	// contributors may be used to endorse or promote products derived from
	// this software without specific prior written permission.
	//
	// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
	// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
	// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
	// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
	// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
	// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
	// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
	// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
	// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
	// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
	// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

	// stackwalker_arm64.cc: arm64-specific stackwalker.
	//
	// See stackwalker_arm64.h for documentation.
	//
	// Author: Mark Mentovai, Ted Mielczarek, Jim Blandy, Colin Blundell

	#include <vector>

	#include "common/scoped_ptr.h"
	#include "google_breakpad/processor/call_stack.h"
	#include "google_breakpad/processor/memory_region.h"
	#include "google_breakpad/processor/source_line_resolver_interface.h"
	#include "google_breakpad/processor/stack_frame_cpu.h"
	#include "processor/cfi_frame_info.h"
	#include "processor/logging.h"
	#include "processor/stackwalker_arm64.h"

	namespace google_breakpad {


	StackwalkerARM64::StackwalkerARM64(const SystemInfo* system_info,
	const MDRawContextARM64* context,
	MemoryRegion* memory,
	const CodeModules* modules,
	StackFrameSymbolizer* resolver_helper)
	: Stackwalker(system_info, memory, modules, resolver_helper),
	context_(context),
	context_frame_validity_(StackFrameARM64::CONTEXT_VALID_ALL) { }


	StackFrame* StackwalkerARM64::GetContextFrame() {
	if (!context_) {
	BPLOG(ERROR) << "Can't get context frame without context";
	return NULL;
	}

	StackFrameARM64* frame = new StackFrameARM64();

	// The instruction pointer is stored directly in a register (x32), so pull it
	// straight out of the CPU context structure.
	frame->context = *context_;
	frame->context_validity = context_frame_validity_;
	frame->trust = StackFrame::FRAME_TRUST_CONTEXT;
	frame->instruction = frame->context.iregs[MD_CONTEXT_ARM64_REG_PC];

	return frame;
	}

	StackFrameARM64* StackwalkerARM64::GetCallerByCFIFrameInfo(
	const vector<StackFrame*> &frames,
	CFIFrameInfo* cfi_frame_info) {
	StackFrameARM64* last_frame = static_cast<StackFrameARM64*>(frames.back());

	static const char* register_names[] = {
	"x0", "x1", "x2", "x3", "x4", "x5", "x6", "x7",
	"x8", "x9", "x10", "x11", "x12", "x13", "x14", "x15",
	"x16", "x17", "x18", "x19", "x20", "x21", "x22", "x23",
	"x24", "x25", "x26", "x27", "x28", "x29", "x30", "sp",
	"pc", NULL
	};

	// Populate a dictionary with the valid register values in last_frame.
	CFIFrameInfo::RegisterValueMap<uint64_t> callee_registers;
	for (int i = 0; register_names[i]; i++) {
	if (last_frame->context_validity & StackFrameARM64::RegisterValidFlag(i))
	callee_registers[register_names[i]] = last_frame->context.iregs[i];
	}

	// Use the STACK CFI data to recover the caller's register values.
	CFIFrameInfo::RegisterValueMap<uint64_t> caller_registers;
	if (!cfi_frame_info->FindCallerRegs(callee_registers, *memory_,
	&caller_registers)) {
	return NULL;
	}
	// Construct a new stack frame given the values the CFI recovered.
	scoped_ptr<StackFrameARM64> frame(new StackFrameARM64());
	for (int i = 0; register_names[i]; i++) {
	CFIFrameInfo::RegisterValueMap<uint64_t>::iterator entry =
	caller_registers.find(register_names[i]);
	if (entry != caller_registers.end()) {
	// We recovered the value of this register; fill the context with the
	// value from caller_registers.
	frame->context_validity \|= StackFrameARM64::RegisterValidFlag(i);
	frame->context.iregs[i] = entry->second;
	} else if (19 <= i && i <= 29 && (last_frame->context_validity &
	StackFrameARM64::RegisterValidFlag(i))) {
	// If the STACK CFI data doesn't mention some callee-saves register, and
	// it is valid in the callee, assume the callee has not yet changed it.
	// Registers r19 through r29 are callee-saves, according to the Procedure
	// Call Standard for the ARM AARCH64 Architecture, which the Linux ABI
	// follows.
	frame->context_validity \|= StackFrameARM64::RegisterValidFlag(i);
	frame->context.iregs[i] = last_frame->context.iregs[i];
	}
	}
	// If the CFI doesn't recover the PC explicitly, then use .ra.
	if (!(frame->context_validity & StackFrameARM64::CONTEXT_VALID_PC)) {
	CFIFrameInfo::RegisterValueMap<uint64_t>::iterator entry =
	caller_registers.find(".ra");
	if (entry != caller_registers.end()) {
	frame->context_validity \|= StackFrameARM64::CONTEXT_VALID_PC;
	frame->context.iregs[MD_CONTEXT_ARM64_REG_PC] = entry->second;
	}
	}
	// If the CFI doesn't recover the SP explicitly, then use .cfa.
	if (!(frame->context_validity & StackFrameARM64::CONTEXT_VALID_SP)) {
	CFIFrameInfo::RegisterValueMap<uint64_t>::iterator entry =
	caller_registers.find(".cfa");
	if (entry != caller_registers.end()) {
	frame->context_validity \|= StackFrameARM64::CONTEXT_VALID_SP;
	frame->context.iregs[MD_CONTEXT_ARM64_REG_SP] = entry->second;
	}
	}

	// If we didn't recover the PC and the SP, then the frame isn't very useful.
	static const uint64_t essentials = (StackFrameARM64::CONTEXT_VALID_SP
	\| StackFrameARM64::CONTEXT_VALID_PC);
	if ((frame->context_validity & essentials) != essentials)
	return NULL;

	frame->trust = StackFrame::FRAME_TRUST_CFI;
	return frame.release();
	}

	StackFrameARM64* StackwalkerARM64::GetCallerByStackScan(
	const vector<StackFrame*> &frames) {
	StackFrameARM64* last_frame = static_cast<StackFrameARM64*>(frames.back());
	uint64_t last_sp = last_frame->context.iregs[MD_CONTEXT_ARM64_REG_SP];
	uint64_t caller_sp, caller_pc;

	if (!ScanForReturnAddress(last_sp, &caller_sp, &caller_pc,
	frames.size() == 1 /* is_context_frame */)) {
	// No plausible return address was found.
	return NULL;
	}

	// ScanForReturnAddress found a reasonable return address. Advance
	// %sp to the location above the one where the return address was
	// found.
	caller_sp += 8;

	// Create a new stack frame (ownership will be transferred to the caller)
	// and fill it in.
	StackFrameARM64* frame = new StackFrameARM64();

	frame->trust = StackFrame::FRAME_TRUST_SCAN;
	frame->context = last_frame->context;
	frame->context.iregs[MD_CONTEXT_ARM64_REG_PC] = caller_pc;
	frame->context.iregs[MD_CONTEXT_ARM64_REG_SP] = caller_sp;
	frame->context_validity = StackFrameARM64::CONTEXT_VALID_PC \|
	StackFrameARM64::CONTEXT_VALID_SP;

	return frame;
	}

	StackFrameARM64* StackwalkerARM64::GetCallerByFramePointer(
	const vector<StackFrame*> &frames) {
	StackFrameARM64* last_frame = static_cast<StackFrameARM64*>(frames.back());

	uint64_t last_fp = last_frame->context.iregs[MD_CONTEXT_ARM64_REG_FP];

	uint64_t caller_fp = 0;
	if (last_fp && !memory_->GetMemoryAtAddress(last_fp, &caller_fp)) {
	BPLOG(ERROR) << "Unable to read caller_fp from last_fp: 0x"
	<< std::hex << last_fp;
	return NULL;
	}

	uint64_t caller_lr = 0;
	if (last_fp && !memory_->GetMemoryAtAddress(last_fp + 8, &caller_lr)) {
	BPLOG(ERROR) << "Unable to read caller_lr from last_fp + 8: 0x"
	<< std::hex << (last_fp + 8);
	return NULL;
	}

	uint64_t caller_sp = last_fp ? last_fp + 16 :
	last_frame->context.iregs[MD_CONTEXT_ARM64_REG_SP];

	// Create a new stack frame (ownership will be transferred to the caller)
	// and fill it in.
	StackFrameARM64* frame = new StackFrameARM64();

	frame->trust = StackFrame::FRAME_TRUST_FP;
	frame->context = last_frame->context;
	frame->context.iregs[MD_CONTEXT_ARM64_REG_FP] = caller_fp;
	frame->context.iregs[MD_CONTEXT_ARM64_REG_SP] = caller_sp;
	frame->context.iregs[MD_CONTEXT_ARM64_REG_PC] =
	last_frame->context.iregs[MD_CONTEXT_ARM64_REG_LR];
	frame->context.iregs[MD_CONTEXT_ARM64_REG_LR] = caller_lr;
	frame->context_validity = StackFrameARM64::CONTEXT_VALID_PC \|
	StackFrameARM64::CONTEXT_VALID_LR \|
	StackFrameARM64::CONTEXT_VALID_FP \|
	StackFrameARM64::CONTEXT_VALID_SP;
	return frame;
	}

	StackFrame* StackwalkerARM64::GetCallerFrame(const CallStack* stack,
	bool stack_scan_allowed) {
	if (!memory_ \|\| !stack) {
	BPLOG(ERROR) << "Can't get caller frame without memory or stack";
	return NULL;
	}

	const vector<StackFrame> &frames = stack->frames();
	StackFrameARM64* last_frame = static_cast<StackFrameARM64*>(frames.back());
	scoped_ptr<StackFrameARM64> frame;

	// See if there is DWARF call frame information covering this address.
	scoped_ptr<CFIFrameInfo> cfi_frame_info(
	frame_symbolizer_->FindCFIFrameInfo(last_frame));
	if (cfi_frame_info.get())
	frame.reset(GetCallerByCFIFrameInfo(frames, cfi_frame_info.get()));

	// If CFI failed, or there wasn't CFI available, fall back to frame pointer.
	if (!frame.get())
	frame.reset(GetCallerByFramePointer(frames));

	// If everything failed, fall back to stack scanning.
	if (stack_scan_allowed && !frame.get())
	frame.reset(GetCallerByStackScan(frames));

	// If nothing worked, tell the caller.
	if (!frame.get())
	return NULL;

	// Should we terminate the stack walk? (end-of-stack or broken invariant)
	if (TerminateWalk(frame->context.iregs[MD_CONTEXT_ARM64_REG_PC],
	frame->context.iregs[MD_CONTEXT_ARM64_REG_SP],
	last_frame->context.iregs[MD_CONTEXT_ARM64_REG_SP],
	frames.size() == 1)) {
	return NULL;
	}

	// The new frame's context's PC is the return address, which is one
	// instruction past the instruction that caused us to arrive at the callee.
	// ARM64 instructions have a uniform 4-byte encoding, so subtracting 4 off
	// the return address gets back to the beginning of the call instruction.
	// Callers that require the exact return address value may access
	// frame->context.iregs[MD_CONTEXT_ARM64_REG_PC].
	frame->instruction = frame->context.iregs[MD_CONTEXT_ARM64_REG_PC] - 4;

	return frame.release();
	}


	} // namespace google_breakpad