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// Copyright 2019 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "base/profiler/stack_sampler_impl.h"
#include <utility>
#include "base/logging.h"
#include "base/profiler/profile_builder.h"
#include "base/profiler/thread_delegate.h"
#include "base/profiler/unwinder.h"
// IMPORTANT NOTE: Some functions within this implementation are invoked while
// the target thread is suspended so it must not do any allocation from the
// heap, including indirectly via use of DCHECK/CHECK or other logging
// statements. Otherwise this code can deadlock on heap locks acquired by the
// target thread before it was suspended. These functions are commented with "NO
namespace base {
std::unique_ptr<ThreadDelegate> thread_delegate,
std::unique_ptr<Unwinder> native_unwinder,
ModuleCache* module_cache,
StackSamplerTestDelegate* test_delegate)
: thread_delegate_(std::move(thread_delegate)),
test_delegate_(test_delegate) {}
StackSamplerImpl::~StackSamplerImpl() = default;
void StackSamplerImpl::AddAuxUnwinder(std::unique_ptr<Unwinder> unwinder) {
aux_unwinder_ = std::move(unwinder);
void StackSamplerImpl::RecordStackFrames(StackBuffer* stack_buffer,
ProfileBuilder* profile_builder) {
RegisterContext thread_context;
uintptr_t stack_top;
bool success =
CopyStack(stack_buffer, &stack_top, profile_builder, &thread_context);
if (!success)
if (test_delegate_)
WalkStack(module_cache_, &thread_context, stack_top,
native_unwinder_.get(), aux_unwinder_.get()));
// static
std::vector<Frame> StackSamplerImpl::WalkStackForTesting(
ModuleCache* module_cache,
RegisterContext* thread_context,
uintptr_t stack_top,
Unwinder* native_unwinder,
Unwinder* aux_unwinder) {
return WalkStack(module_cache, thread_context, stack_top, native_unwinder,
// Suspends the thread, copies its stack, top address of the stack copy, and
// register context, records the current metadata, then resumes the thread.
// Returns true on success, and returns the copied state via the params. NO HEAP
// ALLOCATIONS within the ScopedSuspendThread scope.
bool StackSamplerImpl::CopyStack(StackBuffer* stack_buffer,
uintptr_t* stack_top,
ProfileBuilder* profile_builder,
RegisterContext* thread_context) {
const uintptr_t top = thread_delegate_->GetStackBaseAddress();
uintptr_t bottom = 0;
const uint8_t* stack_copy_bottom = nullptr;
// Allocation of the ScopedSuspendThread object itself is OK since it
// necessarily occurs before the thread is suspended by the object.
std::unique_ptr<ThreadDelegate::ScopedSuspendThread> suspend_thread =
if (!suspend_thread->WasSuccessful())
return false;
if (!thread_delegate_->GetThreadContext(thread_context))
return false;
bottom = RegisterContextStackPointer(thread_context);
// The StackBuffer allocation is expected to be at least as large as the
// largest stack region allocation on the platform, but check just in case
// it isn't *and* the actual stack itself exceeds the buffer allocation
// size.
if ((top - bottom) > stack_buffer->size())
return false;
if (!thread_delegate_->CanCopyStack(bottom))
return false;
stack_copy_bottom = CopyStackContentsAndRewritePointers(
reinterpret_cast<uint8_t*>(bottom), reinterpret_cast<uintptr_t*>(top),
*stack_top = reinterpret_cast<uintptr_t>(stack_copy_bottom) + (top - bottom);
for (uintptr_t* reg :
thread_delegate_->GetRegistersToRewrite(thread_context)) {
*reg = RewritePointerIfInOriginalStack(reinterpret_cast<uint8_t*>(bottom),
stack_copy_bottom, *reg);
return true;
// static
std::vector<Frame> StackSamplerImpl::WalkStack(ModuleCache* module_cache,
RegisterContext* thread_context,
uintptr_t stack_top,
Unwinder* native_unwinder,
Unwinder* aux_unwinder) {
std::vector<Frame> stack;
// Reserve enough memory for most stacks, to avoid repeated
// allocations. Approximately 99.9% of recorded stacks are 128 frames or
// fewer.
// Record the first frame from the context values.
size_t prior_stack_size;
UnwindResult result;
do {
// Choose an authoritative unwinder for the current module. Use the aux
// unwinder if it thinks it can unwind from the current frame, otherwise use
// the native unwinder.
Unwinder* unwinder =
aux_unwinder && aux_unwinder->CanUnwindFrom(&stack.back())
? aux_unwinder
: native_unwinder;
prior_stack_size = stack.size();
result =
unwinder->TryUnwind(thread_context, stack_top, module_cache, &stack);
// The native unwinder should be the only one that returns COMPLETED
// since the stack starts in native code.
DCHECK(result != UnwindResult::COMPLETED || unwinder == native_unwinder);
} while (result != UnwindResult::ABORTED &&
result != UnwindResult::COMPLETED &&
// Give up if the authoritative unwinder for the module was unable to
// unwind.
stack.size() > prior_stack_size);
return stack;
// If the value at |pointer| points to the original stack, rewrite it to point
// to the corresponding location in the copied stack. NO HEAP ALLOCATIONS.
// static
uintptr_t RewritePointerIfInOriginalStack(const uint8_t* original_stack_bottom,
const uintptr_t* original_stack_top,
const uint8_t* stack_copy_bottom,
uintptr_t pointer) {
auto original_stack_bottom_uint =
auto original_stack_top_uint =
auto stack_copy_bottom_uint = reinterpret_cast<uintptr_t>(stack_copy_bottom);
if (pointer < original_stack_bottom_uint ||
pointer >= original_stack_top_uint)
return pointer;
return stack_copy_bottom_uint + (pointer - original_stack_bottom_uint);
// Copies the stack to a buffer while rewriting possible pointers to locations
// within the stack to point to the corresponding locations in the copy. This is
// necessary to handle stack frames with dynamic stack allocation, where a
// pointer to the beginning of the dynamic allocation area is stored on the
// stack and/or in a non-volatile register.
// Eager rewriting of anything that looks like a pointer to the stack, as done
// in this function, does not adversely affect the stack unwinding. The only
// other values on the stack the unwinding depends on are return addresses,
// which should not point within the stack memory. The rewriting is guaranteed
// to catch all pointers because the stacks are guaranteed by the ABI to be
// sizeof(uintptr_t*) aligned.
// |original_stack_bottom| and |original_stack_top| are different pointer types
// due on their differing guaranteed alignments -- the bottom may only be 1-byte
// aligned while the top is aligned to double the pointer width.
// Returns a pointer to the bottom address in the copied stack. This value
// matches the alignment of |original_stack_bottom| to ensure that the stack
// contents have the same alignment as in the original stack. As a result the
// value will be different than |stack_buffer_bottom| if |original_stack_bottom|
// is not aligned to double the pointer width.
// static
const uint8_t* CopyStackContentsAndRewritePointers(
const uint8_t* original_stack_bottom,
const uintptr_t* original_stack_top,
uintptr_t* stack_buffer_bottom) {
const uint8_t* byte_src = original_stack_bottom;
// The first address in the stack with pointer alignment. Pointer-aligned
// values from this point to the end of the stack are possibly rewritten using
// RewritePointerIfInOriginalStack(). Bytes before this cannot be a pointer
// because they occupy less space than a pointer would.
const uint8_t* first_aligned_address = reinterpret_cast<uint8_t*>(
(reinterpret_cast<uintptr_t>(byte_src) + sizeof(uintptr_t) - 1) &
~(sizeof(uintptr_t) - 1));
// The stack copy bottom, which is offset from |stack_buffer_bottom| by the
// same alignment as in the original stack. This guarantees identical
// alignment between values in the original stack and the copy. This uses the
// platform stack alignment rather than pointer alignment so that the stack
// copy is aligned to platform expectations.
uint8_t* stack_copy_bottom =
reinterpret_cast<uint8_t*>(stack_buffer_bottom) +
(reinterpret_cast<uintptr_t>(byte_src) &
(StackSampler::StackBuffer::kPlatformStackAlignment - 1));
uint8_t* byte_dst = stack_copy_bottom;
// Copy bytes verbatim up to the first aligned address.
for (; byte_src < first_aligned_address; ++byte_src, ++byte_dst)
*byte_dst = *byte_src;
// Copy the remaining stack by pointer-sized values, rewriting anything that
// looks like a pointer into the stack.
const uintptr_t* src = reinterpret_cast<const uintptr_t*>(byte_src);
uintptr_t* dst = reinterpret_cast<uintptr_t*>(byte_dst);
for (; src < original_stack_top; ++src, ++dst) {
*dst = RewritePointerIfInOriginalStack(
original_stack_bottom, original_stack_top, stack_copy_bottom, *src);
return stack_copy_bottom;
} // namespace base