base/profiler/native_stack_sampler_mac.cc

// Copyright 2017 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/native_stack_sampler.h"

#include <dlfcn.h>
#include <libkern/OSByteOrder.h>
#include <libunwind.h>
#include <mach-o/compact_unwind_encoding.h>
#include <mach-o/getsect.h>
#include <mach-o/swap.h>
#include <mach/kern_return.h>
#include <mach/mach.h>
#include <mach/thread_act.h>
#include <mach/vm_map.h>
#include <pthread.h>
#include <sys/resource.h>
#include <sys/syslimits.h>

#include <algorithm>
#include <memory>

#include "base/logging.h"
#include "base/mac/mach_logging.h"
#include "base/macros.h"
#include "base/memory/ptr_util.h"
#include "base/strings/string_number_conversions.h"

extern "C" {
void _sigtramp(int, int, struct sigset*);
}

namespace base {

using Frame = StackSamplingProfiler::Frame;
using InternalFrame = StackSamplingProfiler::InternalFrame;
using Module = StackSamplingProfiler::Module;
using InternalModule = StackSamplingProfiler::InternalModule;
using ProfileBuilder = StackSamplingProfiler::ProfileBuilder;

namespace {

// ModuleCacheEntry records a module's address range (half-open) in memory and
// the module itself.
struct ModuleCacheEntry {
  ModuleCacheEntry(uintptr_t start,
                   uintptr_t end,
                   InternalModule internal_module)
      : base_address(start),
        end_address(end),
        internal_module(std::move(internal_module)){};

  // Base address of the represented module.
  uintptr_t base_address;

  // First address off the end of the represented module.
  uintptr_t end_address;

  // Module information.
  InternalModule internal_module;
};

// Module identifiers ---------------------------------------------------------

// Returns the unique build ID for a module loaded at |module_addr|. Returns the
// empty string if the function fails to get the build ID.
//
// Build IDs are created by the concatenation of the module's GUID (Windows) /
// UUID (Mac) and an "age" field that indicates how many times that GUID/UUID
// has been reused. In Windows binaries, the "age" field is present in the
// module header, but on the Mac, UUIDs are never reused and so the "age" value
// appended to the UUID is always 0.
std::string GetUniqueId(const void* module_addr) {
  const mach_header_64* mach_header =
      reinterpret_cast<const mach_header_64*>(module_addr);
  DCHECK_EQ(MH_MAGIC_64, mach_header->magic);

  size_t offset = sizeof(mach_header_64);
  size_t offset_limit = sizeof(mach_header_64) + mach_header->sizeofcmds;

  for (uint32_t i = 0; i < mach_header->ncmds; ++i) {
    if (offset + sizeof(load_command) >= offset_limit)
      return std::string();

    const load_command* current_cmd = reinterpret_cast<const load_command*>(
        reinterpret_cast<const uint8_t*>(mach_header) + offset);

    if (offset + current_cmd->cmdsize > offset_limit) {
      // This command runs off the end of the command list. This is malformed.
      return std::string();
    }

    if (current_cmd->cmd == LC_UUID) {
      if (current_cmd->cmdsize < sizeof(uuid_command)) {
        // This "UUID command" is too small. This is malformed.
        return std::string();
      }

      const uuid_command* uuid_cmd =
          reinterpret_cast<const uuid_command*>(current_cmd);
      static_assert(sizeof(uuid_cmd->uuid) == sizeof(uuid_t),
                    "UUID field of UUID command should be 16 bytes.");
      // The ID is comprised of the UUID concatenated with the Mac's "age" value
      // which is always 0.
      return HexEncode(&uuid_cmd->uuid, sizeof(uuid_cmd->uuid)) + "0";
    }
    offset += current_cmd->cmdsize;
  }
  return std::string();
}

// Returns the size of the _TEXT segment of the module loaded at |module_addr|.
size_t GetModuleTextSize(const void* module_addr) {
  const mach_header_64* mach_header =
      reinterpret_cast<const mach_header_64*>(module_addr);
  DCHECK_EQ(MH_MAGIC_64, mach_header->magic);

  unsigned long module_size;
  getsegmentdata(mach_header, SEG_TEXT, &module_size);

  return module_size;
}

// Stack walking --------------------------------------------------------------

// Fills |state| with |target_thread|'s context.
//
// Note that this is called while a thread is suspended. Make very very sure
// that no shared resources (e.g. memory allocators) are used for the duration
// of this function.
bool GetThreadState(thread_act_t target_thread, x86_thread_state64_t* state) {
  auto count = static_cast<mach_msg_type_number_t>(x86_THREAD_STATE64_COUNT);
  return thread_get_state(target_thread, x86_THREAD_STATE64,
                          reinterpret_cast<thread_state_t>(state),
                          &count) == KERN_SUCCESS;
}

// If the value at |pointer| points to the original stack, rewrites it to point
// to the corresponding location in the copied stack.
//
// Note that this is called while a thread is suspended. Make very very sure
// that no shared resources (e.g. memory allocators) are used for the duration
// of this function.
uintptr_t RewritePointerIfInOriginalStack(
    const uintptr_t* original_stack_bottom,
    const uintptr_t* original_stack_top,
    uintptr_t* stack_copy_bottom,
    uintptr_t pointer) {
  auto original_stack_bottom_int =
      reinterpret_cast<uintptr_t>(original_stack_bottom);
  auto original_stack_top_int = reinterpret_cast<uintptr_t>(original_stack_top);
  auto stack_copy_bottom_int = reinterpret_cast<uintptr_t>(stack_copy_bottom);

  if (pointer < original_stack_bottom_int || pointer >= original_stack_top_int)
    return pointer;

  return stack_copy_bottom_int + (pointer - original_stack_bottom_int);
}

// 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(void*) aligned.
//
// Note that this is called while a thread is suspended. Make very very sure
// that no shared resources (e.g. memory allocators) are used for the duration
// of this function.
void CopyStackAndRewritePointers(uintptr_t* stack_copy_bottom,
                                 const uintptr_t* original_stack_bottom,
                                 const uintptr_t* original_stack_top,
                                 x86_thread_state64_t* thread_state)
    NO_SANITIZE("address") {
  size_t count = original_stack_top - original_stack_bottom;
  for (size_t pos = 0; pos < count; ++pos) {
    stack_copy_bottom[pos] = RewritePointerIfInOriginalStack(
        original_stack_bottom, original_stack_top, stack_copy_bottom,
        original_stack_bottom[pos]);
  }

  uint64_t* rewrite_registers[] = {&thread_state->__rbx, &thread_state->__rbp,
                                   &thread_state->__rsp, &thread_state->__r12,
                                   &thread_state->__r13, &thread_state->__r14,
                                   &thread_state->__r15};
  for (auto* reg : rewrite_registers) {
    *reg = RewritePointerIfInOriginalStack(
        original_stack_bottom, original_stack_top, stack_copy_bottom, *reg);
  }
}

// Extracts the "frame offset" for a given frame from the compact unwind info.
// A frame offset indicates the location of saved non-volatile registers in
// relation to the frame pointer. See |mach-o/compact_unwind_encoding.h| for
// details.
uint32_t GetFrameOffset(int compact_unwind_info) {
  // The frame offset lives in bytes 16-23. This shifts it down by the number of
  // leading zeroes in the mask, then masks with (1 << number of one bits in the
  // mask) - 1, turning 0x00FF0000 into 0x000000FF. Adapted from |EXTRACT_BITS|
  // in libunwind's CompactUnwinder.hpp.
  return (
      (compact_unwind_info >> __builtin_ctz(UNWIND_X86_64_RBP_FRAME_OFFSET)) &
      (((1 << __builtin_popcount(UNWIND_X86_64_RBP_FRAME_OFFSET))) - 1));
}

// True if the unwind from |leaf_frame_rip| may trigger a crash bug in
// unw_init_local. If so, the stack walk should be aborted at the leaf frame.
bool MayTriggerUnwInitLocalCrash(uint64_t leaf_frame_rip) {
  // The issue here is a bug in unw_init_local that, in some unwinds, results in
  // attempts to access memory at the address immediately following the address
  // range of the library. When the library is the last of the mapped libraries
  // that address is in a different memory region. Starting with 10.13.4 beta
  // releases it appears that this region is sometimes either unmapped or mapped
  // without read access, resulting in crashes on the attempted access. It's not
  // clear what circumstances result in this situation; attempts to reproduce on
  // a 10.13.4 beta did not trigger the issue.
  //
  // The workaround is to check if the memory address that would be accessed is
  // readable, and if not, abort the stack walk before calling unw_init_local.
  // As of 2018/03/19 about 0.1% of non-idle stacks on the UI and GPU main
  // threads have a leaf frame in the last library. Since the issue appears to
  // only occur some of the time it's expected that the quantity of lost samples
  // will be lower than 0.1%, possibly significantly lower.
  //
  // TODO(lgrey): Add references above to LLVM/Radar bugs on unw_init_local once
  // filed.
  Dl_info info;
  if (dladdr(reinterpret_cast<const void*>(leaf_frame_rip), &info) == 0)
    return false;
  uint64_t unused;
  vm_size_t size = sizeof(unused);
  return vm_read_overwrite(current_task(),
                           reinterpret_cast<vm_address_t>(info.dli_fbase) +
                               GetModuleTextSize(info.dli_fbase),
                           sizeof(unused),
                           reinterpret_cast<vm_address_t>(&unused), &size) != 0;
}

// Check if the cursor contains a valid-looking frame pointer for frame pointer
// unwinds. If the stack frame has a frame pointer, stepping the cursor will
// involve indexing memory access off of that pointer. In that case,
// sanity-check the frame pointer register to ensure it's within bounds.
//
// Additionally, the stack frame might be in a prologue or epilogue, which can
// cause a crash when the unwinder attempts to access non-volatile registers
// that have not yet been pushed, or have already been popped from the
// stack. libwunwind will try to restore those registers using an offset from
// the frame pointer. However, since we copy the stack from RSP up, any
// locations below the stack pointer are before the beginning of the stack
// buffer. Account for this by checking that the expected location is above the
// stack pointer, and rejecting the sample if it isn't.
bool HasValidRbp(unw_cursor_t* unwind_cursor, uintptr_t stack_top) {
  unw_proc_info_t proc_info;
  unw_get_proc_info(unwind_cursor, &proc_info);
  if ((proc_info.format & UNWIND_X86_64_MODE_MASK) ==
      UNWIND_X86_64_MODE_RBP_FRAME) {
    unw_word_t rsp, rbp;
    unw_get_reg(unwind_cursor, UNW_X86_64_RSP, &rsp);
    unw_get_reg(unwind_cursor, UNW_X86_64_RBP, &rbp);
    uint32_t offset = GetFrameOffset(proc_info.format) * sizeof(unw_word_t);
    if (rbp < offset || (rbp - offset) < rsp || rbp > stack_top)
      return false;
  }
  return true;
}

const char* LibSystemKernelName() {
  static char path[PATH_MAX];
  static char* name = nullptr;
  if (name)
    return name;

  Dl_info info;
  dladdr(reinterpret_cast<void*>(_exit), &info);
  strlcpy(path, info.dli_fname, PATH_MAX);
  name = path;

#if !defined(ADDRESS_SANITIZER)
  DCHECK_EQ(std::string(name),
            std::string("/usr/lib/system/libsystem_kernel.dylib"));
#endif
  return name;
}

void GetSigtrampRange(uintptr_t* start, uintptr_t* end) {
  auto address = reinterpret_cast<uintptr_t>(&_sigtramp);
  DCHECK(address != 0);

  *start = address;

  unw_context_t context;
  unw_cursor_t cursor;
  unw_proc_info_t info;

  unw_getcontext(&context);
  // Set the context's RIP to the beginning of sigtramp,
  // +1 byte to work around a bug in 10.11 (crbug.com/764468).
  context.data[16] = address + 1;
  unw_init_local(&cursor, &context);
  unw_get_proc_info(&cursor, &info);

  DCHECK_EQ(info.start_ip, address);
  *end = info.end_ip;
}

// ScopedSuspendThread --------------------------------------------------------

// Suspends a thread for the lifetime of the object.
class ScopedSuspendThread {
 public:
  explicit ScopedSuspendThread(mach_port_t thread_port)
      : thread_port_(thread_suspend(thread_port) == KERN_SUCCESS
                         ? thread_port
                         : MACH_PORT_NULL) {}

  ~ScopedSuspendThread() {
    if (!was_successful())
      return;

    kern_return_t kr = thread_resume(thread_port_);
    MACH_CHECK(kr == KERN_SUCCESS, kr) << "thread_resume";
  }

  bool was_successful() const { return thread_port_ != MACH_PORT_NULL; }

 private:
  mach_port_t thread_port_;

  DISALLOW_COPY_AND_ASSIGN(ScopedSuspendThread);
};

// NativeStackSamplerMac ------------------------------------------------------

class NativeStackSamplerMac : public NativeStackSampler {
 public:
  NativeStackSamplerMac(mach_port_t thread_port,
                        NativeStackSamplerTestDelegate* test_delegate);
  ~NativeStackSamplerMac() override;

  // StackSamplingProfiler::NativeStackSampler:
  void ProfileRecordingStarting() override;
  std::vector<InternalFrame> RecordStackFrames(
      StackBuffer* stack_buffer,
      ProfileBuilder* profile_builder) override;

 private:
  // Returns the InternalModule containing |instruction_pointer|, adding it to
  // module_cache_entry_ if it's not already present.
  InternalModule GetInternalModule(uintptr_t instruction_pointer);

  // Walks the stack represented by |unwind_context|, calling back to the
  // provided lambda for each frame. Returns false if an error occurred,
  // otherwise returns true.
  template <typename StackFrameCallback, typename ContinueUnwindPredicate>
  bool WalkStackFromContext(unw_context_t* unwind_context,
                            size_t* frame_count,
                            const StackFrameCallback& callback,
                            const ContinueUnwindPredicate& continue_unwind);

  // Walks the stack represented by |thread_state|, calling back to the
  // provided lambda for each frame.
  template <typename StackFrameCallback, typename ContinueUnwindPredicate>
  void WalkStack(const x86_thread_state64_t& thread_state,
                 const StackFrameCallback& callback,
                 const ContinueUnwindPredicate& continue_unwind);

  // Weak reference: Mach port for thread being profiled.
  mach_port_t thread_port_;

  NativeStackSamplerTestDelegate* const test_delegate_;

  // The stack base address corresponding to |thread_handle_|.
  const void* const thread_stack_base_address_;

  // Maps a module's address range to the module.
  std::vector<ModuleCacheEntry> module_cache_entry_;

  // The address range of |_sigtramp|, the signal trampoline function.
  uintptr_t sigtramp_start_;
  uintptr_t sigtramp_end_;

  DISALLOW_COPY_AND_ASSIGN(NativeStackSamplerMac);
};

NativeStackSamplerMac::NativeStackSamplerMac(
    mach_port_t thread_port,
    NativeStackSamplerTestDelegate* test_delegate)
    : thread_port_(thread_port),
      test_delegate_(test_delegate),
      thread_stack_base_address_(
          pthread_get_stackaddr_np(pthread_from_mach_thread_np(thread_port))) {
  GetSigtrampRange(&sigtramp_start_, &sigtramp_end_);
  // This class suspends threads, and those threads might be suspended in dyld.
  // Therefore, for all the system functions that might be linked in dynamically
  // that are used while threads are suspended, make calls to them to make sure
  // that they are linked up.
  x86_thread_state64_t thread_state;
  GetThreadState(thread_port_, &thread_state);
}

NativeStackSamplerMac::~NativeStackSamplerMac() {}

void NativeStackSamplerMac::ProfileRecordingStarting() {
  module_cache_entry_.clear();
}

std::vector<InternalFrame> NativeStackSamplerMac::RecordStackFrames(
    StackBuffer* stack_buffer,
    ProfileBuilder* profile_builder) {
  x86_thread_state64_t thread_state;

  const std::vector<InternalFrame> empty_internal_frames;

  // Copy the stack.

  uintptr_t new_stack_top = 0;
  {
    // IMPORTANT NOTE: Do not do ANYTHING in this in this scope that might
    // allocate memory, including indirectly via use of DCHECK/CHECK or other
    // logging statements. Otherwise this code can deadlock on heap locks in the
    // default heap acquired by the target thread before it was suspended.
    ScopedSuspendThread suspend_thread(thread_port_);
    if (!suspend_thread.was_successful())
      return empty_internal_frames;

    if (!GetThreadState(thread_port_, &thread_state))
      return empty_internal_frames;

    auto stack_top = reinterpret_cast<uintptr_t>(thread_stack_base_address_);
    uintptr_t stack_bottom = thread_state.__rsp;
    if (stack_bottom >= stack_top)
      return empty_internal_frames;

    uintptr_t stack_size = stack_top - stack_bottom;
    if (stack_size > stack_buffer->size())
      return empty_internal_frames;

    profile_builder->RecordAnnotations();

    CopyStackAndRewritePointers(
        reinterpret_cast<uintptr_t*>(stack_buffer->buffer()),
        reinterpret_cast<uintptr_t*>(stack_bottom),
        reinterpret_cast<uintptr_t*>(stack_top), &thread_state);

    new_stack_top =
        reinterpret_cast<uintptr_t>(stack_buffer->buffer()) + stack_size;
  }  // ScopedSuspendThread

  if (test_delegate_)
    test_delegate_->OnPreStackWalk();

  // Walk the stack and record it.

  // Reserve enough memory for most stacks, to avoid repeated allocations.
  // Approximately 99.9% of recorded stacks are 128 frames or fewer.
  std::vector<InternalFrame> internal_frames;
  internal_frames.reserve(128);

  // Avoid an out-of-bounds read bug in libunwind that can crash us in some
  // circumstances. If we're subject to that case, just record the first frame
  // and bail. See MayTriggerUnwInitLocalCrash for details.
  uintptr_t rip = thread_state.__rip;
  if (MayTriggerUnwInitLocalCrash(rip)) {
    internal_frames.emplace_back(rip, GetInternalModule(rip));
    return internal_frames;
  }

  const auto continue_predicate = [this,
                                   new_stack_top](unw_cursor_t* unwind_cursor) {
    // Don't continue if we're in sigtramp. Unwinding this from another thread
    // is very fragile. It's a complex DWARF unwind that needs to restore the
    // entire thread context which was saved by the kernel when the interrupt
    // occurred.
    unw_word_t rip;
    unw_get_reg(unwind_cursor, UNW_REG_IP, &rip);
    if (rip >= sigtramp_start_ && rip < sigtramp_end_)
      return false;

    // Don't continue if rbp appears to be invalid (due to a previous bad
    // unwind).
    return HasValidRbp(unwind_cursor, new_stack_top);
  };

  WalkStack(
      thread_state,
      [&internal_frames](uintptr_t frame_ip, InternalModule internal_module) {
        internal_frames.emplace_back(frame_ip, std::move(internal_module));
      },
      continue_predicate);

  return internal_frames;
}

InternalModule NativeStackSamplerMac::GetInternalModule(
    uintptr_t instruction_pointer) {
  // Check if |instruction_pointer| is in the address range of a module we've
  // already seen.
  auto loc =
      std::find_if(module_cache_entry_.begin(), module_cache_entry_.end(),
                   [instruction_pointer](const ModuleCacheEntry& entry) {
                     return instruction_pointer >= entry.base_address &&
                            instruction_pointer < entry.end_address;
                   });
  if (loc != module_cache_entry_.end())
    return loc->internal_module;

  Dl_info inf;
  if (!dladdr(reinterpret_cast<const void*>(instruction_pointer), &inf))
    return InternalModule();

  auto base_module_address = reinterpret_cast<uintptr_t>(inf.dli_fbase);

  InternalModule internal_module(
      base_module_address, GetUniqueId(inf.dli_fbase), FilePath(inf.dli_fname));

  module_cache_entry_.emplace_back(
      base_module_address,
      base_module_address + GetModuleTextSize(inf.dli_fbase), internal_module);

  return internal_module;
}

template <typename StackFrameCallback, typename ContinueUnwindPredicate>
bool NativeStackSamplerMac::WalkStackFromContext(
    unw_context_t* unwind_context,
    size_t* frame_count,
    const StackFrameCallback& callback,
    const ContinueUnwindPredicate& continue_unwind) {
  unw_cursor_t unwind_cursor;
  unw_init_local(&unwind_cursor, unwind_context);

  int step_result;
  unw_word_t rip;
  do {
    ++(*frame_count);
    unw_get_reg(&unwind_cursor, UNW_REG_IP, &rip);

    // Ensure IP is in a module.
    //
    // Frameless unwinding (non-DWARF) works by fetching the function's stack
    // size from the unwind encoding or stack, and adding it to the stack
    // pointer to determine the function's return address.
    //
    // If we're in a function prologue or epilogue, the actual stack size may be
    // smaller than it will be during the normal course of execution. When
    // libunwind adds the expected stack size, it will look for the return
    // address in the wrong place. This check should ensure that we bail before
    // trying to deref a bad IP obtained this way in the previous frame.
    InternalModule internal_module = GetInternalModule(rip);
    if (!internal_module.is_valid)
      return false;

    callback(static_cast<uintptr_t>(rip), internal_module);

    if (!continue_unwind(&unwind_cursor))
      return false;

    step_result = unw_step(&unwind_cursor);
  } while (step_result > 0);

  if (step_result != 0)
    return false;

  return true;
}

template <typename StackFrameCallback, typename ContinueUnwindPredicate>
void NativeStackSamplerMac::WalkStack(
    const x86_thread_state64_t& thread_state,
    const StackFrameCallback& callback,
    const ContinueUnwindPredicate& continue_unwind) {
  size_t frame_count = 0;
  // This uses libunwind to walk the stack. libunwind is designed to be used for
  // a thread to walk its own stack. This creates two problems.

  // Problem 1: There is no official way to create a unw_context other than to
  // create it from the current state of the current thread's stack. To get
  // around this, forge a context. A unw_context is just a copy of the 16 main
  // registers followed by the instruction pointer, nothing more.
  // Coincidentally, the first 17 items of the x86_thread_state64_t type are
  // exactly those registers in exactly the same order, so just bulk copy them
  // over.
  unw_context_t unwind_context;
  memcpy(&unwind_context, &thread_state, sizeof(uintptr_t) * 17);
  bool result = WalkStackFromContext(&unwind_context, &frame_count, callback,
                                     continue_unwind);

  if (!result)
    return;

  if (frame_count == 1) {
    // Problem 2: Because libunwind is designed to be triggered by user code on
    // their own thread, if it hits a library that has no unwind info for the
    // function that is being executed, it just stops. This isn't a problem in
    // the normal case, but in this case, it's quite possible that the stack
    // being walked is stopped in a function that bridges to the kernel and thus
    // is missing the unwind info.

    // For now, just unwind the single case where the thread is stopped in a
    // function in libsystem_kernel.
    uint64_t& rsp = unwind_context.data[7];
    uint64_t& rip = unwind_context.data[16];
    Dl_info info;
    if (dladdr(reinterpret_cast<void*>(rip), &info) != 0 &&
        strcmp(info.dli_fname, LibSystemKernelName()) == 0) {
      rip = *reinterpret_cast<uint64_t*>(rsp);
      rsp += 8;
      WalkStackFromContext(&unwind_context, &frame_count, callback,
                           continue_unwind);
    }
  }
}

}  // namespace

std::unique_ptr<NativeStackSampler> NativeStackSampler::Create(
    PlatformThreadId thread_id,
    NativeStackSamplerTestDelegate* test_delegate) {
  return std::make_unique<NativeStackSamplerMac>(thread_id, test_delegate);
}

size_t NativeStackSampler::GetStackBufferSize() {
  // In platform_thread_mac's GetDefaultThreadStackSize(), RLIMIT_STACK is used
  // for all stacks, not just the main thread's, so it is good for use here.
  struct rlimit stack_rlimit;
  if (getrlimit(RLIMIT_STACK, &stack_rlimit) == 0 &&
      stack_rlimit.rlim_cur != RLIM_INFINITY) {
    return stack_rlimit.rlim_cur;
  }

  // If getrlimit somehow fails, return the default macOS main thread stack size
  // of 8 MB (DFLSSIZ in <i386/vmparam.h>) with extra wiggle room.
  return 12 * 1024 * 1024;
}

}  // namespace base