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//===-- asan_fake_stack.h ---------------------------------------*- C++ -*-===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// This file is a part of AddressSanitizer, an address sanity checker.
// ASan-private header for, implements FakeStack.
#include "sanitizer_common/sanitizer_common.h"
namespace __asan {
// Fake stack frame contains local variables of one function.
struct FakeFrame {
uptr magic; // Modified by the instrumented code.
uptr descr; // Modified by the instrumented code.
uptr pc; // Modified by the instrumented code.
uptr real_stack;
// For each thread we create a fake stack and place stack objects on this fake
// stack instead of the real stack. The fake stack is not really a stack but
// a fast malloc-like allocator so that when a function exits the fake stack
// is not popped but remains there for quite some time until gets used again.
// So, we poison the objects on the fake stack when function returns.
// It helps us find use-after-return bugs.
// The FakeStack objects is allocated by a single mmap call and has no other
// pointers. The size of the fake stack depends on the actual thread stack size
// and thus can not be a constant.
// stack_size is a power of two greater or equal to the thread's stack size;
// we store it as its logarithm (stack_size_log).
// FakeStack has kNumberOfSizeClasses (11) size classes, each size class
// is a power of two, starting from 64 bytes. Each size class occupies
// stack_size bytes and thus can allocate
// NumberOfFrames=(stack_size/BytesInSizeClass) fake frames (also a power of 2).
// For each size class we have NumberOfFrames allocation flags,
// each flag indicates whether the given frame is currently allocated.
// All flags for size classes 0 .. 10 are stored in a single contiguous region
// followed by another contiguous region which contains the actual memory for
// size classes. The addresses are computed by GetFlags and GetFrame without
// any memory accesses solely based on 'this' and stack_size_log.
// Allocate() flips the appropriate allocation flag atomically, thus achieving
// async-signal safety.
// This allocator does not have quarantine per se, but it tries to allocate the
// frames in round robin fasion to maximize the delay between a deallocation
// and the next allocation.
class FakeStack {
static const uptr kMinStackFrameSizeLog = 6; // Min frame is 64B.
static const uptr kMaxStackFrameSizeLog = 16; // Max stack frame is 64K.
static const uptr kNumberOfSizeClasses =
kMaxStackFrameSizeLog - kMinStackFrameSizeLog + 1;
// CTOR: create the FakeStack as a single mmap-ed object.
static FakeStack *Create(uptr stack_size_log);
void Destroy(int tid);
// stack_size_log is at least 15 (stack_size >= 32K).
static uptr SizeRequiredForFlags(uptr stack_size_log) {
return 1UL << (stack_size_log + 1 - kMinStackFrameSizeLog);
// Each size class occupies stack_size bytes.
static uptr SizeRequiredForFrames(uptr stack_size_log) {
return (1ULL << stack_size_log) * kNumberOfSizeClasses;
// Number of bytes requires for the whole object.
static uptr RequiredSize(uptr stack_size_log) {
return kFlagsOffset + SizeRequiredForFlags(stack_size_log) +
// Offset of the given flag from the first flag.
// The flags for class 0 begin at offset 000000000
// The flags for class 1 begin at offset 100000000
// ....................2................ 110000000
// ....................3................ 111000000
// and so on.
static uptr FlagsOffset(uptr stack_size_log, uptr class_id) {
uptr t = kNumberOfSizeClasses - 1 - class_id;
const uptr all_ones = (1 << (kNumberOfSizeClasses - 1)) - 1;
return ((all_ones >> t) << t) << (stack_size_log - 15);
static uptr NumberOfFrames(uptr stack_size_log, uptr class_id) {
return 1UL << (stack_size_log - kMinStackFrameSizeLog - class_id);
// Divide n by the numbe of frames in size class.
static uptr ModuloNumberOfFrames(uptr stack_size_log, uptr class_id, uptr n) {
return n & (NumberOfFrames(stack_size_log, class_id) - 1);
// The the pointer to the flags of the given class_id.
u8 *GetFlags(uptr stack_size_log, uptr class_id) {
return reinterpret_cast<u8 *>(this) + kFlagsOffset +
FlagsOffset(stack_size_log, class_id);
// Get frame by class_id and pos.
u8 *GetFrame(uptr stack_size_log, uptr class_id, uptr pos) {
return reinterpret_cast<u8 *>(this) + kFlagsOffset +
SizeRequiredForFlags(stack_size_log) +
(1 << stack_size_log) * class_id + BytesInSizeClass(class_id) * pos;
// Allocate the fake frame.
FakeFrame *Allocate(uptr stack_size_log, uptr class_id, uptr real_stack);
// Deallocate the fake frame: read the saved flag address and write 0 there.
static void Deallocate(uptr x, uptr class_id) {
**SavedFlagPtr(x, class_id) = 0;
// Poison the entire FakeStack's shadow with the magic value.
void PoisonAll(u8 magic);
// Return the beginning of the FakeFrame or 0 if the address is not ours.
uptr AddrIsInFakeStack(uptr addr, uptr *frame_beg, uptr *frame_end);
USED uptr AddrIsInFakeStack(uptr addr) {
uptr t1, t2;
return AddrIsInFakeStack(addr, &t1, &t2);
// Number of bytes in a fake frame of this size class.
static uptr BytesInSizeClass(uptr class_id) {
return 1UL << (class_id + kMinStackFrameSizeLog);
// The fake frame is guaranteed to have a right redzone.
// We use the last word of that redzone to store the address of the flag
// that corresponds to the current frame to make faster deallocation.
static u8 **SavedFlagPtr(uptr x, uptr class_id) {
return reinterpret_cast<u8 **>(x + BytesInSizeClass(class_id) - sizeof(x));
uptr stack_size_log() const { return stack_size_log_; }
void HandleNoReturn();
void GC(uptr real_stack);
void ForEachFakeFrame(RangeIteratorCallback callback, void *arg);
FakeStack() { }
static const uptr kFlagsOffset = 4096; // This is were the flags begin.
// Must match the number of uses of DEFINE_STACK_MALLOC_FREE_WITH_CLASS_ID
COMPILER_CHECK(kNumberOfSizeClasses == 11);
static const uptr kMaxStackMallocSize = 1 << kMaxStackFrameSizeLog;
uptr hint_position_[kNumberOfSizeClasses];
uptr stack_size_log_;
// a bit is set if something was allocated from the corresponding size class.
bool needs_gc_;
FakeStack *GetTLSFakeStack();
void SetTLSFakeStack(FakeStack *fs);
} // namespace __asan