ASanStackFrameLayout.cpp

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//===-- ASanStackFrameLayout.cpp - helper for AddressSanitizer ------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// Definition of ComputeASanStackFrameLayout (see ASanStackFrameLayout.h).
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Utils/ASanStackFrameLayout.h"
#include "llvm/ADT/SmallString.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>

namespace llvm {

// We sort the stack variables by alignment (largest first) to minimize
// unnecessary large gaps due to alignment.
// It is tempting to also sort variables by size so that larger variables
// have larger redzones at both ends. But reordering will make report analysis
// harder, especially when temporary unnamed variables are present.
// So, until we can provide more information (type, line number, etc)
// for the stack variables we avoid reordering them too much.
static inline bool CompareVars(const ASanStackVariableDescription &a,
                               const ASanStackVariableDescription &b) {
  return a.Alignment > b.Alignment;
}

// We also force minimal alignment for all vars to kMinAlignment so that vars
// with e.g. alignment 1 and alignment 16 do not get reordered by CompareVars.
static const size_t kMinAlignment = 16;

static size_t RoundUpTo(size_t X, size_t RoundTo) {
  assert((RoundTo & (RoundTo - 1)) == 0);
  return (X + RoundTo - 1) & ~(RoundTo - 1);
}

// The larger the variable Size the larger is the redzone.
// The resulting frame size is a multiple of Alignment.
static size_t VarAndRedzoneSize(size_t Size, size_t Alignment) {
  size_t Res = 0;
  if (Size <= 4)  Res = 16;
  else if (Size <= 16) Res = 32;
  else if (Size <= 128) Res = Size + 32;
  else if (Size <= 512) Res = Size + 64;
  else if (Size <= 4096) Res = Size + 128;
  else                   Res = Size + 256;
  return RoundUpTo(Res, Alignment);
}

void
ComputeASanStackFrameLayout(SmallVectorImpl<ASanStackVariableDescription> &Vars,
                            size_t Granularity, size_t MinHeaderSize,
                            ASanStackFrameLayout *Layout) {
  assert(Granularity >= 8 && Granularity <= 64 &&
         (Granularity & (Granularity - 1)) == 0);
  assert(MinHeaderSize >= 16 && (MinHeaderSize & (MinHeaderSize - 1)) == 0 &&
         MinHeaderSize >= Granularity);
  size_t NumVars = Vars.size();
  assert(NumVars > 0);
  for (size_t i = 0; i < NumVars; i++)
    Vars[i].Alignment = std::max(Vars[i].Alignment, kMinAlignment);

  std::stable_sort(Vars.begin(), Vars.end(), CompareVars);
  SmallString<2048> StackDescriptionStorage;
  raw_svector_ostream StackDescription(StackDescriptionStorage);
  StackDescription << NumVars;
  Layout->FrameAlignment = std::max(Granularity, Vars[0].Alignment);
  SmallVector<uint8_t, 64> &SB(Layout->ShadowBytes);
  SB.clear();
  size_t Offset = std::max(std::max(MinHeaderSize, Granularity),
     Vars[0].Alignment);
  assert((Offset % Granularity) == 0);
  SB.insert(SB.end(), Offset / Granularity, kAsanStackLeftRedzoneMagic);
  for (size_t i = 0; i < NumVars; i++) {
    bool IsLast = i == NumVars - 1;
    size_t Alignment = std::max(Granularity, Vars[i].Alignment);
    (void)Alignment;  // Used only in asserts.
    size_t Size = Vars[i].Size;
    const char *Name = Vars[i].Name;
    assert((Alignment & (Alignment - 1)) == 0);
    assert(Layout->FrameAlignment >= Alignment);
    assert((Offset % Alignment) == 0);
    assert(Size > 0);
    StackDescription << " " << Offset << " " << Size << " " << strlen(Name)
                     << " " << Name;
    size_t NextAlignment = IsLast ? Granularity
                   : std::max(Granularity, Vars[i + 1].Alignment);
    size_t SizeWithRedzone = VarAndRedzoneSize(Vars[i].Size, NextAlignment);
    SB.insert(SB.end(), Size / Granularity, 0);
    if (Size % Granularity)
      SB.insert(SB.end(), Size % Granularity);
    SB.insert(SB.end(), (SizeWithRedzone - Size) / Granularity,
        IsLast ? kAsanStackRightRedzoneMagic
        : kAsanStackMidRedzoneMagic);
    Vars[i].Offset = Offset;
    Offset += SizeWithRedzone;
  }
  if (Offset % MinHeaderSize) {
    size_t ExtraRedzone = MinHeaderSize - (Offset % MinHeaderSize);
    SB.insert(SB.end(), ExtraRedzone / Granularity,
              kAsanStackRightRedzoneMagic);
    Offset += ExtraRedzone;
  }
  Layout->DescriptionString = StackDescription.str();
  Layout->FrameSize = Offset;
  assert((Layout->FrameSize % MinHeaderSize) == 0);
  assert(Layout->FrameSize / Granularity == Layout->ShadowBytes.size());
}

} // llvm namespace