StackProtector.cpp

//===-- StackProtector.cpp - Stack Protector Insertion --------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass inserts stack protectors into functions which need them. A variable
// with a random value in it is stored onto the stack before the local variables
// are allocated. Upon exiting the block, the stored value is checked. If it's
// changed, then there was some sort of violation and the program aborts.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "stack-protector"
#include "llvm/CodeGen/Passes.h"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Function.h"
#include "llvm/Instructions.h"
#include "llvm/Module.h"
#include "llvm/Pass.h"
#include "llvm/ADT/APInt.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Target/TargetLowering.h"
using namespace llvm;

// Enable stack protectors.
static cl::opt<unsigned>
SSPBufferSize("stack-protector-buffer-size", cl::init(8),
              cl::desc("The lower bound for a buffer to be considered for "
                       "stack smashing protection."));

namespace {
  class VISIBILITY_HIDDEN StackProtector : public FunctionPass {
    /// Level - The level of stack protection.
    SSP::StackProtectorLevel Level;

    /// TLI - Keep a pointer of a TargetLowering to consult for determining
    /// target type sizes.
    const TargetLowering *TLI;

    /// FailBB - Holds the basic block to jump to when the stack protector check
    /// fails.
    BasicBlock *FailBB;

    /// StackProtFrameSlot - The place on the stack that the stack protector
    /// guard is kept.
    AllocaInst *StackProtFrameSlot;

    /// StackGuardVar - The global variable for the stack guard.
    GlobalVariable *StackGuardVar;

    Function *F;
    Module *M;

    /// InsertStackProtectorPrologue - Insert code into the entry block that
    /// stores the __stack_chk_guard variable onto the stack.
    void InsertStackProtectorPrologue();

    /// InsertStackProtectorEpilogue - Insert code before the return
    /// instructions checking the stack value that was stored in the
    /// prologue. If it isn't the same as the original value, then call a
    /// "failure" function.
    void InsertStackProtectorEpilogue();

    /// CreateFailBB - Create a basic block to jump to when the stack protector
    /// check fails.
    void CreateFailBB();

    /// RequiresStackProtector - Check whether or not this function needs a
    /// stack protector based upon the stack protector level.
    bool RequiresStackProtector() const;
  public:
    static char ID;             // Pass identification, replacement for typeid.
    StackProtector() : FunctionPass(&ID), Level(SSP::OFF), TLI(0), FailBB(0) {}
    StackProtector(SSP::StackProtectorLevel lvl, const TargetLowering *tli)
      : FunctionPass(&ID), Level(lvl), TLI(tli), FailBB(0) {}

    virtual bool runOnFunction(Function &Fn);
  };
} // end anonymous namespace

char StackProtector::ID = 0;
static RegisterPass<StackProtector>
X("stack-protector", "Insert stack protectors");

FunctionPass *llvm::createStackProtectorPass(SSP::StackProtectorLevel lvl,
                                             const TargetLowering *tli) {
  return new StackProtector(lvl, tli);
}

bool StackProtector::runOnFunction(Function &Fn) {
  F = &Fn;
  M = F->getParent();

  if (!RequiresStackProtector()) return false;
  
  InsertStackProtectorPrologue();
  InsertStackProtectorEpilogue();

  // Cleanup.
  FailBB = 0;
  StackProtFrameSlot = 0;
  StackGuardVar = 0;
  return true;
}

/// InsertStackProtectorPrologue - Insert code into the entry block that stores
/// the __stack_chk_guard variable onto the stack.
void StackProtector::InsertStackProtectorPrologue() {
  BasicBlock &Entry = F->getEntryBlock();
  Instruction &InsertPt = Entry.front();

  const char *StackGuardStr = "__stack_chk_guard";
  StackGuardVar = M->getNamedGlobal(StackGuardStr);

  if (!StackGuardVar)
    StackGuardVar = new GlobalVariable(PointerType::getUnqual(Type::Int8Ty),
                                       false, GlobalValue::ExternalLinkage,
                                       0, StackGuardStr, M);

  StackProtFrameSlot = new AllocaInst(PointerType::getUnqual(Type::Int8Ty),
                                      "StackProt_Frame", &InsertPt);
  LoadInst *LI = new LoadInst(StackGuardVar, "StackGuard", false, &InsertPt);
  new StoreInst(LI, StackProtFrameSlot, false, &InsertPt);
}

/// InsertStackProtectorEpilogue - Insert code before the return instructions
/// checking the stack value that was stored in the prologue. If it isn't the
/// same as the original value, then call a "failure" function.
void StackProtector::InsertStackProtectorEpilogue() {
  // Create the basic block to jump to when the guard check fails.
  CreateFailBB();

  Function::iterator I = F->begin(), E = F->end();
  std::vector<BasicBlock*> ReturnBBs;
  ReturnBBs.reserve(F->size());

  for (; I != E; ++I)
    if (isa<ReturnInst>(I->getTerminator()))
      ReturnBBs.push_back(I);

  if (ReturnBBs.empty()) return; // Odd, but could happen. . .

  // Loop through the basic blocks that have return instructions. Convert this:
  //
  //   return:
  //     ...
  //     ret ...
  //
  // into this:
  //
  //   return:
  //     ...
  //     %1 = load __stack_chk_guard
  //     %2 = load <stored stack guard>
  //     %3 = cmp i1 %1, %2
  //     br i1 %3, label %SPRet, label %CallStackCheckFailBlk
  //
  //   SPRet:
  //     ret ...
  //
  //   CallStackCheckFailBlk:
  //     call void @__stack_chk_fail()
  //     unreachable
  //
  for (std::vector<BasicBlock*>::iterator
         II = ReturnBBs.begin(), IE = ReturnBBs.end(); II != IE; ++II) {
    BasicBlock *BB = *II;
    ReturnInst *RI = cast<ReturnInst>(BB->getTerminator());
    Function::iterator InsPt = BB; ++InsPt; // Insertion point for new BB.

    BasicBlock *NewBB = BasicBlock::Create("SPRet", F, InsPt);

    // Move the return instruction into the new basic block.
    RI->removeFromParent();
    NewBB->getInstList().insert(NewBB->begin(), RI);

    LoadInst *LI2 = new LoadInst(StackGuardVar, "", false, BB);
    LoadInst *LI1 = new LoadInst(StackProtFrameSlot, "", true, BB);
    ICmpInst *Cmp = new ICmpInst(CmpInst::ICMP_EQ, LI1, LI2, "", BB);
    BranchInst::Create(NewBB, FailBB, Cmp, BB);
  }
}

/// CreateFailBB - Create a basic block to jump to when the stack protector
/// check fails.
void StackProtector::CreateFailBB() {
  assert(!FailBB && "Failure basic block already created?!");
  FailBB = BasicBlock::Create("CallStackCheckFailBlk", F);
  std::vector<const Type*> Params;
  Constant *StackChkFail =
    M->getOrInsertFunction("__stack_chk_fail", Type::VoidTy, NULL);
  CallInst::Create(StackChkFail, "", FailBB);
  new UnreachableInst(FailBB);
}

/// RequiresStackProtector - Check whether or not this function needs a stack
/// protector based upon the stack protector level.
bool StackProtector::RequiresStackProtector() const {
  switch (Level) {
  default: return false;
  case SSP::ALL: return true;
  case SSP::SOME: {
    // If the size of the local variables allocated on the stack is greater than
    // SSPBufferSize, then we require a stack protector.
    uint64_t StackSize = 0;
    const TargetData *TD = TLI->getTargetData();

    for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) {
      BasicBlock *BB = I;

      for (BasicBlock::iterator
             II = BB->begin(), IE = BB->end(); II != IE; ++II)
        if (AllocaInst *AI = dyn_cast<AllocaInst>(II)) {
          if (ConstantInt *CI = dyn_cast<ConstantInt>(AI->getArraySize())) {
            uint64_t Bytes = TD->getTypeSizeInBits(AI->getAllocatedType()) / 8;
            const APInt &Size = CI->getValue();
            StackSize += Bytes * Size.getZExtValue();

            if (SSPBufferSize <= StackSize)
              return true;
          }
        }
    }

    return false;
  }
  }
}