//===-- 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/Analysis/Dominators.h"
#include "llvm/Attributes.h"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Function.h"
#include "llvm/Instructions.h"
#include "llvm/Intrinsics.h"
#include "llvm/Module.h"
#include "llvm/Pass.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Target/TargetLowering.h"
using namespace llvm;
// SSPBufferSize - The lower bound for a buffer to be considered for stack
// smashing protection.
static cl::opt<unsigned>
SSPBufferSize("stack-protector-buffer-size", cl::init(8),
cl::desc("Lower bound for a buffer to be considered for "
"stack protection"));
namespace {
class StackProtector : public FunctionPass {
/// TLI - Keep a pointer of a TargetLowering to consult for determining
/// target type sizes.
const TargetLowering *TLI;
Function *F;
Module *M;
DominatorTree* DT;
/// InsertStackProtectors - Insert code into the prologue and epilogue of
/// the function.
///
/// - The prologue code loads and stores the stack guard onto the stack.
/// - The epilogue checks the value stored in the prologue against the
/// original value. It calls __stack_chk_fail if they differ.
bool InsertStackProtectors();
/// CreateFailBB - Create a basic block to jump to when the stack protector
/// check fails.
BasicBlock *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), TLI(0) {
initializeStackProtectorPass(*PassRegistry::getPassRegistry());
}
StackProtector(const TargetLowering *tli)
: FunctionPass(ID), TLI(tli) {
initializeStackProtectorPass(*PassRegistry::getPassRegistry());
}
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.addPreserved<DominatorTree>();
}
virtual bool runOnFunction(Function &Fn);
};
} // end anonymous namespace
char StackProtector::ID = 0;
INITIALIZE_PASS(StackProtector, "stack-protector",
"Insert stack protectors", false, false)
FunctionPass *llvm::createStackProtectorPass(const TargetLowering *tli) {
return new StackProtector(tli);
}
bool StackProtector::runOnFunction(Function &Fn) {
F = &Fn;
M = F->getParent();
DT = getAnalysisIfAvailable<DominatorTree>();
if (!RequiresStackProtector()) return false;
return InsertStackProtectors();
}
/// RequiresStackProtector - Check whether or not this function needs a stack
/// protector based upon the stack protector level. The heuristic we use is to
/// add a guard variable to functions that call alloca, and functions with
/// buffers larger than SSPBufferSize bytes.
bool StackProtector::RequiresStackProtector() const {
if (F->hasFnAttr(Attribute::StackProtectReq))
return true;
if (!F->hasFnAttr(Attribute::StackProtect))
return false;
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 (AI->isArrayAllocation())
// This is a call to alloca with a variable size. Emit stack
// protectors.
return true;
if (ArrayType *AT = dyn_cast<ArrayType>(AI->getAllocatedType())) {
// We apparently only care about character arrays.
if (!AT->getElementType()->isIntegerTy(8))
continue;
// If an array has more than SSPBufferSize bytes of allocated space,
// then we emit stack protectors.
if (SSPBufferSize <= TD->getTypeAllocSize(AT))
return true;
}
}
}
return false;
}
/// InsertStackProtectors - Insert code into the prologue and epilogue of the
/// function.
///
/// - The prologue code loads and stores the stack guard onto the stack.
/// - The epilogue checks the value stored in the prologue against the original
/// value. It calls __stack_chk_fail if they differ.
bool StackProtector::InsertStackProtectors() {
BasicBlock *FailBB = 0; // The basic block to jump to if check fails.
BasicBlock *FailBBDom = 0; // FailBB's dominator.
AllocaInst *AI = 0; // Place on stack that stores the stack guard.
Value *StackGuardVar = 0; // The stack guard variable.
for (Function::iterator I = F->begin(), E = F->end(); I != E; ) {
BasicBlock *BB = I++;
ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator());
if (!RI) continue;
if (!FailBB) {
// Insert code into the entry block that stores the __stack_chk_guard
// variable onto the stack:
//
// entry:
// StackGuardSlot = alloca i8*
// StackGuard = load __stack_chk_guard
// call void @llvm.stackprotect.create(StackGuard, StackGuardSlot)
//
PointerType *PtrTy = Type::getInt8PtrTy(RI->getContext());
unsigned AddressSpace, Offset;
if (TLI->getStackCookieLocation(AddressSpace, Offset)) {
Constant *OffsetVal =
ConstantInt::get(Type::getInt32Ty(RI->getContext()), Offset);
StackGuardVar = ConstantExpr::getIntToPtr(OffsetVal,
PointerType::get(PtrTy, AddressSpace));
} else {
StackGuardVar = M->getOrInsertGlobal("__stack_chk_guard", PtrTy);
}
BasicBlock &Entry = F->getEntryBlock();
Instruction *InsPt = &Entry.front();
AI = new AllocaInst(PtrTy, "StackGuardSlot", InsPt);
LoadInst *LI = new LoadInst(StackGuardVar, "StackGuard", false, InsPt);
Value *Args[] = { LI, AI };
CallInst::
Create(Intrinsic::getDeclaration(M, Intrinsic::stackprotector),
Args, "", InsPt);
// Create the basic block to jump to when the guard check fails.
FailBB = CreateFailBB();
}
// For each block with a return instruction, convert this:
//
// return:
// ...
// ret ...
//
// into this:
//
// return:
// ...
// %1 = load __stack_chk_guard
// %2 = load StackGuardSlot
// %3 = cmp i1 %1, %2
// br i1 %3, label %SP_return, label %CallStackCheckFailBlk
//
// SP_return:
// ret ...
//
// CallStackCheckFailBlk:
// call void @__stack_chk_fail()
// unreachable
// Split the basic block before the return instruction.
BasicBlock *NewBB = BB->splitBasicBlock(RI, "SP_return");
if (DT && DT->isReachableFromEntry(BB)) {
DT->addNewBlock(NewBB, BB);
FailBBDom = FailBBDom ? DT->findNearestCommonDominator(FailBBDom, BB) :BB;
}
// Remove default branch instruction to the new BB.
BB->getTerminator()->eraseFromParent();
// Move the newly created basic block to the point right after the old basic
// block so that it's in the "fall through" position.
NewBB->moveAfter(BB);
// Generate the stack protector instructions in the old basic block.
LoadInst *LI1 = new LoadInst(StackGuardVar, "", false, BB);
LoadInst *LI2 = new LoadInst(AI, "", true, BB);
ICmpInst *Cmp = new ICmpInst(*BB, CmpInst::ICMP_EQ, LI1, LI2, "");
BranchInst::Create(NewBB, FailBB, Cmp, BB);
}
// Return if we didn't modify any basic blocks. I.e., there are no return
// statements in the function.
if (!FailBB) return false;
if (DT && FailBBDom)
DT->addNewBlock(FailBB, FailBBDom);
return true;
}
/// CreateFailBB - Create a basic block to jump to when the stack protector
/// check fails.
BasicBlock *StackProtector::CreateFailBB() {
BasicBlock *FailBB = BasicBlock::Create(F->getContext(),
"CallStackCheckFailBlk", F);
Constant *StackChkFail =
M->getOrInsertFunction("__stack_chk_fail",
Type::getVoidTy(F->getContext()), NULL);
CallInst::Create(StackChkFail, "", FailBB);
new UnreachableInst(F->getContext(), FailBB);
return FailBB;
}