1 //===-- StackProtector.cpp - Stack Protector Insertion --------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass inserts stack protectors into functions which need them. A variable
11 // with a random value in it is stored onto the stack before the local variables
12 // are allocated. Upon exiting the block, the stored value is checked. If it's
13 // changed, then there was some sort of violation and the program aborts.
15 //===----------------------------------------------------------------------===//
17 #define DEBUG_TYPE "stack-protector"
18 #include "llvm/CodeGen/Passes.h"
19 #include "llvm/Analysis/Dominators.h"
20 #include "llvm/Attributes.h"
21 #include "llvm/Constants.h"
22 #include "llvm/DerivedTypes.h"
23 #include "llvm/Function.h"
24 #include "llvm/Instructions.h"
25 #include "llvm/Intrinsics.h"
26 #include "llvm/Module.h"
27 #include "llvm/Pass.h"
28 #include "llvm/Support/CommandLine.h"
29 #include "llvm/Target/TargetData.h"
30 #include "llvm/Target/TargetLowering.h"
31 #include "llvm/ADT/Triple.h"
34 // SSPBufferSize - The lower bound for a buffer to be considered for stack
35 // smashing protection.
36 static cl::opt<unsigned>
37 SSPBufferSize("stack-protector-buffer-size", cl::init(8),
38 cl::desc("Lower bound for a buffer to be considered for "
42 class StackProtector : public FunctionPass {
43 /// TLI - Keep a pointer of a TargetLowering to consult for determining
44 /// target type sizes.
45 const TargetLowering *TLI;
52 /// InsertStackProtectors - Insert code into the prologue and epilogue of
55 /// - The prologue code loads and stores the stack guard onto the stack.
56 /// - The epilogue checks the value stored in the prologue against the
57 /// original value. It calls __stack_chk_fail if they differ.
58 bool InsertStackProtectors();
60 /// CreateFailBB - Create a basic block to jump to when the stack protector
62 BasicBlock *CreateFailBB();
64 /// RequiresStackProtector - Check whether or not this function needs a
65 /// stack protector based upon the stack protector level.
66 bool RequiresStackProtector() const;
68 static char ID; // Pass identification, replacement for typeid.
69 StackProtector() : FunctionPass(ID), TLI(0) {
70 initializeStackProtectorPass(*PassRegistry::getPassRegistry());
72 StackProtector(const TargetLowering *tli)
73 : FunctionPass(ID), TLI(tli) {
74 initializeStackProtectorPass(*PassRegistry::getPassRegistry());
77 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
78 AU.addPreserved<DominatorTree>();
81 virtual bool runOnFunction(Function &Fn);
83 } // end anonymous namespace
85 char StackProtector::ID = 0;
86 INITIALIZE_PASS(StackProtector, "stack-protector",
87 "Insert stack protectors", false, false)
89 FunctionPass *llvm::createStackProtectorPass(const TargetLowering *tli) {
90 return new StackProtector(tli);
93 bool StackProtector::runOnFunction(Function &Fn) {
96 DT = getAnalysisIfAvailable<DominatorTree>();
98 if (!RequiresStackProtector()) return false;
100 return InsertStackProtectors();
103 /// RequiresStackProtector - Check whether or not this function needs a stack
104 /// protector based upon the stack protector level. The heuristic we use is to
105 /// add a guard variable to functions that call alloca, and functions with
106 /// buffers larger than SSPBufferSize bytes.
107 bool StackProtector::RequiresStackProtector() const {
108 if (F->hasFnAttr(Attribute::StackProtectReq))
111 if (!F->hasFnAttr(Attribute::StackProtect))
114 const TargetData *TD = TLI->getTargetData();
115 const TargetMachine &TM = TLI->getTargetMachine();
116 Triple Trip(TM.getTargetTriple());
118 for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) {
121 for (BasicBlock::iterator
122 II = BB->begin(), IE = BB->end(); II != IE; ++II)
123 if (AllocaInst *AI = dyn_cast<AllocaInst>(II)) {
124 if (AI->isArrayAllocation())
125 // This is a call to alloca with a variable size. Emit stack
129 if (ArrayType *AT = dyn_cast<ArrayType>(AI->getAllocatedType())) {
130 // If we're on a non-Darwin platform, don't add stack protectors
131 // unless the array is a character array.
132 if (!Trip.isOSDarwin() && !AT->getElementType()->isIntegerTy(8))
135 // If an array has more than SSPBufferSize bytes of allocated space,
136 // then we emit stack protectors.
137 if (SSPBufferSize <= TD->getTypeAllocSize(AT))
146 /// InsertStackProtectors - Insert code into the prologue and epilogue of the
149 /// - The prologue code loads and stores the stack guard onto the stack.
150 /// - The epilogue checks the value stored in the prologue against the original
151 /// value. It calls __stack_chk_fail if they differ.
152 bool StackProtector::InsertStackProtectors() {
153 BasicBlock *FailBB = 0; // The basic block to jump to if check fails.
154 BasicBlock *FailBBDom = 0; // FailBB's dominator.
155 AllocaInst *AI = 0; // Place on stack that stores the stack guard.
156 Value *StackGuardVar = 0; // The stack guard variable.
158 for (Function::iterator I = F->begin(), E = F->end(); I != E; ) {
159 BasicBlock *BB = I++;
160 ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator());
164 // Insert code into the entry block that stores the __stack_chk_guard
165 // variable onto the stack:
168 // StackGuardSlot = alloca i8*
169 // StackGuard = load __stack_chk_guard
170 // call void @llvm.stackprotect.create(StackGuard, StackGuardSlot)
172 PointerType *PtrTy = Type::getInt8PtrTy(RI->getContext());
173 unsigned AddressSpace, Offset;
174 if (TLI->getStackCookieLocation(AddressSpace, Offset)) {
175 Constant *OffsetVal =
176 ConstantInt::get(Type::getInt32Ty(RI->getContext()), Offset);
178 StackGuardVar = ConstantExpr::getIntToPtr(OffsetVal,
179 PointerType::get(PtrTy, AddressSpace));
181 StackGuardVar = M->getOrInsertGlobal("__stack_chk_guard", PtrTy);
184 BasicBlock &Entry = F->getEntryBlock();
185 Instruction *InsPt = &Entry.front();
187 AI = new AllocaInst(PtrTy, "StackGuardSlot", InsPt);
188 LoadInst *LI = new LoadInst(StackGuardVar, "StackGuard", false, InsPt);
190 Value *Args[] = { LI, AI };
192 Create(Intrinsic::getDeclaration(M, Intrinsic::stackprotector),
195 // Create the basic block to jump to when the guard check fails.
196 FailBB = CreateFailBB();
199 // For each block with a return instruction, convert this:
209 // %1 = load __stack_chk_guard
210 // %2 = load StackGuardSlot
211 // %3 = cmp i1 %1, %2
212 // br i1 %3, label %SP_return, label %CallStackCheckFailBlk
217 // CallStackCheckFailBlk:
218 // call void @__stack_chk_fail()
221 // Split the basic block before the return instruction.
222 BasicBlock *NewBB = BB->splitBasicBlock(RI, "SP_return");
224 if (DT && DT->isReachableFromEntry(BB)) {
225 DT->addNewBlock(NewBB, BB);
226 FailBBDom = FailBBDom ? DT->findNearestCommonDominator(FailBBDom, BB) :BB;
229 // Remove default branch instruction to the new BB.
230 BB->getTerminator()->eraseFromParent();
232 // Move the newly created basic block to the point right after the old basic
233 // block so that it's in the "fall through" position.
234 NewBB->moveAfter(BB);
236 // Generate the stack protector instructions in the old basic block.
237 LoadInst *LI1 = new LoadInst(StackGuardVar, "", false, BB);
238 LoadInst *LI2 = new LoadInst(AI, "", true, BB);
239 ICmpInst *Cmp = new ICmpInst(*BB, CmpInst::ICMP_EQ, LI1, LI2, "");
240 BranchInst::Create(NewBB, FailBB, Cmp, BB);
243 // Return if we didn't modify any basic blocks. I.e., there are no return
244 // statements in the function.
245 if (!FailBB) return false;
248 DT->addNewBlock(FailBB, FailBBDom);
253 /// CreateFailBB - Create a basic block to jump to when the stack protector
255 BasicBlock *StackProtector::CreateFailBB() {
256 BasicBlock *FailBB = BasicBlock::Create(F->getContext(),
257 "CallStackCheckFailBlk", F);
258 Constant *StackChkFail =
259 M->getOrInsertFunction("__stack_chk_fail",
260 Type::getVoidTy(F->getContext()), NULL);
261 CallInst::Create(StackChkFail, "", FailBB);
262 new UnreachableInst(F->getContext(), FailBB);