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 /// ContainsProtectableArray - Check whether the type either is an array or
65 /// contains an array of sufficient size so that we need stack protectors
67 bool ContainsProtectableArray(Type *Ty, bool InStruct = false) const;
69 /// RequiresStackProtector - Check whether or not this function needs a
70 /// stack protector based upon the stack protector level.
71 bool RequiresStackProtector() const;
73 static char ID; // Pass identification, replacement for typeid.
74 StackProtector() : FunctionPass(ID), TLI(0) {
75 initializeStackProtectorPass(*PassRegistry::getPassRegistry());
77 StackProtector(const TargetLowering *tli)
78 : FunctionPass(ID), TLI(tli) {
79 initializeStackProtectorPass(*PassRegistry::getPassRegistry());
82 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
83 AU.addPreserved<DominatorTree>();
86 virtual bool runOnFunction(Function &Fn);
88 } // end anonymous namespace
90 char StackProtector::ID = 0;
91 INITIALIZE_PASS(StackProtector, "stack-protector",
92 "Insert stack protectors", false, false)
94 FunctionPass *llvm::createStackProtectorPass(const TargetLowering *tli) {
95 return new StackProtector(tli);
98 bool StackProtector::runOnFunction(Function &Fn) {
101 DT = getAnalysisIfAvailable<DominatorTree>();
103 if (!RequiresStackProtector()) return false;
105 return InsertStackProtectors();
108 /// ContainsProtectableArray - Check whether the type either is an array or
109 /// contains a char array of sufficient size so that we need stack protectors
111 bool StackProtector::ContainsProtectableArray(Type *Ty, bool InStruct) const {
112 if (!Ty) return false;
113 if (ArrayType *AT = dyn_cast<ArrayType>(Ty)) {
114 if (!AT->getElementType()->isIntegerTy(8)) {
115 const TargetMachine &TM = TLI->getTargetMachine();
116 Triple Trip(TM.getTargetTriple());
118 // If we're on a non-Darwin platform or we're inside of a structure, don't
119 // add stack protectors unless the array is a character array.
120 if (InStruct || !Trip.isOSDarwin())
124 // If an array has more than SSPBufferSize bytes of allocated space, then we
125 // emit stack protectors.
126 if (SSPBufferSize <= TLI->getTargetData()->getTypeAllocSize(AT))
130 const StructType *ST = dyn_cast<StructType>(Ty);
131 if (!ST) return false;
133 for (StructType::element_iterator I = ST->element_begin(),
134 E = ST->element_end(); I != E; ++I)
135 if (ContainsProtectableArray(*I, true))
141 /// RequiresStackProtector - Check whether or not this function needs a stack
142 /// protector based upon the stack protector level. The heuristic we use is to
143 /// add a guard variable to functions that call alloca, and functions with
144 /// buffers larger than SSPBufferSize bytes.
145 bool StackProtector::RequiresStackProtector() const {
146 if (F->hasFnAttr(Attribute::StackProtectReq))
149 if (!F->hasFnAttr(Attribute::StackProtect))
152 for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) {
155 for (BasicBlock::iterator
156 II = BB->begin(), IE = BB->end(); II != IE; ++II)
157 if (AllocaInst *AI = dyn_cast<AllocaInst>(II)) {
158 if (AI->isArrayAllocation())
159 // This is a call to alloca with a variable size. Emit stack
163 if (ContainsProtectableArray(AI->getAllocatedType()))
171 /// InsertStackProtectors - Insert code into the prologue and epilogue of the
174 /// - The prologue code loads and stores the stack guard onto the stack.
175 /// - The epilogue checks the value stored in the prologue against the original
176 /// value. It calls __stack_chk_fail if they differ.
177 bool StackProtector::InsertStackProtectors() {
178 BasicBlock *FailBB = 0; // The basic block to jump to if check fails.
179 BasicBlock *FailBBDom = 0; // FailBB's dominator.
180 AllocaInst *AI = 0; // Place on stack that stores the stack guard.
181 Value *StackGuardVar = 0; // The stack guard variable.
183 for (Function::iterator I = F->begin(), E = F->end(); I != E; ) {
184 BasicBlock *BB = I++;
185 ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator());
189 // Insert code into the entry block that stores the __stack_chk_guard
190 // variable onto the stack:
193 // StackGuardSlot = alloca i8*
194 // StackGuard = load __stack_chk_guard
195 // call void @llvm.stackprotect.create(StackGuard, StackGuardSlot)
197 PointerType *PtrTy = Type::getInt8PtrTy(RI->getContext());
198 unsigned AddressSpace, Offset;
199 if (TLI->getStackCookieLocation(AddressSpace, Offset)) {
200 Constant *OffsetVal =
201 ConstantInt::get(Type::getInt32Ty(RI->getContext()), Offset);
203 StackGuardVar = ConstantExpr::getIntToPtr(OffsetVal,
204 PointerType::get(PtrTy, AddressSpace));
206 StackGuardVar = M->getOrInsertGlobal("__stack_chk_guard", PtrTy);
209 BasicBlock &Entry = F->getEntryBlock();
210 Instruction *InsPt = &Entry.front();
212 AI = new AllocaInst(PtrTy, "StackGuardSlot", InsPt);
213 LoadInst *LI = new LoadInst(StackGuardVar, "StackGuard", false, InsPt);
215 Value *Args[] = { LI, AI };
217 Create(Intrinsic::getDeclaration(M, Intrinsic::stackprotector),
220 // Create the basic block to jump to when the guard check fails.
221 FailBB = CreateFailBB();
224 // For each block with a return instruction, convert this:
234 // %1 = load __stack_chk_guard
235 // %2 = load StackGuardSlot
236 // %3 = cmp i1 %1, %2
237 // br i1 %3, label %SP_return, label %CallStackCheckFailBlk
242 // CallStackCheckFailBlk:
243 // call void @__stack_chk_fail()
246 // Split the basic block before the return instruction.
247 BasicBlock *NewBB = BB->splitBasicBlock(RI, "SP_return");
249 if (DT && DT->isReachableFromEntry(BB)) {
250 DT->addNewBlock(NewBB, BB);
251 FailBBDom = FailBBDom ? DT->findNearestCommonDominator(FailBBDom, BB) :BB;
254 // Remove default branch instruction to the new BB.
255 BB->getTerminator()->eraseFromParent();
257 // Move the newly created basic block to the point right after the old basic
258 // block so that it's in the "fall through" position.
259 NewBB->moveAfter(BB);
261 // Generate the stack protector instructions in the old basic block.
262 LoadInst *LI1 = new LoadInst(StackGuardVar, "", false, BB);
263 LoadInst *LI2 = new LoadInst(AI, "", true, BB);
264 ICmpInst *Cmp = new ICmpInst(*BB, CmpInst::ICMP_EQ, LI1, LI2, "");
265 BranchInst::Create(NewBB, FailBB, Cmp, BB);
268 // Return if we didn't modify any basic blocks. I.e., there are no return
269 // statements in the function.
270 if (!FailBB) return false;
273 DT->addNewBlock(FailBB, FailBBDom);
278 /// CreateFailBB - Create a basic block to jump to when the stack protector
280 BasicBlock *StackProtector::CreateFailBB() {
281 BasicBlock *FailBB = BasicBlock::Create(F->getContext(),
282 "CallStackCheckFailBlk", F);
283 Constant *StackChkFail =
284 M->getOrInsertFunction("__stack_chk_fail",
285 Type::getVoidTy(F->getContext()), NULL);
286 CallInst::Create(StackChkFail, "", FailBB);
287 new UnreachableInst(F->getContext(), FailBB);