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/Attributes.h"
20 #include "llvm/Constants.h"
21 #include "llvm/DerivedTypes.h"
22 #include "llvm/Function.h"
23 #include "llvm/Instructions.h"
24 #include "llvm/Intrinsics.h"
25 #include "llvm/Module.h"
26 #include "llvm/Pass.h"
27 #include "llvm/Support/CommandLine.h"
28 #include "llvm/Target/TargetData.h"
29 #include "llvm/Target/TargetLowering.h"
32 // SSPBufferSize - The lower bound for a buffer to be considered for stack
33 // smashing protection.
34 static cl::opt<unsigned>
35 SSPBufferSize("stack-protector-buffer-size", cl::init(8),
36 cl::desc("Lower bound for a buffer to be considered for "
40 class StackProtector : public FunctionPass {
41 /// TLI - Keep a pointer of a TargetLowering to consult for determining
42 /// target type sizes.
43 const TargetLowering *TLI;
48 /// InsertStackProtectors - Insert code into the prologue and epilogue of
51 /// - The prologue code loads and stores the stack guard onto the stack.
52 /// - The epilogue checks the value stored in the prologue against the
53 /// original value. It calls __stack_chk_fail if they differ.
54 bool InsertStackProtectors();
56 /// CreateFailBB - Create a basic block to jump to when the stack protector
58 BasicBlock *CreateFailBB();
60 /// RequiresStackProtector - Check whether or not this function needs a
61 /// stack protector based upon the stack protector level.
62 bool RequiresStackProtector() const;
64 static char ID; // Pass identification, replacement for typeid.
65 StackProtector() : FunctionPass(ID), TLI(0) {}
66 StackProtector(const TargetLowering *tli)
67 : FunctionPass(ID), TLI(tli) {}
69 virtual bool runOnFunction(Function &Fn);
71 } // end anonymous namespace
73 char StackProtector::ID = 0;
74 INITIALIZE_PASS(StackProtector, "stack-protector",
75 "Insert stack protectors", false, false);
77 FunctionPass *llvm::createStackProtectorPass(const TargetLowering *tli) {
78 return new StackProtector(tli);
81 bool StackProtector::runOnFunction(Function &Fn) {
85 if (!RequiresStackProtector()) return false;
87 return InsertStackProtectors();
90 /// RequiresStackProtector - Check whether or not this function needs a stack
91 /// protector based upon the stack protector level. The heuristic we use is to
92 /// add a guard variable to functions that call alloca, and functions with
93 /// buffers larger than SSPBufferSize bytes.
94 bool StackProtector::RequiresStackProtector() const {
95 if (F->hasFnAttr(Attribute::StackProtectReq))
98 if (!F->hasFnAttr(Attribute::StackProtect))
101 const TargetData *TD = TLI->getTargetData();
103 for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) {
106 for (BasicBlock::iterator
107 II = BB->begin(), IE = BB->end(); II != IE; ++II)
108 if (AllocaInst *AI = dyn_cast<AllocaInst>(II)) {
109 if (AI->isArrayAllocation())
110 // This is a call to alloca with a variable size. Emit stack
114 if (const ArrayType *AT = dyn_cast<ArrayType>(AI->getAllocatedType())) {
115 // We apparently only care about character arrays.
116 if (!AT->getElementType()->isIntegerTy(8))
119 // If an array has more than SSPBufferSize bytes of allocated space,
120 // then we emit stack protectors.
121 if (SSPBufferSize <= TD->getTypeAllocSize(AT))
130 /// InsertStackProtectors - Insert code into the prologue and epilogue of the
133 /// - The prologue code loads and stores the stack guard onto the stack.
134 /// - The epilogue checks the value stored in the prologue against the original
135 /// value. It calls __stack_chk_fail if they differ.
136 bool StackProtector::InsertStackProtectors() {
137 BasicBlock *FailBB = 0; // The basic block to jump to if check fails.
138 AllocaInst *AI = 0; // Place on stack that stores the stack guard.
139 Value *StackGuardVar = 0; // The stack guard variable.
141 for (Function::iterator I = F->begin(), E = F->end(); I != E; ) {
142 BasicBlock *BB = I++;
144 ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator());
148 // Insert code into the entry block that stores the __stack_chk_guard
149 // variable onto the stack:
152 // StackGuardSlot = alloca i8*
153 // StackGuard = load __stack_chk_guard
154 // call void @llvm.stackprotect.create(StackGuard, StackGuardSlot)
156 const PointerType *PtrTy = Type::getInt8PtrTy(RI->getContext());
157 unsigned AddressSpace, Offset;
158 if (TLI->getStackCookieLocation(AddressSpace, Offset)) {
159 Constant *OffsetVal =
160 ConstantInt::get(Type::getInt32Ty(RI->getContext()), Offset);
162 StackGuardVar = ConstantExpr::getIntToPtr(OffsetVal,
163 PointerType::get(PtrTy, AddressSpace));
165 StackGuardVar = M->getOrInsertGlobal("__stack_chk_guard", PtrTy);
168 BasicBlock &Entry = F->getEntryBlock();
169 Instruction *InsPt = &Entry.front();
171 AI = new AllocaInst(PtrTy, "StackGuardSlot", InsPt);
172 LoadInst *LI = new LoadInst(StackGuardVar, "StackGuard", false, InsPt);
174 Value *Args[] = { LI, AI };
176 Create(Intrinsic::getDeclaration(M, Intrinsic::stackprotector),
177 &Args[0], array_endof(Args), "", InsPt);
179 // Create the basic block to jump to when the guard check fails.
180 FailBB = CreateFailBB();
183 // For each block with a return instruction, convert this:
193 // %1 = load __stack_chk_guard
194 // %2 = load StackGuardSlot
195 // %3 = cmp i1 %1, %2
196 // br i1 %3, label %SP_return, label %CallStackCheckFailBlk
201 // CallStackCheckFailBlk:
202 // call void @__stack_chk_fail()
205 // Split the basic block before the return instruction.
206 BasicBlock *NewBB = BB->splitBasicBlock(RI, "SP_return");
208 // Remove default branch instruction to the new BB.
209 BB->getTerminator()->eraseFromParent();
211 // Move the newly created basic block to the point right after the old basic
212 // block so that it's in the "fall through" position.
213 NewBB->moveAfter(BB);
215 // Generate the stack protector instructions in the old basic block.
216 LoadInst *LI1 = new LoadInst(StackGuardVar, "", false, BB);
217 LoadInst *LI2 = new LoadInst(AI, "", true, BB);
218 ICmpInst *Cmp = new ICmpInst(*BB, CmpInst::ICMP_EQ, LI1, LI2, "");
219 BranchInst::Create(NewBB, FailBB, Cmp, BB);
222 // Return if we didn't modify any basic blocks. I.e., there are no return
223 // statements in the function.
224 if (!FailBB) return false;
229 /// CreateFailBB - Create a basic block to jump to when the stack protector
231 BasicBlock *StackProtector::CreateFailBB() {
232 BasicBlock *FailBB = BasicBlock::Create(F->getContext(),
233 "CallStackCheckFailBlk", F);
234 Constant *StackChkFail =
235 M->getOrInsertFunction("__stack_chk_fail",
236 Type::getVoidTy(F->getContext()), NULL);
237 CallInst::Create(StackChkFail, "", FailBB);
238 new UnreachableInst(F->getContext(), FailBB);