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("The lower bound for a buffer to be considered for "
37 "stack smashing protection."));
40 class VISIBILITY_HIDDEN 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 static RegisterPass<StackProtector>
75 X("stack-protector", "Insert stack protectors");
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 /// InsertStackProtectors - Insert code into the prologue and epilogue of the
93 /// - The prologue code loads and stores the stack guard onto the stack.
94 /// - The epilogue checks the value stored in the prologue against the original
95 /// value. It calls __stack_chk_fail if they differ.
96 bool StackProtector::InsertStackProtectors() {
97 // Loop through the basic blocks that have return instructions. Convert this:
107 // %1 = load __stack_chk_guard
108 // %2 = load <stored stack guard>
109 // %3 = cmp i1 %1, %2
110 // br i1 %3, label %SP_return, label %CallStackCheckFailBlk
115 // CallStackCheckFailBlk:
116 // call void @__stack_chk_fail()
119 BasicBlock *FailBB = 0; // The basic block to jump to if check fails.
120 AllocaInst *AI = 0; // Place on stack that stores the stack guard.
121 Constant *StackGuardVar = 0; // The stack guard variable.
123 for (Function::iterator I = F->begin(), E = F->end(); I != E; ) {
126 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
128 // Insert code into the entry block that stores the __stack_chk_guard
129 // variable onto the stack.
130 PointerType *PtrTy = PointerType::getUnqual(Type::Int8Ty);
131 StackGuardVar = M->getOrInsertGlobal("__stack_chk_guard", PtrTy);
133 BasicBlock &Entry = F->getEntryBlock();
134 Instruction *InsPt = &Entry.front();
136 AI = new AllocaInst(PtrTy, "StackGuardSlot", InsPt);
137 LoadInst *LI = new LoadInst(StackGuardVar, "StackGuard", false, InsPt);
139 Value *Args[] = { LI, AI };
141 Create(Intrinsic::getDeclaration(M, Intrinsic::stackprotector_create),
142 &Args[0], array_endof(Args), "", InsPt);
144 // Create the basic block to jump to when the guard check fails.
145 FailBB = CreateFailBB();
148 ++I; // Skip to the next block so that we don't resplit the return block.
150 // Split the basic block before the return instruction.
151 BasicBlock *NewBB = BB->splitBasicBlock(RI, "SP_return");
153 // Move the newly created basic block to the point right after the old
154 // basic block so that it's in the "fall through" position.
155 NewBB->removeFromParent();
156 F->getBasicBlockList().insert(I, NewBB);
158 // Generate the stack protector instructions in the old basic block.
159 LoadInst *LI1 = new LoadInst(StackGuardVar, "", false, BB);
160 CallInst *CI = CallInst::
161 Create(Intrinsic::getDeclaration(M, Intrinsic::stackprotector_check),
163 ICmpInst *Cmp = new ICmpInst(CmpInst::ICMP_EQ, CI, LI1, "", BB);
164 BranchInst::Create(NewBB, FailBB, Cmp, BB);
170 // Return if we didn't modify any basic blocks. I.e., there are no return
171 // statements in the function.
172 if (!FailBB) return false;
177 /// CreateFailBB - Create a basic block to jump to when the stack protector
179 BasicBlock *StackProtector::CreateFailBB() {
180 BasicBlock *FailBB = BasicBlock::Create("CallStackCheckFailBlk", F);
181 Constant *StackChkFail =
182 M->getOrInsertFunction("__stack_chk_fail", Type::VoidTy, NULL);
183 CallInst::Create(StackChkFail, "", FailBB);
184 new UnreachableInst(FailBB);
188 /// RequiresStackProtector - Check whether or not this function needs a stack
189 /// protector based upon the stack protector level. The heuristic we use is to
190 /// add a guard variable to functions that call alloca, and functions with
191 /// buffers larger than 8 bytes.
192 bool StackProtector::RequiresStackProtector() const {
193 if (F->hasFnAttr(Attribute::StackProtectReq))
196 if (F->hasFnAttr(Attribute::StackProtect)) {
197 const TargetData *TD = TLI->getTargetData();
199 for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) {
202 for (BasicBlock::iterator
203 II = BB->begin(), IE = BB->end(); II != IE; ++II)
204 if (AllocaInst *AI = dyn_cast<AllocaInst>(II)) {
205 if (AI->isArrayAllocation())
206 // This is a call to alloca with a variable size. Emit stack
210 if (const ArrayType *AT = dyn_cast<ArrayType>(AI->getAllocatedType()))
211 // If an array has more than 8 bytes of allocated space, then we
212 // emit stack protectors.
213 if (SSPBufferSize <= TD->getABITypeSize(AT))