lib/CodeGen/StackProtector.cpp

   1 //===-- StackProtector.cpp - Stack Protector Insertion --------------------===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  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.
  14 //
  15 //===----------------------------------------------------------------------===//
  16
  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 using namespace llvm;
  32
  33 // SSPBufferSize - The lower bound for a buffer to be considered for stack
  34 // smashing protection.
  35 static cl::opt<unsigned>
  36 SSPBufferSize("stack-protector-buffer-size", cl::init(8),
  37               cl::desc("Lower bound for a buffer to be considered for "
  38                        "stack protection"));
  39
  40 namespace {
  41   class StackProtector : public FunctionPass {
  42     /// TLI - Keep a pointer of a TargetLowering to consult for determining
  43     /// target type sizes.
  44     const TargetLowering *TLI;
  45
  46     Function *F;
  47     Module *M;
  48
  49     DominatorTree* DT;
  50
  51     /// InsertStackProtectors - Insert code into the prologue and epilogue of
  52     /// the function.
  53     ///
  54     ///  - The prologue code loads and stores the stack guard onto the stack.
  55     ///  - The epilogue checks the value stored in the prologue against the
  56     ///    original value. It calls __stack_chk_fail if they differ.
  57     bool InsertStackProtectors();
  58
  59     /// CreateFailBB - Create a basic block to jump to when the stack protector
  60     /// check fails.
  61     BasicBlock *CreateFailBB();
  62
  63     /// RequiresStackProtector - Check whether or not this function needs a
  64     /// stack protector based upon the stack protector level.
  65     bool RequiresStackProtector() const;
  66   public:
  67     static char ID;             // Pass identification, replacement for typeid.
  68     StackProtector() : FunctionPass(ID), TLI(0) {
  69       initializeStackProtectorPass(*PassRegistry::getPassRegistry());
  70     }
  71     StackProtector(const TargetLowering *tli)
  72       : FunctionPass(ID), TLI(tli) {
  73         initializeStackProtectorPass(*PassRegistry::getPassRegistry());
  74       }
  75
  76     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  77       AU.addPreserved<DominatorTree>();
  78     }
  79
  80     virtual bool runOnFunction(Function &Fn);
  81   };
  82 } // end anonymous namespace
  83
  84 char StackProtector::ID = 0;
  85 INITIALIZE_PASS(StackProtector, "stack-protector",
  86                 "Insert stack protectors", false, false)
  87
  88 FunctionPass *llvm::createStackProtectorPass(const TargetLowering *tli) {
  89   return new StackProtector(tli);
  90 }
  91
  92 bool StackProtector::runOnFunction(Function &Fn) {
  93   F = &Fn;
  94   M = F->getParent();
  95   DT = getAnalysisIfAvailable<DominatorTree>();
  96
  97   if (!RequiresStackProtector()) return false;
  98
  99   return InsertStackProtectors();
 100 }
 101
 102 /// RequiresStackProtector - Check whether or not this function needs a stack
 103 /// protector based upon the stack protector level. The heuristic we use is to
 104 /// add a guard variable to functions that call alloca, and functions with
 105 /// buffers larger than SSPBufferSize bytes.
 106 bool StackProtector::RequiresStackProtector() const {
 107   if (F->hasFnAttr(Attribute::StackProtectReq))
 108     return true;
 109
 110   if (!F->hasFnAttr(Attribute::StackProtect))
 111     return false;
 112
 113   const TargetData *TD = TLI->getTargetData();
 114
 115   for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) {
 116     BasicBlock *BB = I;
 117
 118     for (BasicBlock::iterator
 119            II = BB->begin(), IE = BB->end(); II != IE; ++II)
 120       if (AllocaInst *AI = dyn_cast<AllocaInst>(II)) {
 121         if (AI->isArrayAllocation())
 122           // This is a call to alloca with a variable size. Emit stack
 123           // protectors.
 124           return true;
 125
 126         if (const ArrayType *AT = dyn_cast<ArrayType>(AI->getAllocatedType())) {
 127           // We apparently only care about character arrays.
 128           if (!AT->getElementType()->isIntegerTy(8))
 129             continue;
 130
 131           // If an array has more than SSPBufferSize bytes of allocated space,
 132           // then we emit stack protectors.
 133           if (SSPBufferSize <= TD->getTypeAllocSize(AT))
 134             return true;
 135         }
 136       }
 137   }
 138
 139   return false;
 140 }
 141
 142 /// InsertStackProtectors - Insert code into the prologue and epilogue of the
 143 /// function.
 144 ///
 145 ///  - The prologue code loads and stores the stack guard onto the stack.
 146 ///  - The epilogue checks the value stored in the prologue against the original
 147 ///    value. It calls __stack_chk_fail if they differ.
 148 bool StackProtector::InsertStackProtectors() {
 149   BasicBlock *FailBB = 0;       // The basic block to jump to if check fails.
 150   BasicBlock *FailBBDom = 0;    // FailBB's dominator.
 151   AllocaInst *AI = 0;           // Place on stack that stores the stack guard.
 152   Value *StackGuardVar = 0;  // The stack guard variable.
 153
 154   for (Function::iterator I = F->begin(), E = F->end(); I != E; ) {
 155     BasicBlock *BB = I++;
 156     ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator());
 157     if (!RI) continue;
 158
 159     if (!FailBB) {
 160       // Insert code into the entry block that stores the __stack_chk_guard
 161       // variable onto the stack:
 162       //
 163       //   entry:
 164       //     StackGuardSlot = alloca i8*
 165       //     StackGuard = load __stack_chk_guard
 166       //     call void @llvm.stackprotect.create(StackGuard, StackGuardSlot)
 167       //
 168       const PointerType *PtrTy = Type::getInt8PtrTy(RI->getContext());
 169       unsigned AddressSpace, Offset;
 170       if (TLI->getStackCookieLocation(AddressSpace, Offset)) {
 171         Constant *OffsetVal =
 172           ConstantInt::get(Type::getInt32Ty(RI->getContext()), Offset);
 173
 174         StackGuardVar = ConstantExpr::getIntToPtr(OffsetVal,
 175                                       PointerType::get(PtrTy, AddressSpace));
 176       } else {
 177         StackGuardVar = M->getOrInsertGlobal("__stack_chk_guard", PtrTy);
 178       }
 179
 180       BasicBlock &Entry = F->getEntryBlock();
 181       Instruction *InsPt = &Entry.front();
 182
 183       AI = new AllocaInst(PtrTy, "StackGuardSlot", InsPt);
 184       LoadInst *LI = new LoadInst(StackGuardVar, "StackGuard", false, InsPt);
 185
 186       Value *Args[] = { LI, AI };
 187       CallInst::
 188         Create(Intrinsic::getDeclaration(M, Intrinsic::stackprotector),
 189                &Args[0], array_endof(Args), "", InsPt);
 190
 191       // Create the basic block to jump to when the guard check fails.
 192       FailBB = CreateFailBB();
 193     }
 194
 195     // For each block with a return instruction, convert this:
 196     //
 197     //   return:
 198     //     ...
 199     //     ret ...
 200     //
 201     // into this:
 202     //
 203     //   return:
 204     //     ...
 205     //     %1 = load __stack_chk_guard
 206     //     %2 = load StackGuardSlot
 207     //     %3 = cmp i1 %1, %2
 208     //     br i1 %3, label %SP_return, label %CallStackCheckFailBlk
 209     //
 210     //   SP_return:
 211     //     ret ...
 212     //
 213     //   CallStackCheckFailBlk:
 214     //     call void @__stack_chk_fail()
 215     //     unreachable
 216
 217     // Split the basic block before the return instruction.
 218     BasicBlock *NewBB = BB->splitBasicBlock(RI, "SP_return");
 219
 220     if (DT && DT->isReachableFromEntry(BB)) {
 221       DT->addNewBlock(NewBB, BB);
 222       FailBBDom = FailBBDom ? DT->findNearestCommonDominator(FailBBDom, BB) :BB;
 223     }
 224
 225     // Remove default branch instruction to the new BB.
 226     BB->getTerminator()->eraseFromParent();
 227
 228     // Move the newly created basic block to the point right after the old basic
 229     // block so that it's in the "fall through" position.
 230     NewBB->moveAfter(BB);
 231
 232     // Generate the stack protector instructions in the old basic block.
 233     LoadInst *LI1 = new LoadInst(StackGuardVar, "", false, BB);
 234     LoadInst *LI2 = new LoadInst(AI, "", true, BB);
 235     ICmpInst *Cmp = new ICmpInst(*BB, CmpInst::ICMP_EQ, LI1, LI2, "");
 236     BranchInst::Create(NewBB, FailBB, Cmp, BB);
 237   }
 238
 239   // Return if we didn't modify any basic blocks. I.e., there are no return
 240   // statements in the function.
 241   if (!FailBB) return false;
 242
 243   if (DT && FailBBDom)
 244     DT->addNewBlock(FailBB, FailBBDom);
 245
 246   return true;
 247 }
 248
 249 /// CreateFailBB - Create a basic block to jump to when the stack protector
 250 /// check fails.
 251 BasicBlock *StackProtector::CreateFailBB() {
 252   BasicBlock *FailBB = BasicBlock::Create(F->getContext(),
 253                                           "CallStackCheckFailBlk", F);
 254   Constant *StackChkFail =
 255     M->getOrInsertFunction("__stack_chk_fail",
 256                            Type::getVoidTy(F->getContext()), NULL);
 257   CallInst::Create(StackChkFail, "", FailBB);
 258   new UnreachableInst(F->getContext(), FailBB);
 259   return FailBB;
 260 }