StackProtector.cpp revision 80f6a507d4e11ba066ad0e53e12ad25ad8cf07ba 
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"
31using namespace llvm;
32
33// SSPBufferSize - The lower bound for a buffer to be considered for stack
34// smashing protection.
35static cl::opt<unsigned>
36SSPBufferSize("stack-protector-buffer-size", cl::init(8),
37              cl::desc("Lower bound for a buffer to be considered for "
38                       "stack protection"));
39
40namespace {
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
84char StackProtector::ID = 0;
85INITIALIZE_PASS(StackProtector, "stack-protector",
86                "Insert stack protectors", false, false)
87
88FunctionPass *llvm::createStackProtectorPass(const TargetLowering *tli) {
89  return new StackProtector(tli);
90}
91
92bool 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.
106bool 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.
148bool 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
157    ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator());
158    if (!RI) continue;
159
160    if (!FailBB) {
161      // Insert code into the entry block that stores the __stack_chk_guard
162      // variable onto the stack:
163      //
164      //   entry:
165      //     StackGuardSlot = alloca i8*
166      //     StackGuard = load __stack_chk_guard
167      //     call void @llvm.stackprotect.create(StackGuard, StackGuardSlot)
168      //
169      const PointerType *PtrTy = Type::getInt8PtrTy(RI->getContext());
170      unsigned AddressSpace, Offset;
171      if (TLI->getStackCookieLocation(AddressSpace, Offset)) {
172        Constant *OffsetVal =
173          ConstantInt::get(Type::getInt32Ty(RI->getContext()), Offset);
174
175        StackGuardVar = ConstantExpr::getIntToPtr(OffsetVal,
176                                      PointerType::get(PtrTy, AddressSpace));
177      } else {
178        StackGuardVar = M->getOrInsertGlobal("__stack_chk_guard", PtrTy);
179      }
180
181      BasicBlock &Entry = F->getEntryBlock();
182      Instruction *InsPt = &Entry.front();
183
184      AI = new AllocaInst(PtrTy, "StackGuardSlot", InsPt);
185      LoadInst *LI = new LoadInst(StackGuardVar, "StackGuard", false, InsPt);
186
187      Value *Args[] = { LI, AI };
188      CallInst::
189        Create(Intrinsic::getDeclaration(M, Intrinsic::stackprotector),
190               &Args[0], array_endof(Args), "", InsPt);
191
192      // Create the basic block to jump to when the guard check fails.
193      FailBB = CreateFailBB();
194      if (DT)
195        FailBBDom = DT->isReachableFromEntry(BB) ? BB : 0;
196    }
197
198    // For each block with a return instruction, convert this:
199    //
200    //   return:
201    //     ...
202    //     ret ...
203    //
204    // into this:
205    //
206    //   return:
207    //     ...
208    //     %1 = load __stack_chk_guard
209    //     %2 = load StackGuardSlot
210    //     %3 = cmp i1 %1, %2
211    //     br i1 %3, label %SP_return, label %CallStackCheckFailBlk
212    //
213    //   SP_return:
214    //     ret ...
215    //
216    //   CallStackCheckFailBlk:
217    //     call void @__stack_chk_fail()
218    //     unreachable
219
220    // Split the basic block before the return instruction.
221    BasicBlock *NewBB = BB->splitBasicBlock(RI, "SP_return");
222    if (DT) {
223      DT->addNewBlock(NewBB, DT->isReachableFromEntry(BB) ? BB : 0);
224      FailBBDom = DT->findNearestCommonDominator(FailBBDom, BB);
225    }
226
227    // Remove default branch instruction to the new BB.
228    BB->getTerminator()->eraseFromParent();
229
230    // Move the newly created basic block to the point right after the old basic
231    // block so that it's in the "fall through" position.
232    NewBB->moveAfter(BB);
233
234    // Generate the stack protector instructions in the old basic block.
235    LoadInst *LI1 = new LoadInst(StackGuardVar, "", false, BB);
236    LoadInst *LI2 = new LoadInst(AI, "", true, BB);
237    ICmpInst *Cmp = new ICmpInst(*BB, CmpInst::ICMP_EQ, LI1, LI2, "");
238    BranchInst::Create(NewBB, FailBB, Cmp, BB);
239  }
240
241  // Return if we didn't modify any basic blocks. I.e., there are no return
242  // statements in the function.
243  if (!FailBB) return false;
244
245  if (DT)
246    DT->addNewBlock(FailBB, FailBBDom);
247
248  return true;
249}
250
251/// CreateFailBB - Create a basic block to jump to when the stack protector
252/// check fails.
253BasicBlock *StackProtector::CreateFailBB() {
254  BasicBlock *FailBB = BasicBlock::Create(F->getContext(),
255                                          "CallStackCheckFailBlk", F);
256  Constant *StackChkFail =
257    M->getOrInsertFunction("__stack_chk_fail",
258                           Type::getVoidTy(F->getContext()), NULL);
259  CallInst::Create(StackChkFail, "", FailBB);
260  new UnreachableInst(F->getContext(), FailBB);
261  return FailBB;
262}
263