StackProtector.cpp revision 6d86f3cdfc8d750d73f4a711ec74300fcb8644cb 
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#include "llvm/ADT/Triple.h"
32using namespace llvm;
33
34// SSPBufferSize - The lower bound for a buffer to be considered for stack
35// smashing protection.
36static cl::opt<unsigned>
37SSPBufferSize("stack-protector-buffer-size", cl::init(8),
38              cl::desc("Lower bound for a buffer to be considered for "
39                       "stack protection"));
40
41namespace {
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;
46
47    Function *F;
48    Module *M;
49
50    DominatorTree *DT;
51
52    /// InsertStackProtectors - Insert code into the prologue and epilogue of
53    /// the function.
54    ///
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();
59
60    /// CreateFailBB - Create a basic block to jump to when the stack protector
61    /// check fails.
62    BasicBlock *CreateFailBB();
63
64    /// RequiresStackProtector - Check whether or not this function needs a
65    /// stack protector based upon the stack protector level.
66    bool RequiresStackProtector() const;
67  public:
68    static char ID;             // Pass identification, replacement for typeid.
69    StackProtector() : FunctionPass(ID), TLI(0) {
70      initializeStackProtectorPass(*PassRegistry::getPassRegistry());
71    }
72    StackProtector(const TargetLowering *tli)
73      : FunctionPass(ID), TLI(tli) {
74      initializeStackProtectorPass(*PassRegistry::getPassRegistry());
75    }
76
77    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
78      AU.addPreserved<DominatorTree>();
79    }
80
81    virtual bool runOnFunction(Function &Fn);
82  };
83} // end anonymous namespace
84
85char StackProtector::ID = 0;
86INITIALIZE_PASS(StackProtector, "stack-protector",
87                "Insert stack protectors", false, false)
88
89FunctionPass *llvm::createStackProtectorPass(const TargetLowering *tli) {
90  return new StackProtector(tli);
91}
92
93bool StackProtector::runOnFunction(Function &Fn) {
94  F = &Fn;
95  M = F->getParent();
96  DT = getAnalysisIfAvailable<DominatorTree>();
97
98  if (!RequiresStackProtector()) return false;
99
100  return InsertStackProtectors();
101}
102
103/// RequiresStackProtector - Check whether or not this function needs a stack
104/// protector based upon the stack protector level. The heuristic we use is to
105/// add a guard variable to functions that call alloca, and functions with
106/// buffers larger than SSPBufferSize bytes.
107bool StackProtector::RequiresStackProtector() const {
108  if (F->hasFnAttr(Attribute::StackProtectReq))
109    return true;
110
111  if (!F->hasFnAttr(Attribute::StackProtect))
112    return false;
113
114  const TargetData *TD = TLI->getTargetData();
115  const TargetMachine &TM = TLI->getTargetMachine();
116  Triple Trip(TM.getTargetTriple());
117
118  for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) {
119    BasicBlock *BB = I;
120
121    for (BasicBlock::iterator
122           II = BB->begin(), IE = BB->end(); II != IE; ++II)
123      if (AllocaInst *AI = dyn_cast<AllocaInst>(II)) {
124        if (AI->isArrayAllocation())
125          // This is a call to alloca with a variable size. Emit stack
126          // protectors.
127          return true;
128
129        if (ArrayType *AT = dyn_cast<ArrayType>(AI->getAllocatedType())) {
130          // If we're on a non-Darwin platform, don't add stack protectors
131          // unless the array is a character array.
132          if (!Trip.isOSDarwin() && !AT->getElementType()->isIntegerTy(8))
133            continue;
134
135          // If an array has more than SSPBufferSize bytes of allocated space,
136          // then we emit stack protectors.
137          if (SSPBufferSize <= TD->getTypeAllocSize(AT))
138            return true;
139        }
140      }
141  }
142
143  return false;
144}
145
146/// InsertStackProtectors - Insert code into the prologue and epilogue of the
147/// function.
148///
149///  - The prologue code loads and stores the stack guard onto the stack.
150///  - The epilogue checks the value stored in the prologue against the original
151///    value. It calls __stack_chk_fail if they differ.
152bool StackProtector::InsertStackProtectors() {
153  BasicBlock *FailBB = 0;       // The basic block to jump to if check fails.
154  BasicBlock *FailBBDom = 0;    // FailBB's dominator.
155  AllocaInst *AI = 0;           // Place on stack that stores the stack guard.
156  Value *StackGuardVar = 0;  // The stack guard variable.
157
158  for (Function::iterator I = F->begin(), E = F->end(); I != E; ) {
159    BasicBlock *BB = I++;
160    ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator());
161    if (!RI) continue;
162
163    if (!FailBB) {
164      // Insert code into the entry block that stores the __stack_chk_guard
165      // variable onto the stack:
166      //
167      //   entry:
168      //     StackGuardSlot = alloca i8*
169      //     StackGuard = load __stack_chk_guard
170      //     call void @llvm.stackprotect.create(StackGuard, StackGuardSlot)
171      //
172      PointerType *PtrTy = Type::getInt8PtrTy(RI->getContext());
173      unsigned AddressSpace, Offset;
174      if (TLI->getStackCookieLocation(AddressSpace, Offset)) {
175        Constant *OffsetVal =
176          ConstantInt::get(Type::getInt32Ty(RI->getContext()), Offset);
177
178        StackGuardVar = ConstantExpr::getIntToPtr(OffsetVal,
179                                      PointerType::get(PtrTy, AddressSpace));
180      } else {
181        StackGuardVar = M->getOrInsertGlobal("__stack_chk_guard", PtrTy);
182      }
183
184      BasicBlock &Entry = F->getEntryBlock();
185      Instruction *InsPt = &Entry.front();
186
187      AI = new AllocaInst(PtrTy, "StackGuardSlot", InsPt);
188      LoadInst *LI = new LoadInst(StackGuardVar, "StackGuard", false, InsPt);
189
190      Value *Args[] = { LI, AI };
191      CallInst::
192        Create(Intrinsic::getDeclaration(M, Intrinsic::stackprotector),
193               Args, "", InsPt);
194
195      // Create the basic block to jump to when the guard check fails.
196      FailBB = CreateFailBB();
197    }
198
199    // For each block with a return instruction, convert this:
200    //
201    //   return:
202    //     ...
203    //     ret ...
204    //
205    // into this:
206    //
207    //   return:
208    //     ...
209    //     %1 = load __stack_chk_guard
210    //     %2 = load StackGuardSlot
211    //     %3 = cmp i1 %1, %2
212    //     br i1 %3, label %SP_return, label %CallStackCheckFailBlk
213    //
214    //   SP_return:
215    //     ret ...
216    //
217    //   CallStackCheckFailBlk:
218    //     call void @__stack_chk_fail()
219    //     unreachable
220
221    // Split the basic block before the return instruction.
222    BasicBlock *NewBB = BB->splitBasicBlock(RI, "SP_return");
223
224    if (DT && DT->isReachableFromEntry(BB)) {
225      DT->addNewBlock(NewBB, BB);
226      FailBBDom = FailBBDom ? DT->findNearestCommonDominator(FailBBDom, BB) :BB;
227    }
228
229    // Remove default branch instruction to the new BB.
230    BB->getTerminator()->eraseFromParent();
231
232    // Move the newly created basic block to the point right after the old basic
233    // block so that it's in the "fall through" position.
234    NewBB->moveAfter(BB);
235
236    // Generate the stack protector instructions in the old basic block.
237    LoadInst *LI1 = new LoadInst(StackGuardVar, "", false, BB);
238    LoadInst *LI2 = new LoadInst(AI, "", true, BB);
239    ICmpInst *Cmp = new ICmpInst(*BB, CmpInst::ICMP_EQ, LI1, LI2, "");
240    BranchInst::Create(NewBB, FailBB, Cmp, BB);
241  }
242
243  // Return if we didn't modify any basic blocks. I.e., there are no return
244  // statements in the function.
245  if (!FailBB) return false;
246
247  if (DT && FailBBDom)
248    DT->addNewBlock(FailBB, FailBBDom);
249
250  return true;
251}
252
253/// CreateFailBB - Create a basic block to jump to when the stack protector
254/// check fails.
255BasicBlock *StackProtector::CreateFailBB() {
256  BasicBlock *FailBB = BasicBlock::Create(F->getContext(),
257                                          "CallStackCheckFailBlk", F);
258  Constant *StackChkFail =
259    M->getOrInsertFunction("__stack_chk_fail",
260                           Type::getVoidTy(F->getContext()), NULL);
261  CallInst::Create(StackChkFail, "", FailBB);
262  new UnreachableInst(F->getContext(), FailBB);
263  return FailBB;
264}
265