StackProtector.cpp revision d04a8d4b33ff316ca4cf961e06c9e312eff8e64f 
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/ADT/Triple.h"
20#include "llvm/Analysis/Dominators.h"
21#include "llvm/Attributes.h"
22#include "llvm/Constants.h"
23#include "llvm/DataLayout.h"
24#include "llvm/DerivedTypes.h"
25#include "llvm/Function.h"
26#include "llvm/Instructions.h"
27#include "llvm/Intrinsics.h"
28#include "llvm/Module.h"
29#include "llvm/Pass.h"
30#include "llvm/Support/CommandLine.h"
31#include "llvm/Target/TargetLowering.h"
32#include "llvm/Target/TargetOptions.h"
33using namespace llvm;
34
35namespace {
36  class StackProtector : public FunctionPass {
37    /// TLI - Keep a pointer of a TargetLowering to consult for determining
38    /// target type sizes.
39    const TargetLowering *TLI;
40
41    Function *F;
42    Module *M;
43
44    DominatorTree *DT;
45
46    /// InsertStackProtectors - Insert code into the prologue and epilogue of
47    /// the function.
48    ///
49    ///  - The prologue code loads and stores the stack guard onto the stack.
50    ///  - The epilogue checks the value stored in the prologue against the
51    ///    original value. It calls __stack_chk_fail if they differ.
52    bool InsertStackProtectors();
53
54    /// CreateFailBB - Create a basic block to jump to when the stack protector
55    /// check fails.
56    BasicBlock *CreateFailBB();
57
58    /// ContainsProtectableArray - Check whether the type either is an array or
59    /// contains an array of sufficient size so that we need stack protectors
60    /// for it.
61    bool ContainsProtectableArray(Type *Ty, bool InStruct = false) const;
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/// ContainsProtectableArray - Check whether the type either is an array or
103/// contains a char array of sufficient size so that we need stack protectors
104/// for it.
105bool StackProtector::ContainsProtectableArray(Type *Ty, bool InStruct) const {
106  if (!Ty) return false;
107  if (ArrayType *AT = dyn_cast<ArrayType>(Ty)) {
108    const TargetMachine &TM = TLI->getTargetMachine();
109    if (!AT->getElementType()->isIntegerTy(8)) {
110      Triple Trip(TM.getTargetTriple());
111
112      // If we're on a non-Darwin platform or we're inside of a structure, don't
113      // add stack protectors unless the array is a character array.
114      if (InStruct || !Trip.isOSDarwin())
115          return false;
116    }
117
118    // If an array has more than SSPBufferSize bytes of allocated space, then we
119    // emit stack protectors.
120    if (TM.Options.SSPBufferSize <= TLI->getDataLayout()->getTypeAllocSize(AT))
121      return true;
122  }
123
124  const StructType *ST = dyn_cast<StructType>(Ty);
125  if (!ST) return false;
126
127  for (StructType::element_iterator I = ST->element_begin(),
128         E = ST->element_end(); I != E; ++I)
129    if (ContainsProtectableArray(*I, true))
130      return true;
131
132  return false;
133}
134
135/// RequiresStackProtector - Check whether or not this function needs a stack
136/// protector based upon the stack protector level. The heuristic we use is to
137/// add a guard variable to functions that call alloca, and functions with
138/// buffers larger than SSPBufferSize bytes.
139bool StackProtector::RequiresStackProtector() const {
140  if (F->getFnAttributes().hasAttribute(Attributes::StackProtectReq))
141    return true;
142
143  if (!F->getFnAttributes().hasAttribute(Attributes::StackProtect))
144    return false;
145
146  for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) {
147    BasicBlock *BB = I;
148
149    for (BasicBlock::iterator
150           II = BB->begin(), IE = BB->end(); II != IE; ++II)
151      if (AllocaInst *AI = dyn_cast<AllocaInst>(II)) {
152        if (AI->isArrayAllocation())
153          // This is a call to alloca with a variable size. Emit stack
154          // protectors.
155          return true;
156
157        if (ContainsProtectableArray(AI->getAllocatedType()))
158          return true;
159      }
160  }
161
162  return false;
163}
164
165/// InsertStackProtectors - Insert code into the prologue and epilogue of the
166/// function.
167///
168///  - The prologue code loads and stores the stack guard onto the stack.
169///  - The epilogue checks the value stored in the prologue against the original
170///    value. It calls __stack_chk_fail if they differ.
171bool StackProtector::InsertStackProtectors() {
172  BasicBlock *FailBB = 0;       // The basic block to jump to if check fails.
173  BasicBlock *FailBBDom = 0;    // FailBB's dominator.
174  AllocaInst *AI = 0;           // Place on stack that stores the stack guard.
175  Value *StackGuardVar = 0;  // The stack guard variable.
176
177  for (Function::iterator I = F->begin(), E = F->end(); I != E; ) {
178    BasicBlock *BB = I++;
179    ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator());
180    if (!RI) continue;
181
182    if (!FailBB) {
183      // Insert code into the entry block that stores the __stack_chk_guard
184      // variable onto the stack:
185      //
186      //   entry:
187      //     StackGuardSlot = alloca i8*
188      //     StackGuard = load __stack_chk_guard
189      //     call void @llvm.stackprotect.create(StackGuard, StackGuardSlot)
190      //
191      PointerType *PtrTy = Type::getInt8PtrTy(RI->getContext());
192      unsigned AddressSpace, Offset;
193      if (TLI->getStackCookieLocation(AddressSpace, Offset)) {
194        Constant *OffsetVal =
195          ConstantInt::get(Type::getInt32Ty(RI->getContext()), Offset);
196
197        StackGuardVar = ConstantExpr::getIntToPtr(OffsetVal,
198                                      PointerType::get(PtrTy, AddressSpace));
199      } else {
200        StackGuardVar = M->getOrInsertGlobal("__stack_chk_guard", PtrTy);
201      }
202
203      BasicBlock &Entry = F->getEntryBlock();
204      Instruction *InsPt = &Entry.front();
205
206      AI = new AllocaInst(PtrTy, "StackGuardSlot", InsPt);
207      LoadInst *LI = new LoadInst(StackGuardVar, "StackGuard", false, InsPt);
208
209      Value *Args[] = { LI, AI };
210      CallInst::
211        Create(Intrinsic::getDeclaration(M, Intrinsic::stackprotector),
212               Args, "", InsPt);
213
214      // Create the basic block to jump to when the guard check fails.
215      FailBB = CreateFailBB();
216    }
217
218    // For each block with a return instruction, convert this:
219    //
220    //   return:
221    //     ...
222    //     ret ...
223    //
224    // into this:
225    //
226    //   return:
227    //     ...
228    //     %1 = load __stack_chk_guard
229    //     %2 = load StackGuardSlot
230    //     %3 = cmp i1 %1, %2
231    //     br i1 %3, label %SP_return, label %CallStackCheckFailBlk
232    //
233    //   SP_return:
234    //     ret ...
235    //
236    //   CallStackCheckFailBlk:
237    //     call void @__stack_chk_fail()
238    //     unreachable
239
240    // Split the basic block before the return instruction.
241    BasicBlock *NewBB = BB->splitBasicBlock(RI, "SP_return");
242
243    if (DT && DT->isReachableFromEntry(BB)) {
244      DT->addNewBlock(NewBB, BB);
245      FailBBDom = FailBBDom ? DT->findNearestCommonDominator(FailBBDom, BB) :BB;
246    }
247
248    // Remove default branch instruction to the new BB.
249    BB->getTerminator()->eraseFromParent();
250
251    // Move the newly created basic block to the point right after the old basic
252    // block so that it's in the "fall through" position.
253    NewBB->moveAfter(BB);
254
255    // Generate the stack protector instructions in the old basic block.
256    LoadInst *LI1 = new LoadInst(StackGuardVar, "", false, BB);
257    LoadInst *LI2 = new LoadInst(AI, "", true, BB);
258    ICmpInst *Cmp = new ICmpInst(*BB, CmpInst::ICMP_EQ, LI1, LI2, "");
259    BranchInst::Create(NewBB, FailBB, Cmp, BB);
260  }
261
262  // Return if we didn't modify any basic blocks. I.e., there are no return
263  // statements in the function.
264  if (!FailBB) return false;
265
266  if (DT && FailBBDom)
267    DT->addNewBlock(FailBB, FailBBDom);
268
269  return true;
270}
271
272/// CreateFailBB - Create a basic block to jump to when the stack protector
273/// check fails.
274BasicBlock *StackProtector::CreateFailBB() {
275  BasicBlock *FailBB = BasicBlock::Create(F->getContext(),
276                                          "CallStackCheckFailBlk", F);
277  Constant *StackChkFail =
278    M->getOrInsertFunction("__stack_chk_fail",
279                           Type::getVoidTy(F->getContext()), NULL);
280  CallInst::Create(StackChkFail, "", FailBB);
281  new UnreachableInst(F->getContext(), FailBB);
282  return FailBB;
283}
284