StackProtector.cpp revision d13db2c59cc94162d6cf0a04187d408bfef6d4a7 
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/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"
30using namespace llvm;
31
32// SSPBufferSize - The lower bound for a buffer to be considered for stack
33// smashing protection.
34static cl::opt<unsigned>
35SSPBufferSize("stack-protector-buffer-size", cl::init(8),
36              cl::desc("Lower bound for a buffer to be considered for "
37                       "stack protection"));
38
39namespace {
40  class StackProtector : public FunctionPass {
41    /// TLI - Keep a pointer of a TargetLowering to consult for determining
42    /// target type sizes.
43    const TargetLowering *TLI;
44
45    Function *F;
46    Module *M;
47
48    /// InsertStackProtectors - Insert code into the prologue and epilogue of
49    /// the function.
50    ///
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();
55
56    /// CreateFailBB - Create a basic block to jump to when the stack protector
57    /// check fails.
58    BasicBlock *CreateFailBB();
59
60    /// RequiresStackProtector - Check whether or not this function needs a
61    /// stack protector based upon the stack protector level.
62    bool RequiresStackProtector() const;
63  public:
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) {}
68
69    virtual bool runOnFunction(Function &Fn);
70  };
71} // end anonymous namespace
72
73char StackProtector::ID = 0;
74INITIALIZE_PASS(StackProtector, "stack-protector",
75                "Insert stack protectors", false, false);
76
77FunctionPass *llvm::createStackProtectorPass(const TargetLowering *tli) {
78  return new StackProtector(tli);
79}
80
81bool StackProtector::runOnFunction(Function &Fn) {
82  F = &Fn;
83  M = F->getParent();
84
85  if (!RequiresStackProtector()) return false;
86
87  return InsertStackProtectors();
88}
89
90/// RequiresStackProtector - Check whether or not this function needs a stack
91/// protector based upon the stack protector level. The heuristic we use is to
92/// add a guard variable to functions that call alloca, and functions with
93/// buffers larger than SSPBufferSize bytes.
94bool StackProtector::RequiresStackProtector() const {
95  if (F->hasFnAttr(Attribute::StackProtectReq))
96    return true;
97
98  if (!F->hasFnAttr(Attribute::StackProtect))
99    return false;
100
101  const TargetData *TD = TLI->getTargetData();
102
103  for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) {
104    BasicBlock *BB = I;
105
106    for (BasicBlock::iterator
107           II = BB->begin(), IE = BB->end(); II != IE; ++II)
108      if (AllocaInst *AI = dyn_cast<AllocaInst>(II)) {
109        if (AI->isArrayAllocation())
110          // This is a call to alloca with a variable size. Emit stack
111          // protectors.
112          return true;
113
114        if (const ArrayType *AT = dyn_cast<ArrayType>(AI->getAllocatedType())) {
115          // We apparently only care about character arrays.
116          if (!AT->getElementType()->isIntegerTy(8))
117            continue;
118
119          // If an array has more than SSPBufferSize bytes of allocated space,
120          // then we emit stack protectors.
121          if (SSPBufferSize <= TD->getTypeAllocSize(AT))
122            return true;
123        }
124      }
125  }
126
127  return false;
128}
129
130/// InsertStackProtectors - Insert code into the prologue and epilogue of the
131/// function.
132///
133///  - The prologue code loads and stores the stack guard onto the stack.
134///  - The epilogue checks the value stored in the prologue against the original
135///    value. It calls __stack_chk_fail if they differ.
136bool StackProtector::InsertStackProtectors() {
137  BasicBlock *FailBB = 0;       // The basic block to jump to if check fails.
138  AllocaInst *AI = 0;           // Place on stack that stores the stack guard.
139  Value *StackGuardVar = 0;  // The stack guard variable.
140
141  for (Function::iterator I = F->begin(), E = F->end(); I != E; ) {
142    BasicBlock *BB = I++;
143
144    ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator());
145    if (!RI) continue;
146
147    if (!FailBB) {
148      // Insert code into the entry block that stores the __stack_chk_guard
149      // variable onto the stack:
150      //
151      //   entry:
152      //     StackGuardSlot = alloca i8*
153      //     StackGuard = load __stack_chk_guard
154      //     call void @llvm.stackprotect.create(StackGuard, StackGuardSlot)
155      //
156      const PointerType *PtrTy = Type::getInt8PtrTy(RI->getContext());
157      unsigned AddressSpace, Offset;
158      if (TLI->getStackCookieLocation(AddressSpace, Offset)) {
159        Constant *OffsetVal =
160          ConstantInt::get(Type::getInt32Ty(RI->getContext()), Offset);
161
162        StackGuardVar = ConstantExpr::getIntToPtr(OffsetVal,
163                                      PointerType::get(PtrTy, AddressSpace));
164      } else {
165        StackGuardVar = M->getOrInsertGlobal("__stack_chk_guard", PtrTy);
166      }
167
168      BasicBlock &Entry = F->getEntryBlock();
169      Instruction *InsPt = &Entry.front();
170
171      AI = new AllocaInst(PtrTy, "StackGuardSlot", InsPt);
172      LoadInst *LI = new LoadInst(StackGuardVar, "StackGuard", false, InsPt);
173
174      Value *Args[] = { LI, AI };
175      CallInst::
176        Create(Intrinsic::getDeclaration(M, Intrinsic::stackprotector),
177               &Args[0], array_endof(Args), "", InsPt);
178
179      // Create the basic block to jump to when the guard check fails.
180      FailBB = CreateFailBB();
181    }
182
183    // For each block with a return instruction, convert this:
184    //
185    //   return:
186    //     ...
187    //     ret ...
188    //
189    // into this:
190    //
191    //   return:
192    //     ...
193    //     %1 = load __stack_chk_guard
194    //     %2 = load StackGuardSlot
195    //     %3 = cmp i1 %1, %2
196    //     br i1 %3, label %SP_return, label %CallStackCheckFailBlk
197    //
198    //   SP_return:
199    //     ret ...
200    //
201    //   CallStackCheckFailBlk:
202    //     call void @__stack_chk_fail()
203    //     unreachable
204
205    // Split the basic block before the return instruction.
206    BasicBlock *NewBB = BB->splitBasicBlock(RI, "SP_return");
207
208    // Remove default branch instruction to the new BB.
209    BB->getTerminator()->eraseFromParent();
210
211    // Move the newly created basic block to the point right after the old basic
212    // block so that it's in the "fall through" position.
213    NewBB->moveAfter(BB);
214
215    // Generate the stack protector instructions in the old basic block.
216    LoadInst *LI1 = new LoadInst(StackGuardVar, "", false, BB);
217    LoadInst *LI2 = new LoadInst(AI, "", true, BB);
218    ICmpInst *Cmp = new ICmpInst(*BB, CmpInst::ICMP_EQ, LI1, LI2, "");
219    BranchInst::Create(NewBB, FailBB, Cmp, BB);
220  }
221
222  // Return if we didn't modify any basic blocks. I.e., there are no return
223  // statements in the function.
224  if (!FailBB) return false;
225
226  return true;
227}
228
229/// CreateFailBB - Create a basic block to jump to when the stack protector
230/// check fails.
231BasicBlock *StackProtector::CreateFailBB() {
232  BasicBlock *FailBB = BasicBlock::Create(F->getContext(),
233                                          "CallStackCheckFailBlk", F);
234  Constant *StackChkFail =
235    M->getOrInsertFunction("__stack_chk_fail",
236                           Type::getVoidTy(F->getContext()), NULL);
237  CallInst::Create(StackChkFail, "", FailBB);
238  new UnreachableInst(F->getContext(), FailBB);
239  return FailBB;
240}
241