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llvm/lib/CodeGen/StackProtector.cpp

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//===-- StackProtector.cpp - Stack Protector Insertion --------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass inserts stack protectors into functions which need them. A variable
// with a random value in it is stored onto the stack before the local variables
// are allocated. Upon exiting the block, the stored value is checked. If it's
// changed, then there was some sort of violation and the program aborts.
//
//===----------------------------------------------------------------------===//
#include "llvm/CodeGen/StackProtector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/BranchProbabilityInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/CodeGen/Analysis.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/IR/Attributes.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GlobalValue.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/MDBuilder.h"
#include "llvm/IR/Module.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Target/TargetSubtargetInfo.h"
#include <cstdlib>
using namespace llvm;
#define DEBUG_TYPE "stack-protector"
STATISTIC(NumFunProtected, "Number of functions protected");
STATISTIC(NumAddrTaken, "Number of local variables that have their address"
" taken.");
static cl::opt<bool> EnableSelectionDAGSP("enable-selectiondag-sp",
cl::init(true), cl::Hidden);
char StackProtector::ID = 0;
INITIALIZE_PASS(StackProtector, "stack-protector", "Insert stack protectors",
false, true)
FunctionPass *llvm::createStackProtectorPass(const TargetMachine *TM) {
return new StackProtector(TM);
}
StackProtector::SSPLayoutKind
StackProtector::getSSPLayout(const AllocaInst *AI) const {
return AI ? Layout.lookup(AI) : SSPLK_None;
}
void StackProtector::adjustForColoring(const AllocaInst *From,
const AllocaInst *To) {
// When coloring replaces one alloca with another, transfer the SSPLayoutKind
// tag from the remapped to the target alloca. The remapped alloca should
// have a size smaller than or equal to the replacement alloca.
SSPLayoutMap::iterator I = Layout.find(From);
if (I != Layout.end()) {
SSPLayoutKind Kind = I->second;
Layout.erase(I);
// Transfer the tag, but make sure that SSPLK_AddrOf does not overwrite
// SSPLK_SmallArray or SSPLK_LargeArray, and make sure that
// SSPLK_SmallArray does not overwrite SSPLK_LargeArray.
I = Layout.find(To);
if (I == Layout.end())
Layout.insert(std::make_pair(To, Kind));
else if (I->second != SSPLK_LargeArray && Kind != SSPLK_AddrOf)
I->second = Kind;
}
}
bool StackProtector::runOnFunction(Function &Fn) {
F = &Fn;
M = F->getParent();
DominatorTreeWrapperPass *DTWP =
getAnalysisIfAvailable<DominatorTreeWrapperPass>();
DT = DTWP ? &DTWP->getDomTree() : nullptr;
TLI = TM->getSubtargetImpl(Fn)->getTargetLowering();
Attribute Attr = Fn.getFnAttribute("stack-protector-buffer-size");
if (Attr.isStringAttribute() &&
Attr.getValueAsString().getAsInteger(10, SSPBufferSize))
return false; // Invalid integer string
if (!RequiresStackProtector())
return false;
++NumFunProtected;
return InsertStackProtectors();
}
/// \param [out] IsLarge is set to true if a protectable array is found and
/// it is "large" ( >= ssp-buffer-size). In the case of a structure with
/// multiple arrays, this gets set if any of them is large.
bool StackProtector::ContainsProtectableArray(Type *Ty, bool &IsLarge,
bool Strong,
bool InStruct) const {
if (!Ty)
return false;
if (ArrayType *AT = dyn_cast<ArrayType>(Ty)) {
if (!AT->getElementType()->isIntegerTy(8)) {
// If we're on a non-Darwin platform or we're inside of a structure, don't
// add stack protectors unless the array is a character array.
// However, in strong mode any array, regardless of type and size,
// triggers a protector.
if (!Strong && (InStruct || !Trip.isOSDarwin()))
return false;
}
// If an array has more than SSPBufferSize bytes of allocated space, then we
// emit stack protectors.
if (SSPBufferSize <= TLI->getDataLayout()->getTypeAllocSize(AT)) {
IsLarge = true;
return true;
}
if (Strong)
// Require a protector for all arrays in strong mode
return true;
}
const StructType *ST = dyn_cast<StructType>(Ty);
if (!ST)
return false;
bool NeedsProtector = false;
for (StructType::element_iterator I = ST->element_begin(),
E = ST->element_end();
I != E; ++I)
if (ContainsProtectableArray(*I, IsLarge, Strong, true)) {
// If the element is a protectable array and is large (>= SSPBufferSize)
// then we are done. If the protectable array is not large, then
// keep looking in case a subsequent element is a large array.
if (IsLarge)
return true;
NeedsProtector = true;
}
return NeedsProtector;
}
bool StackProtector::HasAddressTaken(const Instruction *AI) {
for (const User *U : AI->users()) {
if (const StoreInst *SI = dyn_cast<StoreInst>(U)) {
if (AI == SI->getValueOperand())
return true;
} else if (const PtrToIntInst *SI = dyn_cast<PtrToIntInst>(U)) {
if (AI == SI->getOperand(0))
return true;
} else if (isa<CallInst>(U)) {
return true;
} else if (isa<InvokeInst>(U)) {
return true;
} else if (const SelectInst *SI = dyn_cast<SelectInst>(U)) {
if (HasAddressTaken(SI))
return true;
} else if (const PHINode *PN = dyn_cast<PHINode>(U)) {
// Keep track of what PHI nodes we have already visited to ensure
// they are only visited once.
if (VisitedPHIs.insert(PN).second)
if (HasAddressTaken(PN))
return true;
} else if (const GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(U)) {
if (HasAddressTaken(GEP))
return true;
} else if (const BitCastInst *BI = dyn_cast<BitCastInst>(U)) {
if (HasAddressTaken(BI))
return true;
}
}
return false;
}
/// \brief Check whether or not this function needs a stack protector based
/// upon the stack protector level.
///
/// We use two heuristics: a standard (ssp) and strong (sspstrong).
/// The standard heuristic which will add a guard variable to functions that
/// call alloca with a either a variable size or a size >= SSPBufferSize,
/// functions with character buffers larger than SSPBufferSize, and functions
/// with aggregates containing character buffers larger than SSPBufferSize. The
/// strong heuristic will add a guard variables to functions that call alloca
/// regardless of size, functions with any buffer regardless of type and size,
/// functions with aggregates that contain any buffer regardless of type and
/// size, and functions that contain stack-based variables that have had their
/// address taken.
bool StackProtector::RequiresStackProtector() {
bool Strong = false;
bool NeedsProtector = false;
if (F->hasFnAttribute(Attribute::StackProtectReq)) {
NeedsProtector = true;
Strong = true; // Use the same heuristic as strong to determine SSPLayout
} else if (F->hasFnAttribute(Attribute::StackProtectStrong))
Strong = true;
else if (!F->hasFnAttribute(Attribute::StackProtect))
return false;
for (const BasicBlock &BB : *F) {
for (const Instruction &I : BB) {
if (const AllocaInst *AI = dyn_cast<AllocaInst>(&I)) {
if (AI->isArrayAllocation()) {
// SSP-Strong: Enable protectors for any call to alloca, regardless
// of size.
if (Strong)
return true;
if (const auto *CI = dyn_cast<ConstantInt>(AI->getArraySize())) {
if (CI->getLimitedValue(SSPBufferSize) >= SSPBufferSize) {
// A call to alloca with size >= SSPBufferSize requires
// stack protectors.
Layout.insert(std::make_pair(AI, SSPLK_LargeArray));
NeedsProtector = true;
} else if (Strong) {
// Require protectors for all alloca calls in strong mode.
Layout.insert(std::make_pair(AI, SSPLK_SmallArray));
NeedsProtector = true;
}
} else {
// A call to alloca with a variable size requires protectors.
Layout.insert(std::make_pair(AI, SSPLK_LargeArray));
NeedsProtector = true;
}
continue;
}
bool IsLarge = false;
if (ContainsProtectableArray(AI->getAllocatedType(), IsLarge, Strong)) {
Layout.insert(std::make_pair(AI, IsLarge ? SSPLK_LargeArray
: SSPLK_SmallArray));
NeedsProtector = true;
continue;
}
if (Strong && HasAddressTaken(AI)) {
++NumAddrTaken;
Layout.insert(std::make_pair(AI, SSPLK_AddrOf));
NeedsProtector = true;
}
}
}
}
return NeedsProtector;
}
static bool InstructionWillNotHaveChain(const Instruction *I) {
return !I->mayHaveSideEffects() && !I->mayReadFromMemory() &&
isSafeToSpeculativelyExecute(I);
}
/// Identify if RI has a previous instruction in the "Tail Position" and return
/// it. Otherwise return 0.
///
/// This is based off of the code in llvm::isInTailCallPosition. The difference
/// is that it inverts the first part of llvm::isInTailCallPosition since
/// isInTailCallPosition is checking if a call is in a tail call position, and
/// we are searching for an unknown tail call that might be in the tail call
/// position. Once we find the call though, the code uses the same refactored
/// code, returnTypeIsEligibleForTailCall.
static CallInst *FindPotentialTailCall(BasicBlock *BB, ReturnInst *RI,
const TargetLoweringBase *TLI) {
// Establish a reasonable upper bound on the maximum amount of instructions we
// will look through to find a tail call.
unsigned SearchCounter = 0;
const unsigned MaxSearch = 4;
bool NoInterposingChain = true;
for (BasicBlock::reverse_iterator I = std::next(BB->rbegin()), E = BB->rend();
I != E && SearchCounter < MaxSearch; ++I) {
Instruction *Inst = &*I;
// Skip over debug intrinsics and do not allow them to affect our MaxSearch
// counter.
if (isa<DbgInfoIntrinsic>(Inst))
continue;
// If we find a call and the following conditions are satisifed, then we
// have found a tail call that satisfies at least the target independent
// requirements of a tail call:
//
// 1. The call site has the tail marker.
//
// 2. The call site either will not cause the creation of a chain or if a
// chain is necessary there are no instructions in between the callsite and
// the call which would create an interposing chain.
//
// 3. The return type of the function does not impede tail call
// optimization.
if (CallInst *CI = dyn_cast<CallInst>(Inst)) {
if (CI->isTailCall() &&
(InstructionWillNotHaveChain(CI) || NoInterposingChain) &&
returnTypeIsEligibleForTailCall(BB->getParent(), CI, RI, *TLI))
return CI;
}
// If we did not find a call see if we have an instruction that may create
// an interposing chain.
NoInterposingChain =
NoInterposingChain && InstructionWillNotHaveChain(Inst);
// Increment max search.
SearchCounter++;
}
return nullptr;
}
/// Insert code into the entry block that stores the __stack_chk_guard
/// variable onto the stack:
///
/// entry:
/// StackGuardSlot = alloca i8*
/// StackGuard = load __stack_chk_guard
/// call void @llvm.stackprotect.create(StackGuard, StackGuardSlot)
///
/// Returns true if the platform/triple supports the stackprotectorcreate pseudo
/// node.
static bool CreatePrologue(Function *F, Module *M, ReturnInst *RI,
const TargetLoweringBase *TLI, const Triple &TT,
AllocaInst *&AI, Value *&StackGuardVar) {
bool SupportsSelectionDAGSP = false;
PointerType *PtrTy = Type::getInt8PtrTy(RI->getContext());
unsigned AddressSpace, Offset;
if (TLI->getStackCookieLocation(AddressSpace, Offset)) {
Constant *OffsetVal =
ConstantInt::get(Type::getInt32Ty(RI->getContext()), Offset);
StackGuardVar =
ConstantExpr::getIntToPtr(OffsetVal, PointerType::get(PtrTy,
AddressSpace));
} else if (TT.isOSOpenBSD()) {
StackGuardVar = M->getOrInsertGlobal("__guard_local", PtrTy);
cast<GlobalValue>(StackGuardVar)
->setVisibility(GlobalValue::HiddenVisibility);
} else {
SupportsSelectionDAGSP = true;
StackGuardVar = M->getOrInsertGlobal("__stack_chk_guard", PtrTy);
}
IRBuilder<> B(&F->getEntryBlock().front());
AI = B.CreateAlloca(PtrTy, nullptr, "StackGuardSlot");
LoadInst *LI = B.CreateLoad(StackGuardVar, "StackGuard");
B.CreateCall(Intrinsic::getDeclaration(M, Intrinsic::stackprotector),
{LI, AI});
return SupportsSelectionDAGSP;
}
/// InsertStackProtectors - Insert code into the prologue and epilogue of the
/// function.
///
/// - The prologue code loads and stores the stack guard onto the stack.
/// - The epilogue checks the value stored in the prologue against the original
/// value. It calls __stack_chk_fail if they differ.
bool StackProtector::InsertStackProtectors() {
bool HasPrologue = false;
bool SupportsSelectionDAGSP =
EnableSelectionDAGSP && !TM->Options.EnableFastISel;
AllocaInst *AI = nullptr; // Place on stack that stores the stack guard.
Value *StackGuardVar = nullptr; // The stack guard variable.
for (Function::iterator I = F->begin(), E = F->end(); I != E;) {
BasicBlock *BB = I++;
ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator());
if (!RI)
continue;
if (!HasPrologue) {
HasPrologue = true;
SupportsSelectionDAGSP &=
CreatePrologue(F, M, RI, TLI, Trip, AI, StackGuardVar);
}
if (SupportsSelectionDAGSP) {
// Since we have a potential tail call, insert the special stack check
// intrinsic.
Instruction *InsertionPt = nullptr;
if (CallInst *CI = FindPotentialTailCall(BB, RI, TLI)) {
InsertionPt = CI;
} else {
InsertionPt = RI;
// At this point we know that BB has a return statement so it *DOES*
// have a terminator.
assert(InsertionPt != nullptr &&
"BB must have a terminator instruction at this point.");
}
Function *Intrinsic =
Intrinsic::getDeclaration(M, Intrinsic::stackprotectorcheck);
CallInst::Create(Intrinsic, StackGuardVar, "", InsertionPt);
} else {
// If we do not support SelectionDAG based tail calls, generate IR level
// tail calls.
//
// For each block with a return instruction, convert this:
//
// return:
// ...
// ret ...
//
// into this:
//
// return:
// ...
// %1 = load __stack_chk_guard
// %2 = load StackGuardSlot
// %3 = cmp i1 %1, %2
// br i1 %3, label %SP_return, label %CallStackCheckFailBlk
//
// SP_return:
// ret ...
//
// CallStackCheckFailBlk:
// call void @__stack_chk_fail()
// unreachable
// Create the FailBB. We duplicate the BB every time since the MI tail
// merge pass will merge together all of the various BB into one including
// fail BB generated by the stack protector pseudo instruction.
BasicBlock *FailBB = CreateFailBB();
// Split the basic block before the return instruction.
BasicBlock *NewBB = BB->splitBasicBlock(RI, "SP_return");
// Update the dominator tree if we need to.
if (DT && DT->isReachableFromEntry(BB)) {
DT->addNewBlock(NewBB, BB);
DT->addNewBlock(FailBB, BB);
}
// Remove default branch instruction to the new BB.
BB->getTerminator()->eraseFromParent();
// Move the newly created basic block to the point right after the old
// basic block so that it's in the "fall through" position.
NewBB->moveAfter(BB);
// Generate the stack protector instructions in the old basic block.
IRBuilder<> B(BB);
LoadInst *LI1 = B.CreateLoad(StackGuardVar);
LoadInst *LI2 = B.CreateLoad(AI);
Value *Cmp = B.CreateICmpEQ(LI1, LI2);
unsigned SuccessWeight =
BranchProbabilityInfo::getBranchWeightStackProtector(true);
unsigned FailureWeight =
BranchProbabilityInfo::getBranchWeightStackProtector(false);
MDNode *Weights = MDBuilder(F->getContext())
.createBranchWeights(SuccessWeight, FailureWeight);
B.CreateCondBr(Cmp, NewBB, FailBB, Weights);
}
}
// Return if we didn't modify any basic blocks. i.e., there are no return
// statements in the function.
if (!HasPrologue)
return false;
return true;
}
/// CreateFailBB - Create a basic block to jump to when the stack protector
/// check fails.
BasicBlock *StackProtector::CreateFailBB() {
LLVMContext &Context = F->getContext();
BasicBlock *FailBB = BasicBlock::Create(Context, "CallStackCheckFailBlk", F);
IRBuilder<> B(FailBB);
if (Trip.isOSOpenBSD()) {
Constant *StackChkFail =
M->getOrInsertFunction("__stack_smash_handler",
Type::getVoidTy(Context),
Type::getInt8PtrTy(Context), nullptr);
B.CreateCall(StackChkFail, B.CreateGlobalStringPtr(F->getName(), "SSH"));
} else {
Constant *StackChkFail =
M->getOrInsertFunction("__stack_chk_fail", Type::getVoidTy(Context),
nullptr);
B.CreateCall(StackChkFail, {});
}
B.CreateUnreachable();
return FailBB;
}
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