CrossDSOCFI.cpp

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//===-- CrossDSOCFI.cpp - Externalize this module's CFI checks ------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This pass exports all llvm.bitset's found in the module in the form of a
// __cfi_check function, which can be used to verify cross-DSO call targets.
//
//===----------------------------------------------------------------------===//
 
#include "llvm/Transforms/IPO/CrossDSOCFI.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GlobalObject.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/MDBuilder.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/ProfDataUtils.h"
#include "llvm/TargetParser/Triple.h"
#include "llvm/Transforms/IPO/LowerTypeTests.h"
 
using namespace llvm;
 
#define DEBUG_TYPE "cross-dso-cfi"
 
STATISTIC(NumTypeIds, "Number of unique type identifiers");
 
namespace {
 
struct CrossDSOCFI {
  MDNode *VeryLikelyWeights;
 
  void buildCFICheck(Module &M);
  bool runOnModule(Module &M);
};
 
} // anonymous namespace
 
/// buildCFICheck - emits __cfi_check for the current module.
void CrossDSOCFI::buildCFICheck(Module &M) {
  // FIXME: verify that __cfi_check ends up near the end of the code section,
  // but before the jump slots created in LowerTypeTests.
  SetVector<uint64_t> TypeIds = lowertypetests::findCfiTypeIds(M);
 
  LLVMContext &Ctx = M.getContext();
  FunctionCallee C = M.getOrInsertFunction(
      "__cfi_check", Type::getVoidTy(Ctx), Type::getInt64Ty(Ctx),
      PointerType::getUnqual(Ctx), PointerType::getUnqual(Ctx));
  Function *F = cast<Function>(C.getCallee());
  // Take over the existing function. The frontend emits a weak stub so that the
  // linker knows about the symbol; this pass replaces the function body.
  F->deleteBody();
  F->setAlignment(Align(4096));
  // __cfi_check is synthesized by this pass, so there is no profile data for
  // it.
  setExplicitlyUnknownFunctionEntryCount(*F, DEBUG_TYPE);
 
  Triple T(M.getTargetTriple());
  if (T.isARM() || T.isThumb())
    F->addFnAttr("target-features", "+thumb-mode");
 
  auto args = F->arg_begin();
  Value &CallSiteTypeId = *(args++);
  CallSiteTypeId.setName("CallSiteTypeId");
  Value &Addr = *(args++);
  Addr.setName("Addr");
  Value &CFICheckFailData = *(args++);
  CFICheckFailData.setName("CFICheckFailData");
  assert(args == F->arg_end());
 
  BasicBlock *BB = BasicBlock::Create(Ctx, "entry", F);
  BasicBlock *ExitBB = BasicBlock::Create(Ctx, "exit", F);
 
  BasicBlock *TrapBB = BasicBlock::Create(Ctx, "fail", F);
  IRBuilder<> IRBFail(TrapBB);
  FunctionCallee CFICheckFailFn = M.getOrInsertFunction(
      "__cfi_check_fail", Type::getVoidTy(Ctx), PointerType::getUnqual(Ctx),
      PointerType::getUnqual(Ctx));
  IRBFail.CreateCall(CFICheckFailFn, {&CFICheckFailData, &Addr});
  IRBFail.CreateBr(ExitBB);
 
  IRBuilder<> IRBExit(ExitBB);
  IRBExit.CreateRetVoid();
 
  IRBuilder<> IRB(BB);
  SwitchInst *SI = IRB.CreateSwitch(&CallSiteTypeId, TrapBB, TypeIds.size());
  // The distribution of type ids at the call sites is not known.
  setExplicitlyUnknownBranchWeights(*SI, DEBUG_TYPE);
  for (uint64_t TypeId : TypeIds) {
    ConstantInt *CaseTypeId = ConstantInt::get(Type::getInt64Ty(Ctx), TypeId);
    BasicBlock *TestBB = BasicBlock::Create(Ctx, "test", F);
    IRBuilder<> IRBTest(TestBB);
 
    Value *Test = IRBTest.CreateIntrinsic(
        Intrinsic::type_test,
        {&Addr,
         MetadataAsValue::get(Ctx, ConstantAsMetadata::get(CaseTypeId))});
    CondBrInst *BI = IRBTest.CreateCondBr(Test, ExitBB, TrapBB);
    BI->setMetadata(LLVMContext::MD_prof, VeryLikelyWeights);
 
    SI->addCase(CaseTypeId, TestBB);
    ++NumTypeIds;
  }
}
 
bool CrossDSOCFI::runOnModule(Module &M) {
  VeryLikelyWeights = MDBuilder(M.getContext()).createLikelyBranchWeights();
  if (M.getModuleFlag("Cross-DSO CFI") == nullptr)
    return false;
  buildCFICheck(M);
  return true;
}
 
PreservedAnalyses CrossDSOCFIPass::run(Module &M, ModuleAnalysisManager &AM) {
  CrossDSOCFI Impl;
  bool Changed = Impl.runOnModule(M);
  if (!Changed)
    return PreservedAnalyses::all();
  return PreservedAnalyses::none();
}