BPFCheckAndAdjustIR.cpp

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//===------------ BPFCheckAndAdjustIR.cpp - Check and Adjust IR -----------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// Check IR and adjust IR for verifier friendly codes.
// The following are done for IR checking:
//   - no relocation globals in PHI node.
// The following are done for IR adjustment:
//   - remove __builtin_bpf_passthrough builtins. Target independent IR
//     optimizations are done and those builtins can be removed.
//
//===----------------------------------------------------------------------===//
 
#include "BPF.h"
#include "BPFCORE.h"
#include "BPFTargetMachine.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/IR/DebugInfoMetadata.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/Pass.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
 
#define DEBUG_TYPE "bpf-check-and-opt-ir"
 
using namespace llvm;
 
namespace {
 
class BPFCheckAndAdjustIR final : public ModulePass {
  bool runOnModule(Module &F) override;
 
public:
  static char ID;
  BPFCheckAndAdjustIR() : ModulePass(ID) {}
  virtual void getAnalysisUsage(AnalysisUsage &AU) const override;
 
private:
  void checkIR(Module &M);
  bool adjustIR(Module &M);
  bool removePassThroughBuiltin(Module &M);
  bool removeCompareBuiltin(Module &M);
  bool sinkMinMax(Module &M);
};
} // End anonymous namespace
 
char BPFCheckAndAdjustIR::ID = 0;
INITIALIZE_PASS(BPFCheckAndAdjustIR, DEBUG_TYPE, "BPF Check And Adjust IR",
                false, false)
 
ModulePass *llvm::createBPFCheckAndAdjustIR() {
  return new BPFCheckAndAdjustIR();
}
 
void BPFCheckAndAdjustIR::checkIR(Module &M) {
  // Ensure relocation global won't appear in PHI node
  // This may happen if the compiler generated the following code:
  //   B1:
  //      g1 = @llvm.skb_buff:0:1...
  //      ...
  //      goto B_COMMON
  //   B2:
  //      g2 = @llvm.skb_buff:0:2...
  //      ...
  //      goto B_COMMON
  //   B_COMMON:
  //      g = PHI(g1, g2)
  //      x = load g
  //      ...
  // If anything likes the above "g = PHI(g1, g2)", issue a fatal error.
  for (Function &F : M)
    for (auto &BB : F)
      for (auto &I : BB) {
        PHINode *PN = dyn_cast<PHINode>(&I);
        if (!PN || PN->use_empty())
          continue;
        for (int i = 0, e = PN->getNumIncomingValues(); i < e; ++i) {
          auto *GV = dyn_cast<GlobalVariable>(PN->getIncomingValue(i));
          if (!GV)
            continue;
          if (GV->hasAttribute(BPFCoreSharedInfo::AmaAttr) ||
              GV->hasAttribute(BPFCoreSharedInfo::TypeIdAttr))
            report_fatal_error("relocation global in PHI node");
        }
      }
}
 
bool BPFCheckAndAdjustIR::removePassThroughBuiltin(Module &M) {
  // Remove __builtin_bpf_passthrough()'s which are used to prevent
  // certain IR optimizations. Now major IR optimizations are done,
  // remove them.
  bool Changed = false;
  CallInst *ToBeDeleted = nullptr;
  for (Function &F : M)
    for (auto &BB : F)
      for (auto &I : BB) {
        if (ToBeDeleted) {
          ToBeDeleted->eraseFromParent();
          ToBeDeleted = nullptr;
        }
 
        auto *Call = dyn_cast<CallInst>(&I);
        if (!Call)
          continue;
        auto *GV = dyn_cast<GlobalValue>(Call->getCalledOperand());
        if (!GV)
          continue;
        if (!GV->getName().startswith("llvm.bpf.passthrough"))
          continue;
        Changed = true;
        Value *Arg = Call->getArgOperand(1);
        Call->replaceAllUsesWith(Arg);
        ToBeDeleted = Call;
      }
  return Changed;
}
 
bool BPFCheckAndAdjustIR::removeCompareBuiltin(Module &M) {
  // Remove __builtin_bpf_compare()'s which are used to prevent
  // certain IR optimizations. Now major IR optimizations are done,
  // remove them.
  bool Changed = false;
  CallInst *ToBeDeleted = nullptr;
  for (Function &F : M)
    for (auto &BB : F)
      for (auto &I : BB) {
        if (ToBeDeleted) {
          ToBeDeleted->eraseFromParent();
          ToBeDeleted = nullptr;
        }
 
        auto *Call = dyn_cast<CallInst>(&I);
        if (!Call)
          continue;
        auto *GV = dyn_cast<GlobalValue>(Call->getCalledOperand());
        if (!GV)
          continue;
        if (!GV->getName().startswith("llvm.bpf.compare"))
          continue;
 
        Changed = true;
        Value *Arg0 = Call->getArgOperand(0);
        Value *Arg1 = Call->getArgOperand(1);
        Value *Arg2 = Call->getArgOperand(2);
 
        auto OpVal = cast<ConstantInt>(Arg0)->getValue().getZExtValue();
        CmpInst::Predicate Opcode = (CmpInst::Predicate)OpVal;
 
        auto *ICmp = new ICmpInst(Opcode, Arg1, Arg2);
        ICmp->insertBefore(Call);
 
        Call->replaceAllUsesWith(ICmp);
        ToBeDeleted = Call;
      }
  return Changed;
}
 
struct MinMaxSinkInfo {
  ICmpInst *ICmp;
  Value *Other;
  ICmpInst::Predicate Predicate;
  CallInst *MinMax;
  ZExtInst *ZExt;
  SExtInst *SExt;
 
  MinMaxSinkInfo(ICmpInst *ICmp, Value *Other, ICmpInst::Predicate Predicate)
      : ICmp(ICmp), Other(Other), Predicate(Predicate), MinMax(nullptr),
        ZExt(nullptr), SExt(nullptr) {}
};
 
static bool sinkMinMaxInBB(BasicBlock &BB,
                           const std::function<bool(Instruction *)> &Filter) {
  // Check if V is:
  //   (fn %a %b) or (ext (fn %a %b))
  // Where:
  //   ext := sext | zext
  //   fn  := smin | umin | smax | umax
  auto IsMinMaxCall = [=](Value *V, MinMaxSinkInfo &Info) {
    if (auto *ZExt = dyn_cast<ZExtInst>(V)) {
      V = ZExt->getOperand(0);
      Info.ZExt = ZExt;
    } else if (auto *SExt = dyn_cast<SExtInst>(V)) {
      V = SExt->getOperand(0);
      Info.SExt = SExt;
    }
 
    auto *Call = dyn_cast<CallInst>(V);
    if (!Call)
      return false;
 
    auto *Called = dyn_cast<Function>(Call->getCalledOperand());
    if (!Called)
      return false;
 
    switch (Called->getIntrinsicID()) {
    case Intrinsic::smin:
    case Intrinsic::umin:
    case Intrinsic::smax:
    case Intrinsic::umax:
      break;
    default:
      return false;
    }
 
    if (!Filter(Call))
      return false;
 
    Info.MinMax = Call;
 
    return true;
  };
 
  auto ZeroOrSignExtend = [](IRBuilder<> &Builder, Value *V,
                             MinMaxSinkInfo &Info) {
    if (Info.SExt) {
      if (Info.SExt->getType() == V->getType())
        return V;
      return Builder.CreateSExt(V, Info.SExt->getType());
    }
    if (Info.ZExt) {
      if (Info.ZExt->getType() == V->getType())
        return V;
      return Builder.CreateZExt(V, Info.ZExt->getType());
    }
    return V;
  };
 
  bool Changed = false;
  SmallVector<MinMaxSinkInfo, 2> SinkList;
 
  // Check BB for instructions like:
  //   insn := (icmp %a (fn ...)) | (icmp (fn ...)  %a)
  //
  // Where:
  //   fn := min | max | (sext (min ...)) | (sext (max ...))
  //
  // Put such instructions to SinkList.
  for (Instruction &I : BB) {
    ICmpInst *ICmp = dyn_cast<ICmpInst>(&I);
    if (!ICmp)
      continue;
    if (!ICmp->isRelational())
      continue;
    MinMaxSinkInfo First(ICmp, ICmp->getOperand(1),
                         ICmpInst::getSwappedPredicate(ICmp->getPredicate()));
    MinMaxSinkInfo Second(ICmp, ICmp->getOperand(0), ICmp->getPredicate());
    bool FirstMinMax = IsMinMaxCall(ICmp->getOperand(0), First);
    bool SecondMinMax = IsMinMaxCall(ICmp->getOperand(1), Second);
    if (!(FirstMinMax ^ SecondMinMax))
      continue;
    SinkList.push_back(FirstMinMax ? First : Second);
  }
 
  // Iterate SinkList and replace each (icmp ...) with corresponding
  // `x < a && x < b` or similar expression.
  for (auto &Info : SinkList) {
    ICmpInst *ICmp = Info.ICmp;
    CallInst *MinMax = Info.MinMax;
    Intrinsic::ID IID = MinMax->getCalledFunction()->getIntrinsicID();
    ICmpInst::Predicate P = Info.Predicate;
    if (ICmpInst::isSigned(P) && IID != Intrinsic::smin &&
        IID != Intrinsic::smax)
      continue;
 
    IRBuilder<> Builder(ICmp);
    Value *X = Info.Other;
    Value *A = ZeroOrSignExtend(Builder, MinMax->getArgOperand(0), Info);
    Value *B = ZeroOrSignExtend(Builder, MinMax->getArgOperand(1), Info);
    bool IsMin = IID == Intrinsic::smin || IID == Intrinsic::umin;
    bool IsMax = IID == Intrinsic::smax || IID == Intrinsic::umax;
    bool IsLess = ICmpInst::isLE(P) || ICmpInst::isLT(P);
    bool IsGreater = ICmpInst::isGE(P) || ICmpInst::isGT(P);
    assert(IsMin ^ IsMax);
    assert(IsLess ^ IsGreater);
 
    Value *Replacement;
    Value *LHS = Builder.CreateICmp(P, X, A);
    Value *RHS = Builder.CreateICmp(P, X, B);
    if ((IsLess && IsMin) || (IsGreater && IsMax))
      // x < min(a, b) -> x < a && x < b
      // x > max(a, b) -> x > a && x > b
      Replacement = Builder.CreateLogicalAnd(LHS, RHS);
    else
      // x > min(a, b) -> x > a || x > b
      // x < max(a, b) -> x < a || x < b
      Replacement = Builder.CreateLogicalOr(LHS, RHS);
 
    ICmp->replaceAllUsesWith(Replacement);
 
    Instruction *ToRemove[] = {ICmp, Info.ZExt, Info.SExt, MinMax};
    for (Instruction *I : ToRemove)
      if (I && I->use_empty())
        I->eraseFromParent();
 
    Changed = true;
  }
 
  return Changed;
}
 
// Do the following transformation:
//
//   x < min(a, b) -> x < a && x < b
//   x > min(a, b) -> x > a || x > b
//   x < max(a, b) -> x < a || x < b
//   x > max(a, b) -> x > a && x > b
//
// Such patterns are introduced by LICM.cpp:hoistMinMax()
// transformation and might lead to BPF verification failures for
// older kernels.
//
// To minimize "collateral" changes only do it for icmp + min/max
// calls when icmp is inside a loop and min/max is outside of that
// loop.
//
// Verification failure happens when:
// - RHS operand of some `icmp LHS, RHS` is replaced by some RHS1;
// - verifier can recognize RHS as a constant scalar in some context;
// - verifier can't recognize RHS1 as a constant scalar in the same
//   context;
//
// The "constant scalar" is not a compile time constant, but a register
// that holds a scalar value known to verifier at some point in time
// during abstract interpretation.
//
// See also:
//   https://lore.kernel.org/bpf/20230406164505.1046801-1-yhs@fb.com/
bool BPFCheckAndAdjustIR::sinkMinMax(Module &M) {
  bool Changed = false;
 
  for (Function &F : M) {
    if (F.isDeclaration())
      continue;
 
    LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>(F).getLoopInfo();
    for (Loop *L : LI)
      for (BasicBlock *BB : L->blocks()) {
        // Filter out instructions coming from the same loop
        Loop *BBLoop = LI.getLoopFor(BB);
        auto OtherLoopFilter = [&](Instruction *I) {
          return LI.getLoopFor(I->getParent()) != BBLoop;
        };
        Changed |= sinkMinMaxInBB(*BB, OtherLoopFilter);
      }
  }
 
  return Changed;
}
 
void BPFCheckAndAdjustIR::getAnalysisUsage(AnalysisUsage &AU) const {
  AU.addRequired<LoopInfoWrapperPass>();
}
 
bool BPFCheckAndAdjustIR::adjustIR(Module &M) {
  bool Changed = removePassThroughBuiltin(M);
  Changed = removeCompareBuiltin(M) || Changed;
  Changed = sinkMinMax(M) || Changed;
  return Changed;
}
 
bool BPFCheckAndAdjustIR::runOnModule(Module &M) {
  checkIR(M);
  return adjustIR(M);
}