MCExternalSymbolizer.cpp

//===-- MCExternalSymbolizer.cpp - External symbolizer --------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//

#include "llvm/MC/MCDisassembler/MCExternalSymbolizer.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/MCExpr.h"
#include "llvm/MC/MCInst.h"
#include "llvm/Support/raw_ostream.h"
#include <cstring>

using namespace llvm;

namespace llvm {
class Triple;
}

// This function tries to add a symbolic operand in place of the immediate
// Value in the MCInst. The immediate Value has had any PC adjustment made by
// the caller. If the instruction is a branch instruction then IsBranch is true,
// else false. If the getOpInfo() function was set as part of the
// setupForSymbolicDisassembly() call then that function is called to get any
// symbolic information at the Address for this instruction. If that returns
// non-zero then the symbolic information it returns is used to create an MCExpr
// and that is added as an operand to the MCInst. If getOpInfo() returns zero
// and IsBranch is true then a symbol look up for Value is done and if a symbol
// is found an MCExpr is created with that, else an MCExpr with Value is
// created. This function returns true if it adds an operand to the MCInst and
// false otherwise.
bool MCExternalSymbolizer::tryAddingSymbolicOperand(
    MCInst &MI, raw_ostream &cStream, int64_t Value, uint64_t Address,
    bool IsBranch, uint64_t Offset, uint64_t OpSize, uint64_t InstSize) {
  struct LLVMOpInfo1 SymbolicOp;
  std::memset(&SymbolicOp, '\0', sizeof(struct LLVMOpInfo1));
  SymbolicOp.Value = Value;

  if (!GetOpInfo ||
      !GetOpInfo(DisInfo, Address, Offset, OpSize, InstSize, 1, &SymbolicOp)) {
    // Clear SymbolicOp.Value from above and also all other fields.
    std::memset(&SymbolicOp, '\0', sizeof(struct LLVMOpInfo1));

    // At this point, GetOpInfo() did not find any relocation information about
    // this operand and we are left to use the SymbolLookUp() call back to guess
    // if the Value is the address of a symbol.  In the case this is a branch
    // that always makes sense to guess.  But in the case of an immediate it is
    // a bit more questionable if it is an address of a symbol or some other
    // reference.  So if the immediate Value comes from a width of 1 byte,
    // OpSize, we will not guess it is an address of a symbol.  Because in
    // object files assembled starting at address 0 this usually leads to
    // incorrect symbolication.
    if (!SymbolLookUp || (OpSize == 1 && !IsBranch))
      return false;

    uint64_t ReferenceType;
    if (IsBranch)
       ReferenceType = LLVMDisassembler_ReferenceType_In_Branch;
    else
       ReferenceType = LLVMDisassembler_ReferenceType_InOut_None;
    const char *ReferenceName;
    const char *Name = SymbolLookUp(DisInfo, Value, &ReferenceType, Address,
                                    &ReferenceName);
    if (Name) {
      SymbolicOp.AddSymbol.Name = Name;
      SymbolicOp.AddSymbol.Present = true;
      // If Name is a C++ symbol name put the human readable name in a comment.
      if(ReferenceType == LLVMDisassembler_ReferenceType_DeMangled_Name)
        cStream << ReferenceName;
    }
    // For branches always create an MCExpr so it gets printed as hex address.
    else if (IsBranch) {
      SymbolicOp.Value = Value;
    }
    if(ReferenceType == LLVMDisassembler_ReferenceType_Out_SymbolStub)
      cStream << "symbol stub for: " << ReferenceName;
    else if(ReferenceType == LLVMDisassembler_ReferenceType_Out_Objc_Message)
      cStream << "Objc message: " << ReferenceName;
    if (!Name && !IsBranch)
      return false;
  }

  const MCExpr *Add = nullptr;
  if (SymbolicOp.AddSymbol.Present) {
    if (SymbolicOp.AddSymbol.Name) {
      StringRef Name(SymbolicOp.AddSymbol.Name);
      MCSymbol *Sym = Ctx.getOrCreateSymbol(Name);
      Add = MCSymbolRefExpr::create(Sym, Ctx);
    } else {
      Add = MCConstantExpr::create((int)SymbolicOp.AddSymbol.Value, Ctx);
    }
  }

  const MCExpr *Sub = nullptr;
  if (SymbolicOp.SubtractSymbol.Present) {
      if (SymbolicOp.SubtractSymbol.Name) {
      StringRef Name(SymbolicOp.SubtractSymbol.Name);
      MCSymbol *Sym = Ctx.getOrCreateSymbol(Name);
      Sub = MCSymbolRefExpr::create(Sym, Ctx);
    } else {
      Sub = MCConstantExpr::create((int)SymbolicOp.SubtractSymbol.Value, Ctx);
    }
  }

  const MCExpr *Off = nullptr;
  if (SymbolicOp.Value != 0)
    Off = MCConstantExpr::create(SymbolicOp.Value, Ctx);

  const MCExpr *Expr;
  if (Sub) {
    const MCExpr *LHS;
    if (Add)
      LHS = MCBinaryExpr::createSub(Add, Sub, Ctx);
    else
      LHS = MCUnaryExpr::createMinus(Sub, Ctx);
    if (Off)
      Expr = MCBinaryExpr::createAdd(LHS, Off, Ctx);
    else
      Expr = LHS;
  } else if (Add) {
    if (Off)
      Expr = MCBinaryExpr::createAdd(Add, Off, Ctx);
    else
      Expr = Add;
  } else {
    if (Off)
      Expr = Off;
    else
      Expr = MCConstantExpr::create(0, Ctx);
  }

  Expr = RelInfo->createExprForCAPIVariantKind(Expr, SymbolicOp.VariantKind);
  if (!Expr)
    return false;

  MI.addOperand(MCOperand::createExpr(Expr));
  return true;
}

// This function tries to add a comment as to what is being referenced by a load
// instruction with the base register that is the Pc.  These can often be values
// in a literal pool near the Address of the instruction. The Address of the
// instruction and its immediate Value are used as a possible literal pool entry.
// The SymbolLookUp call back will return the name of a symbol referenced by the
// literal pool's entry if the referenced address is that of a symbol. Or it
// will return a pointer to a literal 'C' string if the referenced address of
// the literal pool's entry is an address into a section with C string literals.
// Or if the reference is to an Objective-C data structure it will return a
// specific reference type for it and a string.
void MCExternalSymbolizer::tryAddingPcLoadReferenceComment(raw_ostream &cStream,
                                                           int64_t Value,
                                                           uint64_t Address) {
  if (SymbolLookUp) {
    uint64_t ReferenceType = LLVMDisassembler_ReferenceType_In_PCrel_Load;
    const char *ReferenceName;
    (void)SymbolLookUp(DisInfo, Value, &ReferenceType, Address, &ReferenceName);
    if(ReferenceType == LLVMDisassembler_ReferenceType_Out_LitPool_SymAddr)
      cStream << "literal pool symbol address: " << ReferenceName;
    else if(ReferenceType ==
            LLVMDisassembler_ReferenceType_Out_LitPool_CstrAddr) {
      cStream << "literal pool for: \"";
      cStream.write_escaped(ReferenceName);
      cStream << "\"";
    }
    else if(ReferenceType ==
            LLVMDisassembler_ReferenceType_Out_Objc_CFString_Ref)
      cStream << "Objc cfstring ref: @\"" << ReferenceName << "\"";
    else if(ReferenceType ==
            LLVMDisassembler_ReferenceType_Out_Objc_Message)
      cStream << "Objc message: " << ReferenceName;
    else if(ReferenceType ==
            LLVMDisassembler_ReferenceType_Out_Objc_Message_Ref)
      cStream << "Objc message ref: " << ReferenceName;
    else if(ReferenceType ==
            LLVMDisassembler_ReferenceType_Out_Objc_Selector_Ref)
      cStream << "Objc selector ref: " << ReferenceName;
    else if(ReferenceType ==
            LLVMDisassembler_ReferenceType_Out_Objc_Class_Ref)
      cStream << "Objc class ref: " << ReferenceName;
  }
}

namespace llvm {
MCSymbolizer *createMCSymbolizer(const Triple &TT, LLVMOpInfoCallback GetOpInfo,
                                 LLVMSymbolLookupCallback SymbolLookUp,
                                 void *DisInfo, MCContext *Ctx,
                                 std::unique_ptr<MCRelocationInfo> &&RelInfo) {
  assert(Ctx && "No MCContext given for symbolic disassembly");

  return new MCExternalSymbolizer(*Ctx, std::move(RelInfo), GetOpInfo,
                                  SymbolLookUp, DisInfo);
}
}