[llvm] r203962 - Correctly handle an ELF symbol defined with "a = b + expr".

Alexander Kornienko alexfh at google.com
Mon Mar 17 16:43:43 PDT 2014


Here's a small repro (clang-3.5-bad is current HEAD, clang-3.5-good is
built after svn up -r203961 lib/MC/MCSymbol.cpp include/llvm/MC/MCSymbol.h
lib/MC/ELFObjectWriter.cpp):

$ cat q.c
void a1();
void a2() __attribute__((alias("a1")));
void a3() __attribute__((alias("a2")));
void a1() {}

$ ~/work/llvm-build/bin/clang-3.5-good -c q.c && mv q.o good.o &&
  ~/work/llvm-build/bin/clang-3.5-bad -c q.c && mv q.o bad.o && objdump -t
good.o bad.o

good.o:     file format elf64-x86-64

SYMBOL TABLE:
0000000000000000 l    df *ABS*  0000000000000000 q.c
0000000000000000 l    d  .text  0000000000000000 .text
0000000000000000 l    d  .data  0000000000000000 .data
0000000000000000 l    d  .bss   0000000000000000 .bss
0000000000000000 l    d  .comment       0000000000000000 .comment
0000000000000000 l    d  .note.GNU-stack        0000000000000000
.note.GNU-stack
0000000000000000 l    d  .eh_frame      0000000000000000 .eh_frame
0000000000000000 g     F .text  0000000000000006 a1
0000000000000000 g     F .text  0000000000000006 a2
0000000000000000 g     F .text  0000000000000006 a3



bad.o:     file format elf64-x86-64

SYMBOL TABLE:
0000000000000000 l    df *ABS*  0000000000000000 q.c
0000000000000000 l    d  .text  0000000000000000 .text
0000000000000000 l    d  .data  0000000000000000 .data
0000000000000000 l    d  .bss   0000000000000000 .bss
0000000000000000 l    d  .comment       0000000000000000 .comment
0000000000000000 l    d  .note.GNU-stack        0000000000000000
.note.GNU-stack
0000000000000000 l    d  .eh_frame      0000000000000000 .eh_frame
0000000000000000 g     F .text  0000000000000006 a1
0000000000000000 g     F .text  0000000000000006 a2
0000000000000000 g       .text  0000000000000000 a3


Difference is in the flags of the symbol a3.

Could you please fix the problem or revert the commit?

Thanks!

On Mon, Mar 17, 2014 at 8:05 PM, Alexander Kornienko <alexfh at google.com>wrote:

> On Mon, Mar 17, 2014 at 5:17 PM, Alexander Kornienko <alexfh at google.com>wrote:
>
>> This commit breaks something in our code base. Unfortunately, I don't
>> know what exactly and I'm struggling to come up with a reasonably-sized
>> reproduction. Once I have something specific, I'll let you know.
>>
>> +David, who may be interested in this.
>
> I've found a translation unit, which is handled differently before and
> after this revision. The difference is that after this revision, the
> functions marked with __attribute__((alias("..."))) loose the "function"
> flag (i.e. they get space instead of 'F' in the third column of the objdump
> -t output). Is this enough or you need more information to get this fixed?
>
>
>>
>>
>> On Fri, Mar 14, 2014 at 9:09 PM, Rafael Espindola <
>> rafael.espindola at gmail.com> wrote:
>>
>>> Author: rafael
>>> Date: Fri Mar 14 15:09:04 2014
>>> New Revision: 203962
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=203962&view=rev
>>> Log:
>>> Correctly handle an ELF symbol defined with "a = b + expr".
>>>
>>> We were marking the symbol as absolute instead of computing b's offset +
>>> the
>>> expression value.
>>>
>>> This fixes pr19126.
>>>
>>> Added:
>>>     llvm/trunk/test/MC/ELF/offset.s
>>> Modified:
>>>     llvm/trunk/include/llvm/MC/MCSymbol.h
>>>     llvm/trunk/lib/MC/ELFObjectWriter.cpp
>>>     llvm/trunk/lib/MC/MCSymbol.cpp
>>>
>>> Modified: llvm/trunk/include/llvm/MC/MCSymbol.h
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/MC/MCSymbol.h?rev=203962&r1=203961&r2=203962&view=diff
>>>
>>> ==============================================================================
>>> --- llvm/trunk/include/llvm/MC/MCSymbol.h (original)
>>> +++ llvm/trunk/include/llvm/MC/MCSymbol.h Fri Mar 14 15:09:04 2014
>>> @@ -18,6 +18,7 @@
>>>  #include "llvm/Support/Compiler.h"
>>>
>>>  namespace llvm {
>>> +  class MCAsmLayout;
>>>    class MCExpr;
>>>    class MCSection;
>>>    class MCContext;
>>> @@ -145,6 +146,11 @@ namespace llvm {
>>>      // itself.
>>>      const MCSymbol &AliasedSymbol() const;
>>>
>>> +    // If this symbol is not a variable, return itself. If it is a
>>> variable,
>>> +    // evaluate it and check if it is of the form Base +
>>> ConstantOffset. If so,
>>> +    // return Base, if not, return nullptr.
>>> +    const MCSymbol *getBaseSymbol(const MCAsmLayout &Layout) const;
>>> +
>>>      void setVariableValue(const MCExpr *Value);
>>>
>>>      /// @}
>>>
>>> Modified: llvm/trunk/lib/MC/ELFObjectWriter.cpp
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/MC/ELFObjectWriter.cpp?rev=203962&r1=203961&r2=203962&view=diff
>>>
>>> ==============================================================================
>>> --- llvm/trunk/lib/MC/ELFObjectWriter.cpp (original)
>>> +++ llvm/trunk/lib/MC/ELFObjectWriter.cpp Fri Mar 14 15:09:04 2014
>>> @@ -271,6 +271,7 @@ class ELFObjectWriter : public MCObjectW
>>>      /// \param RevGroupMap - Maps a signature symbol to the group
>>> section.
>>>      /// \param NumRegularSections - Number of non-relocation sections.
>>>      void ComputeSymbolTable(MCAssembler &Asm,
>>> +                            const MCAsmLayout &Layout,
>>>                              const SectionIndexMapTy &SectionIndexMap,
>>>                              RevGroupMapTy RevGroupMap,
>>>                              unsigned NumRegularSections);
>>> @@ -462,33 +463,43 @@ void ELFObjectWriter::WriteSymbolEntry(M
>>>    }
>>>  }
>>>
>>> -uint64_t ELFObjectWriter::SymbolValue(MCSymbolData &Data,
>>> +uint64_t ELFObjectWriter::SymbolValue(MCSymbolData &OrigData,
>>>                                        const MCAsmLayout &Layout) {
>>> -  if (Data.isCommon() && Data.isExternal())
>>> -    return Data.getCommonAlignment();
>>> -
>>> -  const MCSymbol &Symbol = Data.getSymbol();
>>> +  MCSymbolData *Data = &OrigData;
>>> +  if (Data->isCommon() && Data->isExternal())
>>> +    return Data->getCommonAlignment();
>>> +
>>> +  const MCSymbol *Symbol = &Data->getSymbol();
>>> +
>>> +  uint64_t Res = 0;
>>> +  if (Symbol->isVariable()) {
>>> +    const MCExpr *Expr = Symbol->getVariableValue();
>>> +    MCValue Value;
>>> +    if (!Expr->EvaluateAsRelocatable(Value, &Layout))
>>> +      return 0;
>>> +    if (Value.getSymB())
>>> +      return 0;
>>> +    Res = Value.getConstant();
>>> +
>>> +    const MCSymbolRefExpr *A = Value.getSymA();
>>> +    if (!A)
>>> +      return Res;
>>>
>>> -  if (Symbol.isAbsolute() && Symbol.isVariable()) {
>>> -    if (const MCExpr *Value = Symbol.getVariableValue()) {
>>> -      int64_t IntValue;
>>> -      if (Value->EvaluateAsAbsolute(IntValue, Layout))
>>> -        return (uint64_t)IntValue;
>>> -    }
>>> +    Symbol = &A->getSymbol();
>>> +    Data = &Layout.getAssembler().getSymbolData(*Symbol);
>>>    }
>>>
>>> -  if (!Symbol.isInSection())
>>> +  if (!Symbol->isInSection())
>>>      return 0;
>>>
>>> +  if (!Data->getFragment())
>>> +    return 0;
>>>
>>> -  if (Data.getFragment()) {
>>> -    if (Data.getFlags() & ELF_Other_ThumbFunc)
>>> -      return Layout.getSymbolOffset(&Data)+1;
>>> -    else
>>> -      return Layout.getSymbolOffset(&Data);
>>> -  }
>>> +  Res += Layout.getSymbolOffset(Data);
>>> +  if (Data->getFlags() & ELF_Other_ThumbFunc)
>>> +    ++Res;
>>>
>>> -  return 0;
>>> +  return Res;
>>>  }
>>>
>>>  void ELFObjectWriter::ExecutePostLayoutBinding(MCAssembler &Asm,
>>> @@ -586,7 +597,7 @@ void ELFObjectWriter::WriteSymbol(MCData
>>>    uint8_t Other = MCELF::getOther(OrigData) << (ELF_STO_Shift -
>>> ELF_STV_Shift);
>>>    Other |= Visibility;
>>>
>>> -  uint64_t Value = SymbolValue(Data, Layout);
>>> +  uint64_t Value = SymbolValue(OrigData, Layout);
>>>    uint64_t Size = 0;
>>>
>>>    assert(!(Data.isCommon() && !Data.isExternal()));
>>> @@ -897,10 +908,11 @@ void ELFObjectWriter::ComputeIndexMap(MC
>>>    }
>>>  }
>>>
>>> -void ELFObjectWriter::ComputeSymbolTable(MCAssembler &Asm,
>>> -                                      const SectionIndexMapTy
>>> &SectionIndexMap,
>>> -                                         RevGroupMapTy RevGroupMap,
>>> -                                         unsigned NumRegularSections) {
>>> +void
>>> +ELFObjectWriter::ComputeSymbolTable(MCAssembler &Asm, const MCAsmLayout
>>> &Layout,
>>> +                                    const SectionIndexMapTy
>>> &SectionIndexMap,
>>> +                                    RevGroupMapTy RevGroupMap,
>>> +                                    unsigned NumRegularSections) {
>>>    // FIXME: Is this the correct place to do this?
>>>    // FIXME: Why is an undefined reference to _GLOBAL_OFFSET_TABLE_
>>> needed?
>>>    if (NeedsGOT) {
>>> @@ -948,33 +960,33 @@ void ELFObjectWriter::ComputeSymbolTable
>>>
>>>      ELFSymbolData MSD;
>>>      MSD.SymbolData = it;
>>> -    const MCSymbol &RefSymbol = Symbol.AliasedSymbol();
>>> +    const MCSymbol *BaseSymbol = Symbol.getBaseSymbol(Layout);
>>>
>>>      // Undefined symbols are global, but this is the first place we
>>>      // are able to set it.
>>>      bool Local = isLocal(*it, isSignature, Used);
>>>      if (!Local && MCELF::GetBinding(*it) == ELF::STB_LOCAL) {
>>> -      MCSymbolData &SD = Asm.getSymbolData(RefSymbol);
>>> +      assert(BaseSymbol);
>>> +      MCSymbolData &SD = Asm.getSymbolData(*BaseSymbol);
>>>        MCELF::SetBinding(*it, ELF::STB_GLOBAL);
>>>        MCELF::SetBinding(SD, ELF::STB_GLOBAL);
>>>      }
>>>
>>> -    if (RefSymbol.isUndefined() && !Used && WeakrefUsed)
>>> -      MCELF::SetBinding(*it, ELF::STB_WEAK);
>>> -
>>> -    if (it->isCommon()) {
>>> +    if (!BaseSymbol) {
>>> +      MSD.SectionIndex = ELF::SHN_ABS;
>>> +    } else if (it->isCommon()) {
>>>        assert(!Local);
>>>        MSD.SectionIndex = ELF::SHN_COMMON;
>>> -    } else if (Symbol.isAbsolute() || RefSymbol.isVariable()) {
>>> -      MSD.SectionIndex = ELF::SHN_ABS;
>>> -    } else if (RefSymbol.isUndefined()) {
>>> +    } else if (BaseSymbol->isUndefined()) {
>>>        if (isSignature && !Used)
>>>          MSD.SectionIndex = SectionIndexMap.lookup(RevGroupMap[&Symbol]);
>>>        else
>>>          MSD.SectionIndex = ELF::SHN_UNDEF;
>>> +      if (!Used && WeakrefUsed)
>>> +        MCELF::SetBinding(*it, ELF::STB_WEAK);
>>>      } else {
>>>        const MCSectionELF &Section =
>>> -        static_cast<const MCSectionELF&>(RefSymbol.getSection());
>>> +        static_cast<const MCSectionELF&>(BaseSymbol->getSection());
>>>        MSD.SectionIndex = SectionIndexMap.lookup(&Section);
>>>        if (MSD.SectionIndex >= ELF::SHN_LORESERVE)
>>>          NeedsSymtabShndx = true;
>>> @@ -1560,8 +1572,8 @@ void ELFObjectWriter::WriteObject(MCAsse
>>>    unsigned NumRegularSections = NumUserSections + NumIndexedSections;
>>>
>>>    // Compute symbol table information.
>>> -  ComputeSymbolTable(Asm, SectionIndexMap, RevGroupMap,
>>> NumRegularSections);
>>> -
>>> +  ComputeSymbolTable(Asm, Layout, SectionIndexMap, RevGroupMap,
>>> +                     NumRegularSections);
>>>
>>>    WriteRelocations(Asm, const_cast<MCAsmLayout&>(Layout), RelMap);
>>>
>>>
>>> Modified: llvm/trunk/lib/MC/MCSymbol.cpp
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/MC/MCSymbol.cpp?rev=203962&r1=203961&r2=203962&view=diff
>>>
>>> ==============================================================================
>>> --- llvm/trunk/lib/MC/MCSymbol.cpp (original)
>>> +++ llvm/trunk/lib/MC/MCSymbol.cpp Fri Mar 14 15:09:04 2014
>>> @@ -9,6 +9,7 @@
>>>
>>>  #include "llvm/MC/MCSymbol.h"
>>>  #include "llvm/MC/MCExpr.h"
>>> +#include "llvm/MC/MCValue.h"
>>>  #include "llvm/Support/Debug.h"
>>>  #include "llvm/Support/raw_ostream.h"
>>>  using namespace llvm;
>>> @@ -51,6 +52,22 @@ const MCSymbol &MCSymbol::AliasedSymbol(
>>>    return *S;
>>>  }
>>>
>>> +const MCSymbol *MCSymbol::getBaseSymbol(const MCAsmLayout &Layout)
>>> const {
>>> +  if (!isVariable())
>>> +    return this;
>>> +
>>> +  const MCExpr *Expr = getVariableValue();
>>> +  MCValue Value;
>>> +  if (!Expr->EvaluateAsRelocatable(Value, &Layout))
>>> +    return nullptr;
>>> +
>>> +  if (Value.getSymB())
>>> +    return nullptr;
>>> +  if (const MCSymbolRefExpr *A = Value.getSymA())
>>> +    return &A->getSymbol();
>>> +  return nullptr;
>>> +}
>>> +
>>>  void MCSymbol::setVariableValue(const MCExpr *Value) {
>>>    assert(!IsUsed && "Cannot set a variable that has already been
>>> used.");
>>>    assert(Value && "Invalid variable value!");
>>>
>>> Added: llvm/trunk/test/MC/ELF/offset.s
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/llvm/trunk/test/MC/ELF/offset.s?rev=203962&view=auto
>>>
>>> ==============================================================================
>>> --- llvm/trunk/test/MC/ELF/offset.s (added)
>>> +++ llvm/trunk/test/MC/ELF/offset.s Fri Mar 14 15:09:04 2014
>>> @@ -0,0 +1,58 @@
>>> +// RUN: llvm-mc -filetype=obj -triple x86_64-pc-linux-gnu %s -o - |
>>> llvm-readobj -t - | FileCheck %s
>>> +
>>> +// Test that a variable declared with "var = other_var + cst" is in the
>>> same
>>> +// section as other_var and its value is the value of other_var + cst.
>>> +
>>> +        .data
>>> +        .globl sym_a
>>> +        .byte 42
>>> +sym_a:
>>> +
>>> +// CHECK:       Symbol {
>>> +// CHECK:         Name: sym_a
>>> +// CHECK-NEXT:    Value: 0x1
>>> +// CHECK-NEXT:    Size: 0
>>> +// CHECK-NEXT:    Binding: Global
>>> +// CHECK-NEXT:    Type: None
>>> +// CHECK-NEXT:    Other: 0
>>> +// CHECK-NEXT:    Section: .data
>>> +// CHECK-NEXT:  }
>>> +
>>> +        .long 42
>>> +        .globl sym_b
>>> +sym_b:
>>> +        .globl sym_c
>>> +sym_c = sym_a
>>> +// CHECK:       Symbol {
>>> +// CHECK:         Name: sym_c
>>> +// CHECK-NEXT:    Value: 0x1
>>> +// CHECK-NEXT:    Size: 0
>>> +// CHECK-NEXT:    Binding: Global
>>> +// CHECK-NEXT:    Type: None
>>> +// CHECK-NEXT:    Other: 0
>>> +// CHECK-NEXT:    Section: .data
>>> +// CHECK-NEXT:  }
>>> +
>>> +        .globl sym_d
>>> +sym_d = sym_a + 1
>>> +// CHECK:       Symbol {
>>> +// CHECK:         Name: sym_d
>>> +// CHECK-NEXT:    Value: 0x2
>>> +// CHECK-NEXT:    Size: 0
>>> +// CHECK-NEXT:    Binding: Global
>>> +// CHECK-NEXT:    Type: None
>>> +// CHECK-NEXT:    Other: 0
>>> +// CHECK-NEXT:    Section: .data
>>> +// CHECK-NEXT:  }
>>> +
>>> +        .globl sym_e
>>> +sym_e = sym_a + (sym_b - sym_a) * 3
>>> +// CHECK:       Symbol {
>>> +// CHECK:         Name: sym_e
>>> +// CHECK-NEXT:    Value: 0xD
>>> +// CHECK-NEXT:    Size: 0
>>> +// CHECK-NEXT:    Binding: Global
>>> +// CHECK-NEXT:    Type: None
>>> +// CHECK-NEXT:    Other: 0
>>> +// CHECK-NEXT:    Section: .data
>>> +// CHECK-NEXT:  }
>>>
>>>
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