[llvm-dev] lld mishandling R_X86_64_PC32 relocations

Wed Jun 6 14:07:44 PDT 2018

On Tue, Jun 5, 2018 at 1:50 PM Stephen Checkoway via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi Peter,
>
> I hadn't noticed that line about RELA in the ABI document, thanks! I agree
> that this is a bug in objcopy. I'll file a bug against objcopy.
>
> It seems that lld already supports relocations that don't comply with the
> standard (namely R_X86_64_16 and R_X86_64_8 <
> https://github.com/hjl-tools/x86-psABI/blob/hjl/master/object-files.tex#L554>),
> but I guess that's different than supporting an entirely different format.
>

lld supports the extension because LLVM supports the extension too (
https://llvm.org/docs/Extensions.html#x86). But yeah, as you said, that's a
small extension to the standard, and that's different from supporting an
entirely different format.

> Thanks for looking into this.
>
> Steve
>
> > On Jun 5, 2018, at 14:34, Peter Collingbourne via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> > Hi Stephen,
> >
> > I think the bug is in objcopy, as it is creating a file that does not
> comply with the x86-64 psABI, which requires all relocations to be RELA:
> >
> https://github.com/hjl-tools/x86-psABI/blob/hjl/master/object-files.tex#L429
> > I also tried passing your bug-64.o to ld.gold, and it rejects it with an
> internal error.
> >
> > $ ld.gold -r -o bug-64-2.o bug-64.o
> > ld.gold: internal error in scan_relocatable_relocs, at
> ../../gold/x86_64.cc:5118
> > $ ld.gold  -o bug-64-2.o bug-64.o
> > ld.gold: error: bug-64.o: unsupported REL reloc section
> > ld.gold: internal error in relocate_section, at ../../gold/x86_64.cc:5051
> >
> > Probably what needs to happen is that we need to start rejecting files
> which use the wrong relocation type and a bug needs to be filed against
> objcopy.
> >
> > Peter
> >
> > On Tue, Jun 5, 2018 at 10:33 AM, Stephen Checkoway via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> > Hi,
> >
> > I've tracked down what I believe is a bug in lld's relocation processing
> for R_X86_64_PC32 REL relocations.
> >
> > I'm producing the object file in a slightly unusual way: I'm using
> objcopy on a relocatable i386 ELF object file to convert it to x86_64 which
> transforms a R_386_PC32 into a R_X86_64_PC32.
> >
> > Steps to reproduce:
> >
> > 1. Assemble the attached bug.asm using nasm and note the R_386_PC32 REL.
> >
> > $ nasm -felf32 -o bug.o bug.asm
> > $ x86_64-elf-objdump -dr bug.o | grep -A1 -e '<_start>:'
> > 00000000 <_start>:
> >    0:   e8 fc ff ff ff          call   1 <_start+0x1>
> > $ x86_64-elf-readelf -r bug.o
> >
> > Relocation section '.rel.text._start' at offset 0x260 contains 1 entry:
> >  Offset     Info    Type            Sym.Value  Sym. Name
> > 00000001  00000302 R_386_PC32        00000000   .text.foo
> >
> > 2. Convert bug.o to a 64-bit ELF object file and note the R_X86_64_PC32
> REL (not RELA!)
> >
> > $ x86_64-elf-objcopy -I elf32-i386 -O elf64-x86-64 bug.o bug-64.o
> > $ x86_64-elf-objdump -M i386 -dr bug-64.o | grep -A1 -e '<_start>:'
> > 0000000000000000 <_start>:
> >    0:   e8 fc ff ff ff          call   1 <_start+0x1>
> > $ x86_64-elf-readelf -r bug-64.o
> >
> > Relocation section '.rel.text._start' at offset 0x128 contains 1 entry:
> >   Offset          Info           Type           Sym. Value    Sym. Name
> > 000000000001  000300000002 R_X86_64_PC32     0000000000000000 .text.foo
> >
> > 3. Link with a just-built ld.lld and note that the relocation has been
> misapplied. It's now calling foo+4 rather than foo.
> >
> > $ ./llvm-build/bin/ld.lld -melf_x86_64 -o bug-lld bug-64.o
> > $ x86_64-elf-objdump -M i386 -d bug-64-lld | grep -A1 -e '<_start>:'
> > 0000000000201000 <_start>:
> >   201000:       e8 0f 00 00 00          call   201014 <foo+0x4>
> >
> > If you link with GNU ld instead, the relocation is applied correctly.
> >
> > $ x86_64-elf-ld -melf_x86_64 -o bug-64-ld bug-64.o
> > $ x86_64-elf-objdump -M i386 -d bug-64-ld | grep -A1 -e '<_start>:'
> > 0000000000400080 <_start>:
> >   400080:       e8 0b 00 00 00          call   400090 <foo>
> >
> > Linking the 32-bit object file works correctly with both GNU ld and
> ld.lld.
> >
> > $ ./llvm-build/bin/ld.lld -melf_i386 -o bug-lld bug.o
> > $ x86_64-elf-objdump -d bug-lld | grep -A1 -e '<_start>:'
> > 00011000 <_start>:
> >    11000:       e8 0b 00 00 00          call   11010 <foo>
> > $ x86_64-elf-ld -melf_i386 -o bug-ld bug.o
> > $ x86_64-elf-objdump -d bug-ld | grep -A1 -e '<_start>:'
> > 08048060 <_start>:
> >  8048060:       e8 0b 00 00 00          call   8048070 <foo>
> >
> > I'm not at all familiar with the lld source, but this looks a lot like
> getImplicitAddend() needs to be implemented for the X86_64 class.
> >
> > Alternatively (but much less useful for me) would be an error message
> that REL relocations are not supported on x86-64.
> >
> > I've attached all of the files created above, in case anyone wants to
> examine them.
> >
> > Thank you,
> >
> > Steve
> >
> > --
> > Stephen Checkoway
> >
> >
> >
> >
> > _______________________________________________
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> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
> >
> >
> >
> > --
> > --
> > Peter
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
> --
> Stephen Checkoway
>
>
>
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