[lld] r284010 - Alternative fix for reloc tareting discarded section

[Rui Ueyama via llvm-commits]

(nit: this patch contains a trailing whitespace.)

On Wed, Oct 12, 2016 at 11:25 AM, Rui Ueyama <ruiu at google.com> wrote:

> I wonder if input sections now think they don't belong to any output
> section if they are assigned to output sections using linker scripts. We
> are using OutSec in various places such as getSymVA in Symbols.cpp. Doesn't
> this change affect them?
>
> On Wed, Oct 12, 2016 at 11:18 AM, Eugene Leviant <evgeny.leviant at gmail.com
> > wrote:
>
>> The computeInputSections() function sets OutSec to (-1), see
>> LinkerScript.cpp:216.
>> This is being done to avoid adding input section twice, because
>> computeInputSections() is being called
>> for every input section pattern in a command.
>> As (-1) is not 0, condition S->OutSec in getSymVA() is true, and you
>> get crash in a call to S->OutSec->getVA().
>> To fix this I added piece of code which zeroes OutSec field after all
>> input sections have been added to result vector.
>>
>>
>> 2016-10-12 21:10 GMT+03:00 Rui Ueyama <ruiu at google.com>:
>> > On Wed, Oct 12, 2016 at 5:31 AM, Eugene Leviant via llvm-commits
>> > <llvm-commits at lists.llvm.org> wrote:
>> >>
>> >> Author: evgeny777
>> >> Date: Wed Oct 12 07:31:34 2016
>> >> New Revision: 284010
>> >>
>> >> URL: http://llvm.org/viewvc/llvm-project?rev=284010&view=rev
>> >> Log:
>> >> Alternative fix for reloc tareting discarded section
>> >>
>> >> r283984 introduced a problem of too many warning messages being shown
>> >> when -ffunction-sections and -fdata-sections were used in conjunction
>> >> with --gc-sections linker flag and debugging information present. This
>> >> happens because lot of relocations from .debug_line section may become
>> >> invalid in such case. The newer fix doesn't show any warning message
>> but
>> >> zeroes OutSec pointer in createInputSectionList() to avoid crash, when
>> >> relocations are written
>> >>
>> >> Modified:
>> >>     lld/trunk/ELF/LinkerScript.cpp
>> >>     lld/trunk/ELF/Symbols.cpp
>> >>     lld/trunk/test/ELF/linkerscript/discard-section.s
>> >>
>> >> Modified: lld/trunk/ELF/LinkerScript.cpp
>> >> URL:
>> >> http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/LinkerScri
>> pt.cpp?rev=284010&r1=284009&r2=284010&view=diff
>> >>
>> >> ============================================================
>> ==================
>> >> --- lld/trunk/ELF/LinkerScript.cpp (original)
>> >> +++ lld/trunk/ELF/LinkerScript.cpp Wed Oct 12 07:31:34 2016
>> >> @@ -239,6 +239,12 @@ LinkerScript<ELFT>::createInputSectionLi
>> >>        Ret.push_back(static_cast<InputSectionBase<ELFT> *>(S));
>> >>    }
>> >>
>> >> +  // After we created final list we should now set OutSec pointer to
>> >> null,
>> >> +  // instead of -1. Otherwise we may get a crash when writing relocs,
>> in
>> >> +  // case section is discarded by linker script
>> >> +  for (InputSectionBase<ELFT> *S : Ret)
>> >> +    S->OutSec = nullptr;
>> >
>> >
>> > I was thinking that it is an invariant that InputSection::OutSec always
>> > points to an output section if the input section is included in the
>> output
>> > section, so I'm confused with this code and comment. How does it work?
>> >
>> >> +
>> >>    return Ret;
>> >>  }
>> >>
>> >>
>> >> Modified: lld/trunk/ELF/Symbols.cpp
>> >> URL:
>> >> http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Symbols.cp
>> p?rev=284010&r1=284009&r2=284010&view=diff
>> >>
>> >> ============================================================
>> ==================
>> >> --- lld/trunk/ELF/Symbols.cpp (original)
>> >> +++ lld/trunk/ELF/Symbols.cpp Wed Oct 12 07:31:34 2016
>> >> @@ -25,12 +25,6 @@ using namespace lld;
>> >>  using namespace lld::elf;
>> >>
>> >>  template <class ELFT>
>> >> -static std::string getSectionName(InputSectionBase<ELFT> *S) {
>> >> -  StringRef Filename = S->getFile()->getName();
>> >> -  return (sys::path::filename(Filename) + "(" + S->Name + ")").str();
>> >> -}
>> >> -
>> >> -template <class ELFT>
>> >>  static typename ELFT::uint getSymVA(const SymbolBody &Body,
>> >>                                      typename ELFT::uint &Addend) {
>> >>    typedef typename ELFT::uint uintX_t;
>> >> @@ -60,11 +54,6 @@ static typename ELFT::uint getSymVA(cons
>> >>      if (!SC)
>> >>        return D.Value;
>> >>
>> >> -    if (!SC->Live) {
>> >> -      warn("relocation refers to discarded section '" +
>> >> getSectionName(SC) + "'");
>> >> -      return 0;
>> >> -    }
>> >> -
>> >>      uintX_t Offset = D.Value;
>> >>      if (D.isSection()) {
>> >>        Offset += Addend;
>> >>
>> >> Modified: lld/trunk/test/ELF/linkerscript/discard-section.s
>> >> URL:
>> >> http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/linke
>> rscript/discard-section.s?rev=284010&r1=284009&r2=284010&view=diff
>> >>
>> >> ============================================================
>> ==================
>> >> --- lld/trunk/test/ELF/linkerscript/discard-section.s (original)
>> >> +++ lld/trunk/test/ELF/linkerscript/discard-section.s Wed Oct 12
>> 07:31:34
>> >> 2016
>> >> @@ -1,10 +1,9 @@
>> >>  # REQUIRES: x86
>> >>  # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
>> >>  # RUN: echo "SECTIONS { /DISCARD/ : { *(.aaa*) } }" > %t.script
>> >> -# RUN: ld.lld -o %t1 --script %t.script %t 2>&1 | FileCheck
>> >> --check-prefix=WARN %s
>> >> +# RUN: ld.lld -o %t1 --script %t.script %t
>> >>  # RUN: llvm-objdump -section-headers %t1 | FileCheck %s
>> >>
>> >> -# WARN: relocation refers to discarded section {{.+}}(.aaa)
>> >>  # CHECK-NOT: .aaa
>> >>
>> >>  .section .aaa,"a"
>> >>
>> >>
>> >> _______________________________________________
>> >> llvm-commits mailing list
>> >> llvm-commits at lists.llvm.org
>> >> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>> >
>> >
>>
>
>
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