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

[Rui Ueyama via llvm-commits]

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/
> LinkerScript.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.
> cpp?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/
> linkerscript/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
> >
> >
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20161012/e483a6b8/attachment.html>