[PATCH] D26033: [ELF] Resolve undefined symbols from archives when linking shared objects

[Rui Ueyama via llvm-commits]

On Fri, Oct 28, 2016 at 2:15 AM, Eugene Leviant <evgeny.leviant at gmail.com>
wrote:

> Here is how I stepped on a problem (simplified):
>
> clang a.out.o liba.so libb.so -o executable // got link error here
> when used lld, both gold and ld succeeded
>
> liba,so is built this way:
>
> clang a.o libb.so libstatic.a -o liba.so
>
> I wonder is this some sort of malformed or undocumented behavior of
> ld/gold?
>

So,

  libb.so contains an undefined symbol *foo,*
  libstatic.a contains a defined symbol *foo*, and
  no one except libstatic.a defines *foo*

then it will fail. But, I wonder why libb.so contains an undefined symbol
*foo* in the first place, and why you passed an object file that must be
linked as an archive member rather as a bare .o.


> 2016-10-27 21:16 GMT+03:00 Rui Ueyama <ruiu at google.com>:
> > On Thu, Oct 27, 2016 at 11:08 AM, Rafael Espíndola
> > <rafael.espindola at gmail.com> wrote:
> >>
> >> This is inconsistent with how lld looks at shared libraries.
> >>
> >> We always use --allow-shlib-undefined, which basically means that we
> >> only look at share libraries to see what they provide and trust that
> >> they will have all their dependencies at run time.
> >>
> >> If we absolutely must, we should probably just implement a
> >> --no-allow-shlib-undefined. But I would like a very strong
> >> justification before we do that.
> >
> >
> > I agreed. We intentionally do not try to resolve symbols in DSOs because
> it
> > doesn't make much sense. Such undefined symbols may be resolved not at
> > link-time but at runtime. In scanShlibUndefined, we do not resolves
> symbols,
> > but just mark symbols as ExportDynamic.
> >
> >>
> >>
> >> Cheers,
> >> Rafael
> >>
> >> On 27 October 2016 at 11:01, Eugene Leviant <evgeny.leviant at gmail.com>
> >> wrote:
> >> > evgeny777 created this revision.
> >> > evgeny777 added reviewers: ruiu, rafael.
> >> > evgeny777 added subscribers: grimar, ikudrin, llvm-commits.
> >> > evgeny777 set the repository for this revision to rL LLVM.
> >> > evgeny777 added a project: lld.
> >> >
> >> > When linking DSO lld doesn't resolve symbols from input DSOs in case
> >> > those symbols are lazy. Below is an example:
> >> >
> >> > input.so - has undefined symbol 'print'
> >> > libprint.a - defines 'print'
> >> >
> >> > ld.lld -shared -o output.so input.so libprint.a
> >> >
> >> > Even though libprint.a has definition of 'print', output.so will have
> it
> >> > undefined.
> >> > Later when you link application with output.so, you'll also have to
> >> > provide libprint.a
> >> >
> >> > This patch fixes it.
> >> >
> >> >
> >> > Repository:
> >> >   rL LLVM
> >> >
> >> > https://reviews.llvm.org/D26033
> >> >
> >> > Files:
> >> >   ELF/SymbolTable.cpp
> >> >   test/ELF/Inputs/shared4.s
> >> >   test/ELF/shared-lazy.s
> >> >
> >> >
> >> > Index: test/ELF/shared-lazy.s
> >> > ===================================================================
> >> > --- test/ELF/shared-lazy.s
> >> > +++ test/ELF/shared-lazy.s
> >> > @@ -0,0 +1,10 @@
> >> > +# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
> >> > +# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux
> >> > %p/Inputs/archive.s -o %t2.o
> >> > +# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux
> >> > %p/Inputs/shared4.s -o %t3.o
> >> > +# RUN: llvm-ar rcs %t2.a %t2.o
> >> > +# RUN: ld.lld -shared %t3.o -o %t3.so
> >> > +# RUN: ld.lld -shared %t.o %t2.a %t3.so -o %t.so
> >> > +# RUN: llvm-objdump -t %t.so | FileCheck --check-prefix=DEFINED %s
> >> > +
> >> > +# DEFINED: _start
> >> > +call _entry at plt
> >> > Index: test/ELF/Inputs/shared4.s
> >> > ===================================================================
> >> > --- test/ELF/Inputs/shared4.s
> >> > +++ test/ELF/Inputs/shared4.s
> >> > @@ -0,0 +1,3 @@
> >> > +.globl _shared
> >> > +_shared:
> >> > +  call _start at plt
> >> > Index: ELF/SymbolTable.cpp
> >> > ===================================================================
> >> > --- ELF/SymbolTable.cpp
> >> > +++ ELF/SymbolTable.cpp
> >> > @@ -549,9 +549,12 @@
> >> >  template <class ELFT> void SymbolTable<ELFT>::scanShlibUndefined() {
> >> >    for (SharedFile<ELFT> *File : SharedFiles)
> >> >      for (StringRef U : File->getUndefinedSymbols())
> >> > -      if (SymbolBody *Sym = find(U))
> >> > +      if (SymbolBody *Sym = find(U)) {
> >> >          if (Sym->isDefined())
> >> >            Sym->symbol()->ExportDynamic = true;
> >> > +        else if (Sym->isLazy())
> >> > +          addUndefined(U);
> >> > +      }
> >> >  }
> >> >
> >> >  // This function processes --export-dynamic-symbol and
> --dynamic-list.
> >> >
> >> >
> >
> >
>
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