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

[Rui Ueyama via llvm-commits]

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.
> >
> >
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20161027/ab891f5e/attachment.html>