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

[Rafael Espíndola via llvm-commits]

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.

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.
>
>