Specify how undefined weak symbol should be resolved in executable

[H.J. Lu]

On Mon, Feb 22, 2016 at 8:25 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> Currently when building executable, undefined weak symbol is resolved
> differently, depending on if the relocatable input is compiled with PIC or
> not:
>
> hjl@gnu-tools-1 weak-1]$ cat x.c
> extern void __attribute__ ((weak)) fun (void);
> int main (void)
> {
>   if (&fun != 0)
>     fun ();
>   return 0;
> }
> [hjl@gnu-tools-1 weak-1]$ make
> gcc -B./ -O2 -o x x.c
> gcc -B./ -O2 -o y x.c -fPIC
> readelf -r x
>
> Relocation section '.rela.dyn' at offset 0x328 contains 1 entries:
>   Offset          Info           Type           Sym. Value    Sym. Name + Addend
> 000000600870  000200000006 R_X86_64_GLOB_DAT 0000000000000000 __gmon_start__ + 0
>
> Relocation section '.rela.plt' at offset 0x340 contains 1 entries:
>   Offset          Info           Type           Sym. Value    Sym. Name + Addend
> 000000600890  000100000007 R_X86_64_JUMP_SLO 0000000000000000
> __libc_start_main@GLIBC_2.2.5 + 0
>
> There is no dynamic relocation against fun, whose value is
> resolved to zero at link-time.
>
> readelf -r y
>
> Relocation section '.rela.dyn' at offset 0x348 contains 2 entries:
>   Offset          Info           Type           Sym. Value    Sym. Name + Addend
> 0000006008a0  000100000006 R_X86_64_GLOB_DAT 0000000000000000 fun + 0
> 0000006008a8  000300000006 R_X86_64_GLOB_DAT 0000000000000000 __gmon_start__ + 0
>
> Relocation section '.rela.plt' at offset 0x378 contains 1 entries:
>   Offset          Info           Type           Sym. Value    Sym. Name + Addend
> 0000006008c8  000200000007 R_X86_64_JUMP_SLO 0000000000000000
> __libc_start_main@GLIBC_2.2.5 + 0
> [hjl@gnu-tools-1 weak-1]$
>
> There is dynamic relocation against fun, whose value is resolved
> at runt-time. We should make it consistent, regardless if input
> relocatable file is compiled with PIC or not.
>
> I raised the issue at gABI:
>
> https://groups.google.com/d/msg/generic-abi/Ckq19PfLxyk/eCbXPofkAQAJ
>
> But if gABI change is incompatible with other OSes, we need to
> specify it in GNU ABI.

We should extend GNU ABI to:

When creating executable, if dynamic relocation is available at
run-time, the link
editor should generate dynamic relocations against unresolved weak symbols so
that their values will be resolved at run-time.

-- 
H.J.