Bug#792921: [sparc64] linking against libx264 crashes runtime linker

[James Clarke]

Control: reassign -1 binutils
Control: retitle -1 ld on sparc64 converts R_SPARC_32 to R_SPARC_RELATIVE
Control: affects -1 src:x264
Control: tags -1 upstream patch
Control: forwarded -1 asdf

On Tue, 21 Jul 2015 22:35:49 +0200 Andreas Cadhalpun <andreas.cadhalpun at googlemail.com> wrote:
> On 21.07.2015 21:44, Carlos O'Donell wrote:
> > Does the problem always reproduce or just sometimes?
>
> Always.
>
> > If it's just sometimes then it's much much harder to figure out what's wrong.
>
> If it were just sometimes, I wouldn't have been able to trace it to libx264...
>
> > You'll need a dedicated person to track down exactly what is the
> > concurrency issue and why it's failing.
>
> What I don't understand is why it only fails for libx264, but e.g. libx265 is fine.
> Also, I don't see how the code, where the crash happens, can possibly crash:
> From do-rel.h [1]:
>  85:      const ElfW(Rel) *relative = r;
>  86:      r += nrelative;
> [...]
> 111:            for (; relative < r; ++relative)
> 112:              DO_ELF_MACHINE_REL_RELATIVE (map, l_addr, relative);
>
> gdb claims it crashes at line 111.

It does indeed crash there, although on line 112 not line 111.
DO_ELF_MACHINE_REL_RELATIVE is defined to be elf_machine_rel_relative, which in
this case has been defined to elf_machine_rela_relative, which is an
always-inline function that will just perform some calculations and a single
assignment to the relocation address.

The reason it crashes is that libx264 has R_SPARC_RELATIVE relocations which
are not 8-byte aligned. This is because ld incorrectly converts some R_SPARC_32
relocations into R_SPARC_RELATIVE ones, which is only valid for 32-bit Sparc.

I have attached a patch which seems to fix this particular case; recompiling
libx264, linking main.c with it and running the resulting main no longer
leads to a bus error, and terminates with exit code 0.

James
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