[PATCH] Re: Don't relax undefined symbols on ppc.

[Douglas B Rupp]

On 07/23/2018 07:01 PM, Alan Modra wrote:
> On Mon, Jul 23, 2018 at 07:38:44AM -0700, Douglas B Rupp wrote:
>>
>>
>> On 07/09/2018 06:27 PM, Alan Modra wrote:
>>> On Mon, Jul 09, 2018 at 01:58:33PM -0700, Douglas B Rupp wrote:
>>>> There is a problem with --relax in the partial link trying to relax the
>>>> undefined symbols, leading to the RTP failing at startup with signal 11.
>>>
>>> Please describe the problem in detail.  ld -r --relax non-pic is
>>> supposed to work for ppc32, with a branch to an undefined symbol being
>>> resolved to a trampoline which has relocations to the symbol.  Are the
>>> emitted relocations wrong?
>>>
>>>> I solicit comments on the attached patch, which causes undefined symbols to
>>>> be ignored during the relax process.  Leaving them to be relaxed and
>>>> resolved during the final link.
>>>
>>> If ld -r mashes together sections such that the resulting output
>>> section size exceeds the reach of a branch, then it may not be
>>> possible to for branches to reach trampolines placed at the end of the
>>> section.  That's the problem ld -r --relax is trying to solve, by
>>> placing trampolines at the end of each input section.
>>>
>>
>> The problem is that "tsec" is the undefined section and basically a no-op in
>> the following line, so existing trampolines are found when they shouldn't be
>> found.
>>
>> elf32-ppc.c: if (f->tsec == tsec && f->toff == toff)
> 
> But for undefined syms and relocatable linking, toff is the symbol
> index.  Or at least it is supposed to be.  I do see a bug where the
> reloc addend could be added to toff, but that is unlikely to trigger.
> 
>>
>> The attached patch fixes it.
> 
> I don't see how adding "abfd" to the branch fixup struct can possibly
> help.  "abfd" is a constant for any one invocation of
> ppc_elf_relax_section!
> 

I don't know what to tell you, it does fix the problem.
I can take another look at it at some future date and try to provide 
more detail.
--Doug