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Re: Preventing preemption of 'protected' symbols in GNU ld 2.26
- From: Szabolcs Nagy <szabolcs dot nagy at arm dot com>
- To: Ramana Radhakrishnan <ramana dot gcc at googlemail dot com>, Jeff Law <law at redhat dot com>
- Cc: Cary Coutant <ccoutant at gmail dot com>, "H.J. Lu" <hjl dot tools at gmail dot com>, Joe Groff <jgroff at apple dot com>, Alan Modra <amodra at gmail dot com>, Binutils <binutils at sourceware dot org>, <nd at arm dot com>
- Date: Fri, 15 Apr 2016 17:09:28 +0100
- Subject: Re: Preventing preemption of 'protected' symbols in GNU ld 2.26
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On 31/03/16 14:26, Ramana Radhakrishnan wrote:
> On Thu, Mar 31, 2016 at 1:52 AM, Jeff Law <law@redhat.com> wrote:
>> On 03/30/2016 06:40 PM, Cary Coutant wrote:
>>>>
>>>> It would help me immensely on the GCC side if things if you and Alan
>>>> could
>>>> easily summarize correct behavior and the impact if we were to just
>>>> revert
>>>> HJ's change. A testcase would be amazingly helpful too.
>>>
>>>
>>> It looks like it's not just the one change. There's this patch:
>>>
>>> https://gcc.gnu.org/ml/gcc-patches/2015-07/msg01871.html
>>>
>>> which took the idea that protected can still be pre-empted by a COPY
>>> relocation and extended it to three more targets that use COPY
>>> relocations.
>>>
>>> I wonder how many other patches have been based on the same
>>> misunderstanding?
(sorry i missed this thread)
this was not a misunderstanding.
that patch is necessary for correctness (odr) in
the presence of copy relocations as described in
https://gcc.gnu.org/ml/gcc-patches/2015-09/msg02365.html
and
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55012
this was a long standing code gen bug in gcc and was
about time to fix it (it was also broken in glibc's
dynamic linker, but e.g. not in musl libc).
(i don't see what is the issue with using the copy in
the main executable from a shared library, performance
is not a correctness issue, nor how it is possible to
avoid the copy relocs.)
>>
>> I don't think it was many -- I certainly recall the arm/aarch64 variant.
>> There may have been one other varasm.c change in this space or I might be
>> conflating it with the arm/aarch64 change. Tracking those down is naturally
>> part of this work.
>
> The glibc tests elf/tst-protected1{a,b}.c also need to be reviewed at
> the same time. IIUC, the reason the patch above went in were to fix
> failures on arm / aarch64 with those tests. I haven't yet worked out
> whether all this is the same issue.
>
> CC'ing Szabolcs.
>
> Thanks,
> Ramana
>
>>
>> jeff
>