Re: code sections beyond .text skipped from alternatives_smp_module_add

From: Deep Debroy
Date: Wed Jun 22 2011 - 17:58:20 EST

On Wed, Jun 22, 2011 at 10:27 AM, Deep Debroy <ddebroy@xxxxxxxxx> wrote:
> On Wed, Jun 22, 2011 at 6:21 AM, Konrad Rzeszutek Wilk
> <konrad.wilk@xxxxxxxxxx> wrote:
>>> > Looking at the code, in module_finalize for x86, only .text seems to
>>> > be getting picked for the patching of lock prefixes while other
>>> > sections such as .exit.text or .init.text are not. Is there a reason
>>> > we skip the other *.text code sections from the lock patches? Would
>>> + Gerd Hoffmann who introduced the SMP patching code below back in Jan
>>> 2006 as part of 2.6.15.
>> Whoa, long time ago.
>>> Any comments on why patching of smp_lock prefixes should be restricted
>>> to .text and not other *.text code sections?
>> It could be that at that time the .exit.text or .init.text did not exist.
>> As in, the patching code just hasn't kept up. One way of checking that
>> is just finding the ancient 2.6.15 code and seeing if there is any
>> mention of those extra segments.
> Thanks Konrad. One slight correction: after rechecking the kernel
> sources, it appears the smp lock prefix code first made it's
> appearance in the official trees during 2.6.18. In any case, going
> back even to 2.6.16 sources, layout_sections in module.c specially
> handled .init prefixed sections from the rest i.e. core sections.
> Further, the module struct in include/module/linux.h seems to have had
> members such as init_text_size which suggests atleast .init.text did
> exit back then as well. While I didn't find any crumbs in the code
> that point to the existence of a .exit.text (besides a function
> pointer called exit which most likely ended up in the .exit.text), the
> ELF headers for Centos 5.6 kernel objects (which uses the 2.6.18
> kernel) typically have a .exit.text.
>> Do you have a patch to fix this?
> I can work on that. Just wanted to first make sure that there wasn't
> any specific reason to avoid patching non .text sections.
> Thanks,
> Deep

Some further digging through messages revealed a patch from Randy
Dunlap in June 2006: "[PATCH] ignore smp_locks section warnings from
init/exit code." Given this patch came in after the smp locking
hotpatching mechanism was introduced, there may have been an
assumption that instructions that results in entries in smp_locks
relocations in the object file should not exist in the init/exit.text

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