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Re: [PATCH] Remove unnecessary IFUNC dispatch for __memset_chk.
- From: "H.J. Lu" <hjl dot tools at gmail dot com>
- To: Alexander Monakov <amonakov at ispras dot ru>
- Cc: OndÅej BÃlka <neleai at seznam dot cz>, Zack Weinberg <zackw at panix dot com>, Andreas Schwab <schwab at linux-m68k dot org>, GNU C Library <libc-alpha at sourceware dot org>
- Date: Tue, 11 Aug 2015 12:55:31 -0700
- Subject: Re: [PATCH] Remove unnecessary IFUNC dispatch for __memset_chk.
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On Tue, Aug 11, 2015 at 12:41 PM, Alexander Monakov <amonakov@ispras.ru> wrote:
> On Tue, 11 Aug 2015, H.J. Lu wrote:
>> > Didn't know its explicitly forbidden to use nonstatic resolvers. Do we document that requirement somewhere?
>> > If not then we should add it to documentation.
>>
>> It makes no senses for IFUNC selector to return the address of
>> a global function, which may be preempted at run-time.
>
> That would be imposing a policy while providing a mechanism.
>
> I think there are plausible scenarios where returning a global function from
> an ifunc resolver would be natural.
>
> To give a specific example, imagine an ifunc symbol
> 'printf_fortified_opportunistically', which resolves to 'printf_chk' if it is
> available in the global namespace, or normal 'printf' otherwise.
>
> Such a symbol could be in any DSO, not necessarily libc, and thus it has to be
> able to pick up global symbol definitions.
>
By the nature of preemption, you have NO control over which definition
will be used at run-time. It can be literally anything, including segfault.
--
H.J.