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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: Mon, 17 Aug 2015 08:59:39 -0700
- Subject: Re: [PATCH] Remove unnecessary IFUNC dispatch for __memset_chk.
- Authentication-results: sourceware.org; auth=none
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On Mon, Aug 17, 2015 at 8:54 AM, Alexander Monakov <email@example.com> wrote:
> On Tue, 11 Aug 2015, H.J. Lu wrote:
>> On Tue, Aug 11, 2015 at 1:30 PM, OndÅej BÃlka <firstname.lastname@example.org> wrote:
>> > On Tue, Aug 11, 2015 at 12:55:31PM -0700, H.J. Lu wrote:
>> >> On Tue, Aug 11, 2015 at 12:41 PM, Alexander Monakov <email@example.com> 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.
>> > Could you document that somewhere? Sorry for earlier confusion where I
>> > thought you meant resolver must be static. Its about that users could
>> > have similar ideas like making symbol global for silly reasons like
>> > be able to profile variant with LD_PRELOAD.
>> Something like:
>> If the returned function has non-local binding, the run-time behavior
>> is undefined
>> as it may be preempted at run-time.
> I still don't understand why you point that out specifically for IFUNC.
> Symbol preemption is normal for ELF symbol lookups. To me it sounds like
> saying that "with LD_PRELOAD, the run-time behavior is undefined, as some
> symbols are preempted at run-time".
IFUNC selector should be deterministic at run-time on a given hardware.
If the function returned by IFUNC selector can be preempted, IFUNC isn't
the appropriate approach for this use case.