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Re: [PATCH] Remove unnecessary IFUNC dispatch for __memset_chk.

On Tue, Aug 11, 2015 at 1:30 PM, OndÅej BÃlka <> 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 <> 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.


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