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

[Alexander Monakov <amonakov at ispras dot ru>]

On Mon, 17 Aug 2015, H.J. Lu wrote:
> On Mon, Aug 17, 2015 at 8:54 AM, Alexander Monakov <amonakov@ispras.ru> wrote:
> > On Tue, 11 Aug 2015, H.J. Lu wrote:
> >
> >> On Tue, Aug 11, 2015 at 1:30 PM, OndÅej BÃlka <neleai@seznam.cz> 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 <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.
> >> >>
> >> > 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.

You appear to be arguing from a standpoint that IFUNC is *exclusively* for
choosing a CPU-specific variant.  Why do you reject the possibility of using
IFUNC for different purposes?  I gave one example earlier.

Thanks.
Alexander