[PATCH] Remove unnecessary IFUNC dispatch for __memset_chk.

[Alexander Monakov]

On Mon, 17 Aug 2015, H.J. Lu wrote:

> On Mon, Aug 17, 2015 at 9:33 AM, Alexander Monakov <amonakov@ispras.ru> wrote:
> > 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, Ondřej Bílka <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.
> >
> 
> I said " IFUNC selector should be deterministic at run-time on a given
> hardware."
> You can use it for other purpose as long as it is deterministic.  The
> whole IFUNC
> implementation on x86 is built on this assumption.

Earlier you rejected the idea that a resolver can use an externally visible
symbol.  I just don't see why you say it might be non-deterministic.  One
can inspect the executable, LD_PRELOAD'ed modules, etc. to see whether they
are going to provide an interposing definition of a symbol or not.  What is
non-deterministic in there?

Thanks.
Alexander