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


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".

What is your opinion on changing BFD ld to follow what Gold appears to do:
sort GLOB_DAT relocations so that references to IFUNC symbols come last?

(I might be wrong that Gold deliberately sorts like that; I didn't check in
detail)

Thanks.
Alexander

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