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

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Mon, Aug 17, 2015 at 1:22 PM, Alexander Monakov <amonakov@ispras.ru> wrote:
> On Mon, 17 Aug 2015, H.J. Lu wrote:
>> >> >> > Surely such a counter-argument applies equally to JUMP_SLOT relocations?
>> >> >> > Imagine that 'extern int f(void)' in bug 13302 that you've linked turns
>> >> >> > out to be an IFUNC symbol at runtime.
>> >> >> >
>> >> >> > extern int f(void);
>> >> >> >
>> >> >> > void alt1(void) { }
>> >> >> > void alt2(void) { }
>> >> >> >
>> >> >> > static void (*resolve (void)) (void)
>> >> >> > {
>> >> >> >   return f() ? alt1 : alt2;
>> >> >> > }
>> >> >> >
>> >> >> > void fct(void) __attribute__ ((ifunc ("resolve")));
>> >> >> >
>> >> >>
>> >> >> The difference here is symbol definition for GLOB_DAT is known
>> >> >> only at run-time while relocations are known at link-time.
>> >> >
>> >> > Sorry, I just don't see what point you're trying to make here.
>> >> >
>> >> > Anyway.  Suppose 'resolve()' is compiled with -fno-plt.  Then instead of
>> >> > JUMP_SLOT relocation to 'f' it'll have a GLOB_DAT relocation to 'f', correct?
>> >> > What's going to happen then?
>> >> >
>> >>
>> >> I believe it is OK since GLOB_DAT relocations are performed before
>> >> IRELATIVE relocations in .rela.plt / .rel.plt section.
>> >
>> > OK.  Now further suppose that we are
>> >
>> > a) calling 'fct' with -fno-plt, or
>> > b) taking the address of 'fct' (as in Ondrey's original example)
>> >
>> > In both cases we're going to have a GLOB_DAT relocation to 'fct'.
>> >
>> > So we have a GLOB_DAT relocation to 'fct', which is an IFUNC symbol with
>> > 'resolve' as its resolver, and the resolver invokes 'f', using a GLOB_DAT
>> > relocation.
>> >
>> > Are you saying it's not supposed to work?
>>
>> They won't work because
>>
>>  void alt1(void) { }
>>  void alt2(void) { }
>>
>> which has nothing to do with fct.  They work if alt1 and alt2 are internal
>> to DSO.
>
> OK.  Suppose that alt1 and alt2 are indeed internal.  You're saying that it's
> supposed to work.  But then GLOB_DAT relocation to 'f' should be processed
> prior to GLOB_DAT relocation to 'fct', correct?  If you do it in the opposite
> order, the resolver will not be able to invoke 'f'.
>

It should work.  If not, please open a bug report and CC me.

Thanks.

-- 
H.J.