RFC: Add DT_FLAGS_2 and DF_2_GNU_IFUNC

[Florian Weimer]

On 05/26/2018 12:09 AM, H.J. Lu wrote:

>> This is just the old known issue related to symbol interposition and IFUNC
>> resolvers, where even complete DT_NEEDED information is not sufficient to
>> perform the relocations in the correct order.
>>
>> A single bit will not solve this.  In your reproducer, add an IFUNC resolver
>> to libbar.so, and I'm sure the bug will resurface again.
> 
> Can you add a complete testcase to
> 
> https://sourceware.org/bugzilla/show_bug.cgi?id=23240

Let's not clutter Bugzilla unnecessarily.

Try this with your patched toolchain:


void
bar (char *dst, const char *src, unsigned int size)
{
   __builtin_memmove (dst, src, size);
}

void unused_func (void) __attribute__ ((ifunc ("resolve_unused_func")));

static void *
resolve_unused_func (void)
{
   return 0;
}


It should result in libbar.so having the DF_2_GNU_IFUNC flag (please 
verify with readelf).  I expect that with your ld.so patch, we again 
relocate libbar.so first, leading to exactly the same crash.

I hope this makes it clearer what I'm worried about.

Thanks,
Florian