[RFC] Prevent tailcall optimizations of libdl functions

[Jeff Law]

On 01/25/2017 07:32 AM, Florian Weimer wrote:
> On 01/25/2017 03:08 PM, Rich Felker wrote:
>
>> FWIW this technique is non-conforming, at least for any standard
>> functions it's used on, since it breaks pointer equality. A conforming
>> program can evaluate p1==p2, where p1=dlsym is assigned in one dso and
>> p2=dlsym is assigned in another, and must see equality on a conforming
>> implementation. As such I think this hack needs to be removed from
>> other functions like stat.
>
> Is there a specific requirement in POSIX to that effect, or is it an
> accident as the result of how POSIX and ISO C interact?
>
> As far as I can tell, ISO C specifies function pointer equality by
> saying that pointers compare equal iff they refer to the “same […]
> function”, but the standard does not seem to define what makes two
> functions the same.
>
> (But even if it is fully conforming or acceptable for other reasons, I
> dislike the __dso_handle technique because it is too complex.)
I've always interpreted it as "if I dereference the function pointers, 
do I get into the same function".  It's not proper standards language, 
but you get the basic idea.

The 32bit HPUX runtime had fptr descriptors which were not unique and 
were only used for a subset of indirect calls.  So you couldn't just 
compare function pointers.  You can do call into a special routine to 
compute/extract the actual address for comparison purposes.

Jeff