[RFC] Prevent tailcall optimizations of libdl functions

[Jeff Law]

On 01/25/2017 09:54 AM, Rich Felker wrote:
> On Wed, Jan 25, 2017 at 08:42:44AM -0700, Jeff Law wrote:
>> 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.
>
> That's not a conforming C implementation. C specifies working
> comparison of function pointers.
This was all hidden by the compiler.  At the C level you just compared 
the function pointers.

jeff