Re: [RFC] Prevent tailcall optimizations of libdl functions

[Carlos O'Donell <carlos at redhat dot com>]

On 01/25/2017 11:59 AM, Jeff Law wrote:
> 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.

We do the same for 32-bit hppa on Linux (along with a host of other weird
things).

Honestly I don't know why they didn't just bite the bullet and do unique
OPDs like POWER and make the comparison simpler.

-- 
Cheers,
Carlos.