Re: [PATCH glibc 1/3] glibc: Perform rseq registration at C startup and thread creation (v19)

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On May 26, 2020, at 10:57 AM, Florian Weimer fweimer@redhat.com wrote:

> * Mathieu Desnoyers:
> 
>>> Like the attribute, it needs to come right after the struct keyword, I
>>> think.  (Trailing attributes can be ambiguous, but not in this case.)
>>
>> Nope. _Alignas really _is_ special :-(
>>
>> struct _Alignas (16) blah {
>>         int a;
>> };
>>
>> p.c:1:8: error: expected ‘{’ before ‘_Alignas’
>>  struct _Alignas (16) blah {
> 
> Meh, yet another unnecessary C++ incompatibility.  C does not support
> empty structs, so I assume they didn't see the field requirement as a
> burden.

Indeed, it's weird.

> 
>> One last thing I'm planning to add in sys/rseq.h to cover acessing the
>> rseq_cs pointers with both the UAPI headers and the glibc struct rseq
>> declarations:
>>
>> /* The rseq_cs_ptr macro can be used to access the pointer to the current
>>    rseq critical section descriptor.  */
>> #ifdef __LP64__
>> # define rseq_cs_ptr(rseq) \
>>            ((const struct rseq_cs *) (rseq)->rseq_cs.ptr)
>> #else /* __LP64__ */
>> # define rseq_cs_ptr(rseq) \
>>            ((const struct rseq_cs *) (rseq)->rseq_cs.ptr.ptr32)
>> #endif /* __LP64__ */
>>
>> Does it make sense ?
> 
> Written this way, it's an aliasing violation.  I don't think it's very
> useful.

OK, I'll just remove it.

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com