Re: [PATCH glibc 1/3] glibc: Perform rseq registration at C startup and thread creation (v20)

[Mathieu Desnoyers via Libc-alpha <libc-alpha@sourceware.org>]

----- On Jun 3, 2020, at 8:31 AM, Florian Weimer fweimer@redhat.com wrote:

> * Mathieu Desnoyers:
> 
>> ----- On Jun 3, 2020, at 8:05 AM, Florian Weimer fweimer@redhat.com wrote:
>>
>>> * Mathieu Desnoyers:
>>> 
>>>> +#ifdef __cplusplus
>>>> +# if  __cplusplus >= 201103L
>>>> +#  define __rseq_static_assert(expr, diagnostic) static_assert (expr,
>>>> diagnostic)
>>>> +#  define __rseq_alignof(type)                   alignof (type)
>>>> +#  define __rseq_alignas(x)                      alignas (x)
>>>> +#  define __rseq_tls_storage_class               thread_local
>>>> +# endif
>>>> +#elif (defined __STDC_VERSION__ ? __STDC_VERSION__ : 0) >= 201112L
>>>> +# define __rseq_static_assert(expr, diagnostic)  _Static_assert (expr,
>>>> diagnostic)
>>>> +# define __rseq_alignof(type)                    _Alignof (type)
>>>> +# define __rseq_alignas(x)                       _Alignas (x)
>>>> +# define __rseq_tls_storage_class                _Thread_local
>>>> +#endif
>>> 
>>> This does not seem to work.  I get this with GCC 9:
>>> 
>>> In file included from /tmp/cih_test_gsrKbC.cc:8:0:
>>> ../sysdeps/unix/sysv/linux/sys/rseq.h:42:50: error: attribute ignored
>>> [-Werror=attributes]
>>> #  define __rseq_alignas(x)                      alignas (x)
>>>                                                  ^
>>> ../sysdeps/unix/sysv/linux/sys/rseq.h:122:14: note: in expansion of macro
>>> ‘__rseq_alignas’
>>>     uint32_t __rseq_alignas (32) version;
>>>              ^
>>
>> Is that when compiling C or C++ code ? If it's C code, I would expect
>> "_Alignas" to be used, not "alignas".
>>
>> Which exact version of gcc do you use ?
> 
> C++ code.  CXX was set to this compiler at configure time:
> 
> gcc version 9.3.1 20200408 (Red Hat 9.3.1-2) (GCC)

I think I found the culprit: it should be:

__rseq_alignas (32) uint32_t version;

rather than the other way around.

> 
>>> In any case, these changes really have to go into the UAPI header first.
>>> Only the __thread handling should remain.  Otherwise, we'll have a tough
>>> situation on our hands changing the UAPI header, without introducing
>>> macro definition conflicts.  I'd suggest to stick to the aligned
>>> attribute for the time being, like the current UAPI headers.

OK, so I just remove the __rseq_alignas for now and use "aligned()" instead
like the UAPI header. I plan to keep the other macros for now.

>>
>> OK. Should I do that in a separate patch, or you do it on top of my patchset,
>> or should I re-spin another round of the series ?
> 
> I think the initial commit should mirror the current UAPI header
> contents.
> 
> Keep the macros for the UAPI patch though. 8-)  We can pick up these
> changes once they have been merged into Linux.

OK,

Thanks!

Mathieu

> 
> Thanks,
> Florian

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