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

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Date:   Tue, 26 May 2020 16:57:56 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     libc-alpha <libc-alpha@...rceware.org>,
        Rich Felker <dalias@...c.org>,
        linux-api <linux-api@...r.kernel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ben Maurer <bmaurer@...com>, Dave Watson <davejwatson@...com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul <paulmck@...ux.vnet.ibm.com>, Paul Turner <pjt@...gle.com>,
        Joseph Myers <joseph@...esourcery.com>
Subject: Re: [PATCH glibc 1/3] glibc: Perform rseq registration at C startup and thread creation (v19)

* 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.

> 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.

Thanks,
Florian