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From:   Florian Weimer <fw@deneb.enyo.de>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     libc-alpha <libc-alpha@sourceware.org>, carlos <carlos@redhat.com>,
        Rich Felker <dalias@libc.org>,
        linux-api <linux-api@vger.kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Will Deacon <will.deacon@arm.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ben Maurer <bmaurer@fb.com>, Dave Watson <davejwatson@fb.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Paul <paulmck@linux.vnet.ibm.com>, Paul Turner <pjt@google.com>,
        Joseph Myers <joseph@codesourcery.com>
Subject: Re: [RFC PATCH glibc 4/8] glibc: Perform rseq(2) registration at C startup and thread creation (v15)
References: <20200319144110.3733-1-mathieu.desnoyers@efficios.com>
        <20200319144110.3733-5-mathieu.desnoyers@efficios.com>
        <874kukpf9f.fsf@mid.deneb.enyo.de>
        <2147217200.3240.1584633395285.JavaMail.zimbra@efficios.com>
Date:   Thu, 19 Mar 2020 17:03:39 +0100
In-Reply-To: <2147217200.3240.1584633395285.JavaMail.zimbra@efficios.com>
        (Mathieu Desnoyers's message of "Thu, 19 Mar 2020 11:56:35 -0400
        (EDT)")
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* Mathieu Desnoyers:

>> Can you use __has_include in <sys/rseq.h>, with a copy of the kernel
>> definitions if the kernel header is not available?
>
> Sure. Should I pull a verbatim copy of uapi linux/rseq.h into glibc ?
> If so, where should I put it ?

Probably into <sys/rseq.h>, perhaps with a construct like this
(untested):

#ifdef __has_include
# if __has_include ("linux/rseq.h")
#   define __GLIBC_HAVE_KERNEL_RSEQ
# endif
#else
# include <linux/version.h>
# if LINUX_VERSION_CODE >= KERNEL_VERSION (4, 18, 0)
#   define __GLIBC_HAVE_KERNEL_RSEQ
# endif
#endif

#ifdef __GLIBC_HAVE_KERNEL_RSEQ
# include <linux/rseq.h>
#else

… (fallback goes here)
#endif

We have an ongoing debate whether the fallback definition should use
__u64 or uint64_t.

You also need to add an assert that the compiler supports
__attribute__ ((aligned)) because ignoring it produces an
ABI-incompatible header.  The struct rseq/struct rseq_cs definitions
are broken, they should not try to change the alignment.

PS: I have Internet connection trouble.  Nobody should be worried if I
drop off the net for a while.  I understand this is quite a bad time
for that. 8-(