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From:   Florian Weimer <fweimer@redhat.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        "H . Peter Anvin" <hpa@zytor.com>, Paul Turner <pjt@google.com>,
        linux-api@vger.kernel.org,
        Christian Brauner <christian.brauner@ubuntu.com>,
        David.Laight@ACULAB.COM, carlos@redhat.com,
        Peter Oskolkov <posk@posk.io>
Subject: Re: [PATCH v3 02/23] rseq: Introduce extensible rseq ABI
References: <20220729190225.12726-1-mathieu.desnoyers@efficios.com>
        <20220729190225.12726-3-mathieu.desnoyers@efficios.com>
Date:   Wed, 10 Aug 2022 08:33:58 +0200
In-Reply-To: <20220729190225.12726-3-mathieu.desnoyers@efficios.com> (Mathieu
        Desnoyers's message of "Fri, 29 Jul 2022 15:02:04 -0400")
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* Mathieu Desnoyers:

> Introduce the extensible rseq ABI, where the feature size supported by
> the kernel and the required alignment are communicated to user-space
> through ELF auxiliary vectors.
>
> This allows user-space to call rseq registration with a rseq_len of
> either 32 bytes for the original struct rseq size (which includes
> padding), or larger.
>
> If rseq_len is larger than 32 bytes, then it must be large enough to
> contain the feature size communicated to user-space through ELF
> auxiliary vectors.

I don't think this works with the glibc extension mechanism because
__rseq_size does not change until the padding is exhausted.

I think you'll need to add the suggested flags to the auxiliary vector,
and then we can use that during registration and also communicate these
flags via __rseq_flags.

Size and alignment can be stored in a single auxiliary vector entry.

Thanks,
Florian