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From:   Florian Weimer <fw@deneb.enyo.de>
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 <brauner@kernel.org>,
        David.Laight@ACULAB.COM, carlos@redhat.com,
        Peter Oskolkov <posk@posk.io>,
        Alexander Mikhalitsyn <alexander@mihalicyn.com>,
        Chris Kennelly <ckennelly@google.com>
Subject: Re: [PATCH 05/30] selftests/rseq: Use ELF auxiliary vector for
 extensible rseq
References: <20221122203932.231377-1-mathieu.desnoyers@efficios.com>
        <20221122203932.231377-6-mathieu.desnoyers@efficios.com>
Date:   Wed, 04 Jan 2023 20:14:42 +0100
In-Reply-To: <20221122203932.231377-6-mathieu.desnoyers@efficios.com> (Mathieu
        Desnoyers's message of "Tue, 22 Nov 2022 15:39:07 -0500")
Message-ID: <87a62yun6l.fsf@mid.deneb.enyo.de>
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* Mathieu Desnoyers:

> +static
> +unsigned int get_rseq_feature_size(void)
> +{
> +	unsigned long auxv_rseq_feature_size, auxv_rseq_align;
> +
> +	auxv_rseq_align = getauxval(AT_RSEQ_ALIGN);
> +	assert(!auxv_rseq_align || auxv_rseq_align <= RSEQ_THREAD_AREA_ALLOC_SIZE);
> +
> +	auxv_rseq_feature_size = getauxval(AT_RSEQ_FEATURE_SIZE);
> +	assert(!auxv_rseq_feature_size || auxv_rseq_feature_size <= RSEQ_THREAD_AREA_ALLOC_SIZE);
> +	if (auxv_rseq_feature_size)
> +		return auxv_rseq_feature_size;
> +	else
> +		return ORIG_RSEQ_FEATURE_SIZE;
> +}

Do you intend to use the auxiliary vector as the userspace handshake
for glibc-managed rseq, too?  I don't think it works if the kernel
overtakes glibc.  Or is there some other approach shown in the series
that I missed?

Maybe we should just skip the existing padding and use it only for
some vaguely kernel-internal purpose (say through a vDSO helper), so
that it is less of an issue how to communicate the presence of these
fields to userspace.