linux-kernel - Re: [RFC PATCH 2/4] rseq: Allow extending struct rseq

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Message-ID: <87k0z5xpau.fsf@mid.deneb.enyo.de>
Date:   Wed, 15 Jul 2020 08:31:05 +0200
From:   Florian Weimer <fw@...eb.enyo.de>
To:     Chris Kennelly <ckennelly@...gle.com>
Cc:     Peter Oskolkov <posk@...gle.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Peter Oskolkov <posk@...k.io>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        paulmck <paulmck@...ux.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        "H. Peter Anvin" <hpa@...or.com>, Paul Turner <pjt@...gle.com>,
        linux-api <linux-api@...r.kernel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        carlos <carlos@...hat.com>
Subject: Re: [RFC PATCH 2/4] rseq: Allow extending struct rseq

* Chris Kennelly:

> When glibc provides registration, is the anticipated use case that a
> library would unregister and reregister each thread to "upgrade" it to
> the most modern version of interface it knows about provided by the
> kernel?

Absolutely not, that is likely to break other consumers because an
expected rseq area becomes dormant instead.

> There, I could assume an all-or-nothing registration of the new
> feature--limited only by kernel availability for thread
> homogeneity--but inconsistencies across early adopter libraries would
> mean each thread would have to examine its own TLS to determine if a
> feature were available.

Exactly.  Certain uses of seccomp can also have this effect,
presenting a non-homogeneous view.