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Date:   Wed, 15 Jul 2020 12:59:43 +0200
From:   Christian Brauner <christian.brauner@ubuntu.com>
To:     Florian Weimer <fw@deneb.enyo.de>
Cc:     Chris Kennelly <ckennelly@google.com>,
        Peter Oskolkov <posk@google.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Peter Oskolkov <posk@posk.io>,
        Peter Zijlstra <peterz@infradead.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        paulmck <paulmck@linux.ibm.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Paul Turner <pjt@google.com>,
        linux-api <linux-api@vger.kernel.org>, carlos <carlos@redhat.com>
Subject: Re: [RFC PATCH 2/4] rseq: Allow extending struct rseq
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On Wed, Jul 15, 2020 at 08:31:05AM +0200, Florian Weimer wrote:
> * 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.

Fwiw, I pointed this out in the discussions that led up to this
patchset. I don't see how this can work if threads don't check for their
feature set.

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

Good point. There might be threads with a seccomp filter that would
block rseq features is what you mean, I assume.

Christian