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Date: Fri, 16 Sep 2022 23:32:38 +0200
From: Florian Weimer <fw@...eb.enyo.de>
To: Chris Kennelly <ckennelly@...gle.com>
Cc: libc-coord@...ts.openwall.com, Mathieu Desnoyers
<mathieu.desnoyers@...icios.com>, "carlos@...hat.com"
<carlos@...hat.com>, libc-alpha <libc-alpha@...rceware.org>,
szabolcs.nagy@....com
Subject: Re: Re: RSEQ symbols: __rseq_size, __rseq_flags vs
__rseq_feature_size
* Chris Kennelly:
>> If the kernel does not currently overwrite the padding, glibc can do
>> its own per-thread initialization there to support its malloc
>> implementation (because the padding is undefined today from an
>> application perspective). That is, we would initialize these
>> invisible vCPU IDs the same way we assign arenas today. That would
>> cover this specific malloc use case only, of course.
> If a user program updates to a new kernel before glibc does, would it be
> able to easily take advantage of it?
No, as far as I understand it, there is presently no signaling from
kernel to applications that bypasses the rseq area registration. So
the only thing you could do is to unregister and re-register with a
compatible value. And that is of course quite undefined and assumes
that you can do this early enough during the life-time of each thread.
But if we have the extension handshake, I'll expect us to backport it
quite widely, after some time to verify that it works with CRIU etc.
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