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

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Message-ID: <87mu4028uk.fsf@oldenburg2.str.redhat.com>
Date:   Wed, 15 Jul 2020 15:42:11 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     carlos <carlos@...hat.com>, 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>
Subject: Re: [RFC PATCH 2/4] rseq: Allow extending struct rseq

* Mathieu Desnoyers:

> So indeed it could be done today without upgrading the toolchains by
> writing custom assembler for each architecture to get the thread's
> struct rseq. AFAIU the ABI to access the thread pointer is fixed for
> each architecture, right ?

Yes, determining the thread pointer and access initial-exec TLS
variables is baked into the ABI.

> How would this allow early-rseq-adopter libraries to interact with
> glibc ?

Under all extension proposals I've seen so far, early adopters are
essentially incompatible with glibc rseq registration.  I don't think
you can have it both ways.

Thanks,
Florian