Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
From:   Florian Weimer <fweimer@redhat.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     carlos <carlos@redhat.com>, 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>,
        Christian Brauner <christian.brauner@ubuntu.com>
Subject: Re: [RFC PATCH 2/4] rseq: Allow extending struct rseq
References: <20200714030348.6214-1-mathieu.desnoyers@efficios.com>
        <87mu42bepq.fsf@oldenburg2.str.redhat.com>
        <131549905.11442.1594731035989.JavaMail.zimbra@efficios.com>
        <87a7028d5u.fsf@oldenburg2.str.redhat.com>
        <2452161.11491.1594732791558.JavaMail.zimbra@efficios.com>
        <71f08b3a-56f5-0e0f-53b0-cc680f7e8181@redhat.com>
        <2053637148.14136.1594818777608.JavaMail.zimbra@efficios.com>
        <87y2nk29rp.fsf@oldenburg2.str.redhat.com>
        <882700738.14181.1594819884049.JavaMail.zimbra@efficios.com>
Date:   Wed, 15 Jul 2020 15:42:11 +0200
In-Reply-To: <882700738.14181.1594819884049.JavaMail.zimbra@efficios.com>
        (Mathieu Desnoyers's message of "Wed, 15 Jul 2020 09:31:24 -0400
        (EDT)")
Message-ID: <87mu4028uk.fsf@oldenburg2.str.redhat.com>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.3 (gnu/linux)
MIME-Version: 1.0
Content-Type: text/plain
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

* 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