linux-kernel - Re: [PATCH 1/4] glibc: Perform rseq(2) registration at C startup and thread creation (v7)

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Message-ID: <87pnq4zxyj.fsf@oldenburg2.str.redhat.com>
Date:   Tue, 02 Apr 2019 09:08:04 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Carlos O'Donell <codonell@...hat.com>,
        Paul Burton <paul.burton@...s.com>,
        Will Deacon <will.deacon@....com>,
        Boqun Feng <boqun.feng@...il.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Russell King <linux@...linux.org.uk>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>, carlos <carlos@...hat.com>,
        Joseph Myers <joseph@...esourcery.com>,
        Szabolcs Nagy <szabolcs.nagy@....com>,
        libc-alpha <libc-alpha@...rceware.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ben Maurer <bmaurer@...com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Dave Watson <davejwatson@...com>, Paul Turner <pjt@...gle.com>,
        Rich Felker <dalias@...c.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>
Subject: Re: [PATCH 1/4] glibc: Perform rseq(2) registration at C startup and thread creation (v7)

* Michael Ellerman:

> I'm a bit vague on what we're trying to do here.
>
> But it seems like you want some sort of "eye catcher" prior to the branch?
>
> That value is a valid instruction on current CPUs (rlwimi.
> r5,r24,6,1,9), and even if it wasn't it could become one in future.
>
> If you change it to 0x8053530 that is both a valid instruction and is a
> nop (conditional trap immediate but with no conditions set).

I think we need something that is very unlikely to appear in the
instruction stream.  It's just a marker.  The instruction will never be
executed, and it does not have to be a trap, either (I believe that a
standard trap instruction would be a bad choice).

Thanks,
Florian