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

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Message-ID: <87lg0tosfz.fsf@concordia.ellerman.id.au>
Date:   Tue, 02 Apr 2019 17:02:40 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     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>
Cc:     carlos <carlos@...hat.com>, Florian Weimer <fweimer@...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)

Mathieu Desnoyers <mathieu.desnoyers@...icios.com> writes:
> Hi Carlos,
>
> ----- On Mar 22, 2019, at 4:09 PM, Carlos O'Donell codonell@...hat.com wrote:
...
>
> [...]
>>> +++ b/sysdeps/unix/sysv/linux/powerpc/bits/rseq.h
> [...]
>>> +/* Signature required before each abort handler code.  */
>>> +#define RSEQ_SIG 0x53053053
>> 
>> Why isn't this an opcode specific to power?
>
> On powerpc 32/64, the abort is placed in a __rseq_failure executable section:
>
> #define RSEQ_ASM_DEFINE_ABORT(label, abort_label)                               \
>                 ".pushsection __rseq_failure, \"ax\"\n\t"                       \
>                 ".long " __rseq_str(RSEQ_SIG) "\n\t"                            \
>                 __rseq_str(label) ":\n\t"                                       \
>                 "b %l[" __rseq_str(abort_label) "]\n\t"                         \
>                 ".popsection\n\t"
>
> That section only contains snippets of those trampolines. Arguably, it would be
> good if disassemblers could find valid instructions there. Boqun Feng could perhaps
> shed some light on this signature choice ? Now would be a good time to decide
> once and for all whether a valid instruction would be a better choice.

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).

cheers