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

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Date:   Wed, 27 Mar 2019 10:16:08 +0100
From:   Martin Schwidefsky <schwidefsky@...ibm.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     "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>,
        Russell King <linux@...linux.org.uk>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        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)

On Mon, 25 Mar 2019 11:54:32 -0400 (EDT)
Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:

> >> +++ b/sysdeps/unix/sysv/linux/s390/bits/rseq.h  
> [...]
> >> +
> >> +/* Signature required before each abort handler code.  */
> >> +#define RSEQ_SIG 0x53053053  
> > 
> > Why not a s390 specific value here?  
> 
> s390 also has the abort handler in a __rseq_failure section:
> 
> #define RSEQ_ASM_DEFINE_ABORT(label, teardown, abort_label)             \
>                 ".pushsection __rseq_failure, \"ax\"\n\t"               \
>                 ".long " __rseq_str(RSEQ_SIG) "\n\t"                    \
>                 __rseq_str(label) ":\n\t"                               \
>                 teardown                                                \
>                 "j %l[" __rseq_str(abort_label) "]\n\t"                 \
>                 ".popsection\n\t"
> 
> Same question applies as powerpc: since disassemblers will try to decode
> that instruction, would it be better to define it as a valid one ?
> 
> [...]

A 4-byte sequence starting with 0x53 is decoded as a "diebr" instruction.
And please replace that "j %l[...]" with a "jg %l[...]", the branch target
range of the "j" instruction is 64K, not enough for the general case.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.