Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Wed, 27 Mar 2019 10:16:08 +0100
From:   Martin Schwidefsky <schwidefsky@de.ibm.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     "Carlos O'Donell" <codonell@redhat.com>,
        Paul Burton <paul.burton@mips.com>,
        Will Deacon <will.deacon@arm.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Russell King <linux@armlinux.org.uk>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        carlos <carlos@redhat.com>, Florian Weimer <fweimer@redhat.com>,
        Joseph Myers <joseph@codesourcery.com>,
        Szabolcs Nagy <szabolcs.nagy@arm.com>,
        libc-alpha <libc-alpha@sourceware.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ben Maurer <bmaurer@fb.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Dave Watson <davejwatson@fb.com>, Paul Turner <pjt@google.com>,
        Rich Felker <dalias@libc.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-api <linux-api@vger.kernel.org>
Subject: Re: [PATCH 1/4] glibc: Perform rseq(2) registration at C startup
 and thread creation (v7)
In-Reply-To: <1965431879.7576.1553529272844.JavaMail.zimbra@efficios.com>
References: <20190212194253.1951-1-mathieu.desnoyers@efficios.com>
        <20190212194253.1951-2-mathieu.desnoyers@efficios.com>
        <5166fbe9-cfe0-8554-abc7-4fc844cf2765@redhat.com>
        <1965431879.7576.1553529272844.JavaMail.zimbra@efficios.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Message-Id: <20190327101608.77b0de6f@mschwideX1>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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