From mboxrd@z Thu Jan  1 00:00:00 1970
From: Martin Schwidefsky <schwidefsky@de.ibm.com>
Subject: Re: [PATCH 1/4] glibc: Perform rseq(2) registration at C startup
 and thread creation (v7)
Date: Wed, 27 Mar 2019 10:16:08 +0100
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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>
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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.