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Subject: Re: [PATCH 1/4] glibc: Perform rseq(2) registration at C startup and
 thread creation (v7)
To:     Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     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>
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 <20190327101608.77b0de6f@mschwideX1>
From:   Carlos O'Donell <codonell@redhat.com>
Message-ID: <4021516e-6a1e-166d-a4f6-e961e6f94cc4@redhat.com>
Date:   Wed, 27 Mar 2019 16:38:32 -0400
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On 3/27/19 5:16 AM, Martin Schwidefsky wrote:
> 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.

Why was this particular operated selected?
  
So on s390 the RSEQ_SIG will show up as an unexpected "divide to integer"
instruction that can't be reached by any control flow?

Can we use a NOP with a unique value in an immediate operand?

The goal being to have something that won't confuse during a debug
session, or that the debugger can ignore (like constant pools on Arm)

-- 
Cheers,
Carlos.