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

From: Carlos O'Donell <codonell@redhat.com>
To: Michael Ellerman <mpe@ellerman.id.au>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.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>,
	Martin Schwidefsky <schwidefsky@de.ibm.com>,
	Russell King <linux@armlinux.org.uk>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>
Cc: 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)
Date: Thu, 4 Apr 2019 16:15:53 -0400	[thread overview]
Message-ID: <7412087f-5ef4-0670-503e-f8de6ca3b0bb@redhat.com> (raw)
In-Reply-To: <87lg0tosfz.fsf@concordia.ellerman.id.au>

On 4/2/19 2:02 AM, Michael Ellerman wrote:
> Mathieu Desnoyers <mathieu.desnoyers@efficios.com> writes:
>> Hi Carlos,
>>
>> ----- On Mar 22, 2019, at 4:09 PM, Carlos O'Donell codonell@redhat.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).

The s390 IBM team needs to respond to this and I want to make sure they ACK
the NOP being used here because it impacts them directly.

I'd like to see Martin's opinion on this.

-- 
Cheers,
Carlos.