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

From: Alan Modra <amodra@gmail.com>
To: Michael Ellerman <mpe@ellerman.id.au>
Cc: Carlos O'Donell <codonell@redhat.com>,
	Tulio Magno Quites Machado Filho <tuliom@ascii.art.br>,
	Florian Weimer <fweimer@redhat.com>,
	Michael Meissner <meissner@linux.ibm.com>,
	Peter Bergner <bergner@vnet.ibm.com>,
	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>, carlos <carlos@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: Tue, 9 Apr 2019 18:59:48 +0930	[thread overview]
Message-ID: <20190409092948.GA14424@bubble.grove.modra.org> (raw)
In-Reply-To: <877ec3yffq.fsf@concordia.ellerman.id.au>

On Tue, Apr 09, 2019 at 02:23:53PM +1000, Michael Ellerman wrote:
> I'd much rather we use a trap with a specific immediate value. Otherwise
> someone's going to waste time one day puzzling over why userspace is
> doing mtmsr.

It's data.  We have other data in executable sections.  Anyone who
wonders about odd disassembly just hasn't realized they are
disassembling data.

> It would also complicate things if we ever wanted to emulate mtmsr.

No, because it won't be executed.  If I understand correctly, the only
reason to choose an illegal, trap or privileged insn is to halt
execution earlier rather than later when a program goes off in the
weeds.

> If we want something that is a trap rather than a nop then use 0x0fe50553.
> 
> That's "compare the value in r5 with 0x553 and then trap unconditionally".
> 
> It shows up in objdump as:
> 
>     10000000:	53 05 e5 0f 	twui    r5,1363
> 
> 
> The immediate can be anything, I chose that value to mimic the x86 value
> Mathieu mentioned.
> 
> There's no reason that instruction would ever be generated because the
> immediate value serves no purpose. So it satisfies the "very unlikely
> to appear" criteria AFAICS.

Yes, looks fine to me, except that in VLE mode (do we care?)
".long 0x0fe50553" disassembles as
   0:	0f e5       	se_cmphl r5,r30
   2:	05 53       	se_mullw r3,r5
No illegal/trap/privileged insn there.

".long 0x0fe5000b" might be better to cover VLE.

-- 
Alan Modra
Australia Development Lab, IBM