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

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Date:   Tue, 9 Apr 2019 18:59:48 +0930
From:   Alan Modra <amodra@...il.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Carlos O'Donell <codonell@...hat.com>,
        Tulio Magno Quites Machado Filho <tuliom@...ii.art.br>,
        Florian Weimer <fweimer@...hat.com>,
        Michael Meissner <meissner@...ux.ibm.com>,
        Peter Bergner <bergner@...t.ibm.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Paul Burton <paul.burton@...s.com>,
        Will Deacon <will.deacon@....com>,
        Boqun Feng <boqun.feng@...il.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Russell King <linux@...linux.org.uk>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>, carlos <carlos@...hat.com>,
        Joseph Myers <joseph@...esourcery.com>,
        Szabolcs Nagy <szabolcs.nagy@....com>,
        libc-alpha <libc-alpha@...rceware.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ben Maurer <bmaurer@...com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Dave Watson <davejwatson@...com>, Paul Turner <pjt@...gle.com>,
        Rich Felker <dalias@...c.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>
Subject: Re: [PATCH 1/4] glibc: Perform rseq(2) registration at C startup and
 thread creation (v7)

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