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Date:   Tue, 3 Jul 2018 10:55:46 +0200
From:   Heiko Carstens <heiko.carstens@de.ibm.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-api <linux-api@vger.kernel.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Dave Watson <davejwatson@fb.com>, Paul Turner <pjt@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Russell King <linux@arm.linux.org.uk>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Andi Kleen <andi@firstfloor.org>,
        Chris Lameter <cl@linux.com>, Ben Maurer <bmaurer@fb.com>,
        rostedt <rostedt@goodmis.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Joel Fernandes <joelaf@google.com>, michal.simek@xilinx.com,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>
Subject: Re: [RFC PATCH for 4.18] rseq: use __u64 for rseq_cs fields,
 validate user inputs
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> > > We're piece-wise enabling rseq across architectures anyway, and when the
> > > relevant maintains do this, they can have a look at their
> > > {get,put}_user() implementations and fix them.
> > > 
> > > If you rely on get_user(u64) working, that means microblaze is already
> > > broken, but I suppose it already was, since their rseq enablement patch
> > > is extremely dodgy. Michal?
> > 
> > s390 uses the mvcos instruction to implement get_user(). That instruction
> > is not defined to be atomic, but may copy bytes piecemeal.. I had the
> > impression that the rseq fields are supposed to be updated within the
> > context of a single thread (user + kernel space).
> > 
> > However if another user space thread is allowed to do this as well, then
> > the get_user() approach won't fly on s390.
> > 
> > That leaves the question: does it even make sense for a thread to update
> > the rseq structure of a different thread?
> 
> The problem is interrupts; we need interrupts on the CPU doing the store
> to observe either the old or the new value, not a mix.
> 
> If mvcos does not guarantee that, we're having problems. Is there a
> reason get_user() cannot use a 'regular' load?

Well, that's single instruction semantics. This is something we actually
can guarantee, since the mvcos instruction itself won't be interrupted and
copies all 1/2/4/8 bytes in a row.

So we are talking about that single instructions are required and not
atomic accesses?