linux-kernel - Re: [RFC PATCH for 4.18] rseq: use __u64 for rseq

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Date:   Tue, 3 Jul 2018 14:09:53 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andi Kleen <andi@...stfloor.org>,
        heiko carstens <heiko.carstens@...ibm.com>,
        Andy Lutomirski <luto@...capital.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Dave Watson <davejwatson@...com>, Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Chris Lameter <cl@...ux.com>,
        Ben Maurer <bmaurer@...com>, rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Michal Simek <michal.simek@...inx.com>,
        schwidefsky <schwidefsky@...ibm.com>, gor <gor@...ux.ibm.com>
Subject: Re: [RFC PATCH for 4.18] rseq: use __u64 for rseq_cs fields,
 validate user inputs

----- On Jul 3, 2018, at 1:59 PM, Linus Torvalds torvalds@...ux-foundation.org wrote:

> On Tue, Jul 3, 2018 at 10:49 AM Peter Zijlstra <peterz@...radead.org> wrote:
>>
>> > I can simply document that loads/stores from/to all struct rseq fields
>> > should be thread-local then ?
>>
>> I'm not sure that covers things sufficiently. You really want the
>> userspace load/stores to be single instructions.
> 
> Actually, I think we should try very hard to limit even that to _just_
> the rseq pointer itself.
> 
> Everything else can be filled in ahead of time with non-atomic stores,
> and then the last thing that happens - and the only thing that wants
> that final "one last atomic write" is the rseq pointer write.
> 
> No?

Well a small nit here: the rseq->rseq_cs pointer store is performed
at the _very beginning_ of the rseq critical section. We indeed want
that store to be performed as a single instruction by user-space.

What I think you have in mind as "one last atomic write" is the commit
instruction at the end of the critical section, which does not touch
any field in struct rseq.

> 
> So I'd suggest that the only part we aim to have any "atomic" behavior
> at all is for the individual fields in "struct rseq" itself. So the
> cpu id and the base pointer and the flags. And even they are
> thread-local, so the atomicity is not about the kernel, but about user
> space needing to read and update them in word-sized chunks.
> 
> End result: absolutely nothing is atomic for the kernel.

Yes, +1. If everyone is OK with that I'll go and implement the changes
within the coming day.

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com