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

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Date:   Mon, 2 Jul 2018 19:16:59 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        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>, Andi Kleen <andi@...stfloor.org>,
        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>
Subject: Re: [RFC PATCH for 4.18] rseq: use __u64 for rseq_cs fields,
 validate user inputs

----- On Jul 2, 2018, at 7:06 PM, Linus Torvalds torvalds@...ux-foundation.org wrote:

> On Mon, Jul 2, 2018 at 4:00 PM Mathieu Desnoyers
> <mathieu.desnoyers@...icios.com> wrote:
>>
>> Unfortunately, that rseq->rseq_cs field needs to be updated by user-space
>> with single-copy atomicity. Therefore, we want 32-bit user-space to initialize
>> the padding with 0, and only update the low bits with single-copy atomicity.
> 
> Well... It's actually still single-copy atomicity as a 64-bit value.
> 
> Why? Because it doesn't matter how you write the upper bits. You'll be
> writing the same value to them (zero) anyway.
> 
> So who cares if the write ends up being two instructions, because the
> write to the upper bits doesn't actually *do* anything.
> 
> Hmm?

Are there any kind of guarantees that a __u64 update on a 32-bit architecture
won't be torn into something daft like byte-per-byte stores when performed
from C code ?

I don't worry whether the upper bits get updated or how, but I really care
about not having store tearing of the low bits update.

Thanks,

Mathieu


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