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

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Message-ID: <321013411.10852.1530573953298.JavaMail.zimbra@efficios.com>
Date:   Mon, 2 Jul 2018 19:25:53 -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 7:22 PM, Linus Torvalds torvalds@...ux-foundation.org wrote:

> On Mon, Jul 2, 2018 at 4:17 PM Mathieu Desnoyers
> <mathieu.desnoyers@...icios.com> wrote:
>>
>> 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 ?
> 
> Guarantees? No. Not that there are any guarantees that the same won't
> happen for a plain 32-bit value either.
> 
> Will compilers generate that kind of code? I guess some crazy compiler
> could simply be really bad at handling 64-bit values, and just happen
> to handle 32-bit values better. So in that sense a 64-bit entity is
> certainly a bit riskier. But that would be a really bad compiler, I
> have to say.

Given that the only C code updating that field is rseq_prepare_unload()
(the rest is only ever updated from assembly), we could perhaps mandate
that user-space always update it from assembly, and therefore implement
rseq_prepare_unload as an inline asm which clears rseq->rseq_cs.

Does it sound better than the LINUX_FIELD_u32_u64 macro ?

Thanks,

Mathieu


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