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

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Message-ID: <1020760632.10855.1530577391822.JavaMail.zimbra@efficios.com>
Date:   Mon, 2 Jul 2018 20:23:11 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Chris Lameter <cl@...ux.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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>,
        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 8:19 PM, Chris Lameter cl@...ux.com wrote:

> On Mon, 2 Jul 2018, Mathieu Desnoyers 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 ?
>>
>> 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.
> 
> Platforms with 32 bit word size only guarantee atomicity of a 32 bit
> write or RMV instruction.
> 
> Special instructions may exist on a platform to perform 64 bit atomic
> updates. We use cmpxchg64 f.e. on Intel 32 bit platforms to guarantee
> atomicity8.
> 
> So use the macros that we have to guarantee 64 bit ops and you should be
> fine. See linux/arch/x86/include/asm/atomic64_32.h

We are talking about user-space here. What we need is a single instruction
atomic store, similar to what WRITE_ONCE() does in the kernel. The discussion
is about whether doing the user-space equivalent of a WRITE_ONCE() to a u64
on a 32-bit architecture should be considered to provide single-copy atomicity
on the low 32 bits.

Thanks,

Mathieu


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