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

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Message-ID: <1776351430.10902.1530585009519.JavaMail.zimbra@efficios.com>
Date:   Mon, 2 Jul 2018 22:30:09 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        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>,
        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 10:18 PM, Linus Torvalds torvalds@...ux-foundation.org wrote:

> On Mon, Jul 2, 2018 at 7:01 PM Mathieu Desnoyers
> <mathieu.desnoyers@...icios.com> wrote:
>>
>> One thing to consider is how we will implement the load of that pointer
>> on the kernel side.
> 
> Use "get_user()". It works for 64-bit objects too, and it will be
> atomic in the 32-bit sub-parts on a 32-bit architecture.

Is it really ? Last time we had this discussion, not all architectures
guaranteed that reading a 64-bit integer would happen in two atomic
32-bit sub-parts. This was the main motivation for the LINUX_FIELD_u32_u64()
macro as it stands today (rather than using a union).

> 
> Again: there is no point in trying to be atomic in the full 64 bits
> (when you're running on 32-bit). The upper bits don't have to "match"
> the lower bits. They just have to be zero. So doing it as two loads is
> fine - the same way it's perfectly fine to do it as two stores (since
> the store to the upper bits will always be zero).

I'd be fine with two atomic loads, but I'd rather have a strong
confirmation about this, because last time around there were
architectures where it was not true as far as I recall.

Thanks,

Mathieu


> 
>             Linus

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