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

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Date:   Mon, 2 Jul 2018 19:18:38 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Paul 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 - ARM Linux <linux@....linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>, Peter Anvin <hpa@...or.com>,
        Andi Kleen <andi@...stfloor.org>,
        Christoph Lameter <cl@...ux.com>, Ben Maurer <bmaurer@...com>,
        Steven 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 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.

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).

            Linus