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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 2 Jul 2018 19:18:38 -0700
Message-ID: <CA+55aFxWMjwx-4TGBHt0H6bh6FXC3NKMsjp=eKGw5cgA4wMBUA@mail.gmail.com>
Subject: Re: [RFC PATCH for 4.18] rseq: use __u64 for rseq_cs fields, validate
 user inputs
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     Andy Lutomirski <luto@amacapital.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Dave Watson <davejwatson@fb.com>, Paul Turner <pjt@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Russell King - ARM Linux <linux@arm.linux.org.uk>,
        Ingo Molnar <mingo@redhat.com>, Peter Anvin <hpa@zytor.com>,
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On Mon, Jul 2, 2018 at 7:01 PM Mathieu Desnoyers
<mathieu.desnoyers@efficios.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