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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 2 Jul 2018 16:06:09 -0700
Message-ID: <CA+55aFzZYOatOtjrm7fD7878_SzCAmEoEu74n0+pgd7faH-sTQ@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:     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>,
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        Andy Lutomirski <luto@amacapital.net>,
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On Mon, Jul 2, 2018 at 4:00 PM Mathieu Desnoyers
<mathieu.desnoyers@efficios.com> wrote:
>
> Unfortunately, that rseq->rseq_cs field needs to be updated by user-space
> with single-copy atomicity. Therefore, we want 32-bit user-space to initialize
> the padding with 0, and only update the low bits with single-copy atomicity.

Well... It's actually still single-copy atomicity as a 64-bit value.

Why? Because it doesn't matter how you write the upper bits. You'll be
writing the same value to them (zero) anyway.

So who cares if the write ends up being two instructions, because the
write to the upper bits doesn't actually *do* anything.

Hmm?

                Linus