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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 2 Jul 2018 16:22:12 -0700
Message-ID: <CA+55aFyQwfXxMnx2WQadfZKAnEfkoT+zomrRRToZz8-uWzUccQ@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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        Boqun Feng <boqun.feng@gmail.com>,
        Andy Lutomirski <luto@amacapital.net>,
        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>,
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On Mon, Jul 2, 2018 at 4:17 PM Mathieu Desnoyers
<mathieu.desnoyers@efficios.com> 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 ?

Guarantees? No. Not that there are any guarantees that the same won't
happen for a plain 32-bit value either.

Will compilers generate that kind of code? I guess some crazy compiler
could simply be really bad at handling 64-bit values, and just happen
to handle 32-bit values better. So in that sense a 64-bit entity is
certainly a bit riskier. But that would be a really bad compiler, I
have to say.

                Linus