linux-kernel - Re: [PATCH for 4.18 2/6] rseq: use get_user/put_user rather than __get_user/__put

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Message-ID: <CA+55aFzpjA3r9HdfCU5LrEhSt6S=+RfKuJ2GVc8Wv5Hz-_Me8Q@mail.gmail.com>
Date:   Mon, 9 Jul 2018 11:04:28 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     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>,
        Andy Lutomirski <luto@...capital.net>,
        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>,
        Andrew Hunter <ahh@...gle.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH for 4.18 2/6] rseq: use get_user/put_user rather than __get_user/__put_user

On Mon, Jul 9, 2018 at 10:28 AM Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
>
> So, another twist to this story: ppc32 does not implement u64 get_user().

I was going to say that "that's not possible", since we actually have
64-bit arguments at least in the form of "loff_t __user *".

But when I started looking, it turns out that yeah, we do
"copy_from_user()" on them, and instead made the x86 copy_from_user()
have special cases for constant sizes.

So a 8-byte copy_from_user() is fine. It ends up being a "get_user()"
on x86 anyway.

                Linus