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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 2 Nov 2018 09:14:51 -0700
Message-ID: <CAHk-=wh_bQK5zs+CwQ5eyodq4sQT0eOPp60qzvVL2_EtgETP-g@mail.gmail.com>
Subject: Re: [PULL] vhost: cleanups and fixes
To:     mst@redhat.com
Cc:     mark.rutland@arm.com, Kees Cook <keescook@chromium.org>,
        kvm@vger.kernel.org, virtualization@lists.linux-foundation.org,
        netdev@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
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On Fri, Nov 2, 2018 at 6:04 AM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> I've tried making access_ok mask the parameter it gets.

PLEASE don't do this.

Just use "copy_to/from_user()".

We have had lots of bugs because code bitrots.

And no, the access_ok() checks aren't expensive, not even in a loop.
They *used* to be somewhat expensive compared to the access, but that
simply isn't true any more. The real expense in copy_to_user and
friends are in the user access bit setting (STAC and CLAC on x86),
which easily an order of magnitude more expensive than access_ok().

So just get rid of the double-underscore version. It's basically
always a mis-optimization due to entirely historical reasons. I can
pretty much guarantee that it's not visible in profiles.

                  Linus