Messages in this thread | | | From | Linus Torvalds <> | Date | Fri, 2 Nov 2018 09:14:51 -0700 | Subject | Re: [PULL] vhost: cleanups and fixes |
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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
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