Messages in this thread | | | From | Linus Torvalds <> | Date | Tue, 8 Jan 2019 21:19:52 -0800 | Subject | Re: __get_user slower than get_user (was Re: [RFC PATCH V3 0/5] Hi:) |
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On Tue, Jan 8, 2019 at 8:31 PM Michael S. Tsirkin <mst@redhat.com> wrote: > > Linus, given that you just changed all users of access_ok anyway, do > you still think that the access_ok() conversion to return a speculation > sanitized pointer or NULL is too big a conversion?
I didn't actually change a single access_ok().
I changed the (very few) users of "user_access_begin()" to do an access_ok() in them. There were 8 of them total.
It turns out that two of those cases (the strn*_user() ones) found bugs in the implementation of access_ok() of two architectures, and then looking at the others found that six more architectures also had problems, but those weren't actually because of any access_ok() changes, they were pre-existing issues. So we definitely had unfortunate bugs in access_ok(), but they were mostly the benign kind (ir the "use arguments twice - a real potential bug, but not one that actually likely makes any difference to existing users)
Changing all 600+ users of access_ok() would be painful.
That said, one thing I *would* like to do is to just get rid of __get_user() and __put_user() entirely. Or rather, just make them do exactly the same thing that the normal "get_user()"/"put_user()" functions do.
And then, _within_ the case of get_user()/put_user(), doing the access_ok() as a data dependency rather than a lfence should be easy enough.
Linus
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