On Tue, Jun 02, 2020 at 04:42:03PM -0400, Michael S. Tsirkin wrote: > On Tue, Jun 02, 2020 at 05:30:48PM +0100, Al Viro wrote: > > On Tue, Jun 02, 2020 at 04:45:05AM -0400, Michael S. Tsirkin wrote: > > > So vhost needs to poke at userspace *a lot* in a quick succession. It > > > is thus benefitial to enable userspace access, do our thing, then > > > disable. Except access_ok has already been pre-validated with all the > > > relevant nospec checks, so we don't need that. Add an API to allow > > > userspace access after access_ok and barrier_nospec are done. > > > > This is the wrong way to do it, and this API is certain to be abused > > elsewhere. NAK - we need to sort out vhost-related problems, but > > this is not an acceptable solution. Sorry. > > OK so summarizing what you and Linus both said, we need at > least a way to make sure access_ok (and preferably the barrier too) > is not missed. > > Another comment is about actually checking that performance impact > is significant and worth the complexity and risk. > > Is that a fair summary? > > I'm actually thinking it's doable with a new __unsafe_user type of > pointer, sparse will then catch errors for us.
Er... how would sparse keep track of the range?
