On Tue, Dec 10, 2013 at 02:58:19PM -0800, Darren Hart wrote:
> On Tue, 2013-12-10 at 14:48 -0800, Linus Torvalds wrote:
> > On Tue, Dec 10, 2013 at 2:42 PM, Thomas Gleixner <t...@linutronix.de>
> > wrote:
> > >
> > > And yes, I remember that we do not do an extra check for the fshared
> > > case, because get_user_pages_fast() should do it for us already. If
> > > not we are fubared not only in the futex code.
> >
> > Yeah. It turns out we do do the access check indirectly - by looking
> > at the PAGE_USER bit, even if we don't necessarily check the actual
> > limits. So get_user_pages_fast() is fine.
> >
> > > But there is a subtle detail:
> >
> > Yup, see my email from ten minutes ago, we found the same thing. And
> > that would seem to explain the endless loop, and also the timing
> > (since Dave mentions he started doing large-pages lately).
> >
> > So I think the "__get_user_pages_fast(address, 1, !ro, &page)" thing
> > should work.
> >
> > Dave, can you re-create that trinity run and test that patch? I think
> > we've got this, but it might be nice to leave the hung machine up and
> > running until it's verified.. Although I don't really see what else we
> > could need or get out of it, so..
>
> Would it be possible to limit the options to only pass FUTEX_CMP_REQUEUE
> and a read-only uaddr? That should improve confidence when it doesn't
> fail :-)
easy enough to hack into the code yeah. A bit complicated to come up with
a sensible grammar for a command line parser for such cases sadly.
I'll give the patch a try after dinner.
Dave
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