On Tue, 10 Dec 2013, Linus Torvalds wrote:
> On Tue, Dec 10, 2013 at 2:57 PM, Thomas Gleixner <t...@linutronix.de> wrote:
> >
> > But how does the access_ok() move do anything helpful here?
>
> Just making it all more obvious.
>
> > We really need it for the fastpath !fshared case, but for the fshared
> > case you actively break working code, because you force a VERIFY_WRITE
> > check into it. The VERIFY_WRITE is necessary for !fshared, because
> > there is no way that one thread can map the futex RO and the other RW,
> > right?
>
> Nobody actually uses that argument any more (it goes back to the old
> i386 "let's manually verify that we have write permissions, because
> the CPU doesn't do it for us in the trap handling"), and it should
> probably be removed.
Fair enough.
> But you're right that it's at least misleading. I'd love to remove it
> entirely, because it's not even syntax-checked, and it's confusing.
> But that would be a humongous patch.
Well, we should ask Julia for a coccinelle patch to limit the
wreckage. :)
Seriously, if that VERIFY_WRITE is completely useless we really want
to get rid of it.
> So these days, "access_ok()" literally just checks that the address is
> in the user address space range. And that would seem to always be
> appropriate for futexes, so why not just do it in the generic code?
Agreed, but as long as the VERIFY_WRITE argument is there it needs at
least a big fat comment :)
Thanks,
tglx
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