Re: [PATCH, take4] FUTEX : new PRIVATE futexes

Eric Dumazet
2007-04-11T08:22:26Z

On Wed, 11 Apr 2007 17:22:57 +1000
Nick Piggin <nickpiggin@yahoo.com.au> wrote:

> Eric Dumazet wrote:
> > On Sat, 07 Apr 2007 19:30:14 +1000
> > Nick Piggin <nickpiggin@yahoo.com.au> wrote:
> >
> >
> >>Eric Dumazet wrote:
> >
> >
> >
> >>>- Current mm code have a problem with 64bit futexes, as spoted by Nick :
> >>>
> >>>get_futex_key() does a check against sizeof(u32) regardless of futex being 64bits or not.
> >>>So it is possible a 64bit futex spans two pages of memory...
> >>>I had to change get_futex_key() prototype to be able to do a correct test.
> >>
> >>I wonder if it should be encfocing alignment to keep in on 1 page?
> >
> >
> > I believe I just did that :)
>
> Yes :P What I was trying to say before jumping on a plane is that
> sys_futex/sys_futex64 calls should each check their own address alignment, so
> the deeper parts of the call stack always know alignment is correct.
>
> This will remove all the fsize you pass around, and also sanitise the userspace
> argument much higher in the call stack, which is very preferable and more
> conventional.
>
> Maybe this isn't possible (it's very obvious, so there may be a good reason it
> hasn't been done).

I had this idea as well, but considering get_futex_key() is exported in include/linux/futex.h, I believe some out-of tree thing is using it.

As this external thing certainly is not doing the check itself, to be on the safe side we should enforce it in get_futex_key(). I agree with you : If we want to maximize performance, we could say : The check *must* be done by the caller.