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

Eric Dumazet
2007-04-07T10:01:20Z

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 :)

Before the patch :

Alignment was only 4 bytes for all futexes, but some user app could trigger a kernel bug (since one 64bit futex could sit on two different pages, so possible separate vmas, so the inode refcounting was wrong, and access_ok did not a correct check)

After the patch :

Alignment is 8 bytes for 64 bit futexes, 4 bytes for 32bit futexes.
All futexes are contrained to be in one single page.