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SubjectRe: process 'stuck' at exit.
On Tue, 10 Dec 2013, Linus Torvalds wrote:

> On Tue, Dec 10, 2013 at 1:57 PM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > So it looks like __get_user_pages_fast() fails, and keeps failing.
>
> Hmm.. Is any of the addresses unchecked, perhaps?
> __get_user_pages_fast() does an access_ok() check, while
> get_user_pages_fast() does *not* seem to do one.
>
> That looks a bit dangerous. Yeah, users should have checked the
> address range, but there really is no reason not to do it in
> get_user_pages_fast().
>
> And it looks like the futex code is actually seriously buggered. It
> only does the access_ok() check for the non-shared case.
>
> Why?

The !fshared case is the fast path which does not even reach
get_user_pages_fast().

We had this discussion some time ago already, where the access_ok()
check was missing in the !fshared case or the check was buggered for
some reason. Need to dig up the gory details.

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.

But there is a subtle detail:

err = get_user_pages_fast(address, 1, 1, &page);

So we ask for write access as the write argument is 1. In case that
fails we have that fallback path:

/*
* If write access is not required (eg. FUTEX_WAIT), try
* and get read-only access.
*/
if (err == -EFAULT && rw == VERIFY_READ) {
err = get_user_pages_fast(address, 1, 0, &page);

That's a legitimate use case. And futex_requeue only requests
VERIFY_READ for the !requeue_pi case.

Now, if that map is RO, i.e. we took the fallback path then the THP
one will fail as it has write=1 unconditionally.

if (likely(__get_user_pages_fast(address, 1, 1, &page) == 1))

Thanks,

tglx



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