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futex: Deobfuscate handle_futex_death()

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handle_futex_death() uses futex_atomic_cmpxchg_inatomic() without
disabling page faults. That's ok, but totally non obvious.

We don't hold locks so we actually can and want to fault here, because
the get_user() before futex_atomic_cmpxchg_inatomic() does not
guarantee a R/W mapping.

We could just add a big fat comment to explain this, but actually
changing the code so that the functionality is entirely clear is
better.

Use the helper function which disables page faults around the
futex_atomic_cmpxchg_inatomic() and handle a fault with a call to
fault_in_user_writeable() as all other places in the futex code do as
well.

Pointed-out-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Darren Hart <darren@dvhart.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: David Howells <dhowells@redhat.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: "James E.J. Bottomley" <jejb@parisc-linux.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Chris Metcalf <cmetcalf@tilera.com>
LKML-Reference: <alpine.LFD.2.00.1103141126590.2787@localhost6.localdomain6>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
This commit is contained in:
Thomas Gleixner 2011-03-14 10:34:35 +01:00
parent 995612178c
commit 6e0aa9f8a8
1 changed files with 14 additions and 3 deletions
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17
kernel/futex.c
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@ -2458,9 +2458,20 @@ int handle_futex_death(u32 __user *uaddr, struct task_struct *curr, int pi)
* userspace.
*/
mval = (uval & FUTEX_WAITERS) | FUTEX_OWNER_DIED;
if (futex_atomic_cmpxchg_inatomic(&nval, uaddr, uval, mval))
return -1;
/*
* We are not holding a lock here, but we want to have
* the pagefault_disable/enable() protection because
* we want to handle the fault gracefully. If the
* access fails we try to fault in the futex with R/W
* verification via get_user_pages. get_user() above
* does not guarantee R/W access. If that fails we
* give up and leave the futex locked.
*/
if (cmpxchg_futex_value_locked(&nval, uaddr, uval, mval)) {
if (fault_in_user_writeable(uaddr))
return -1;
goto retry;
}
if (nval != uval)
goto retry;