futex: Sanitize cmpxchg_futex_value_locked API

The cmpxchg_futex_value_locked API was funny in that it returned either
the original, user-exposed futex value OR an error code such as -EFAULT.
This was confusing at best, and could be a source of livelocks in places
that retry the cmpxchg_futex_value_locked after trying to fix the issue
by running fault_in_user_writeable().
    
This change makes the cmpxchg_futex_value_locked API more similar to the
get_futex_value_locked one, returning an error code and updating the
original value through a reference argument.
    
Signed-off-by: Michel Lespinasse <walken@google.com>
Acked-by: Chris Metcalf <cmetcalf@tilera.com>  [tile]
Acked-by: Tony Luck <tony.luck@intel.com>  [ia64]
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Michal Simek <monstr@monstr.eu>  [microblaze]
Acked-by: David Howells <dhowells@redhat.com> [frv]
Cc: Darren Hart <darren@dvhart.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Russell King <linux@arm.linux.org.uk>
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: Linus Torvalds <torvalds@linux-foundation.org>
LKML-Reference: <20110311024851.GC26122@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 6 insertions, 3 deletions

diff --git a/arch/ia64/include/asm/futex.h b/arch/ia64/include/asm/futex.h
index c7f0f062239..b0728404dad 100644
--- a/arch/ia64/include/asm/futex.h
+++ b/arch/ia64/include/asm/futex.h
@@ -100,23 +100,26 @@ futex_atomic_op_inuser (int encoded_op, int __user *uaddr)
 }
 
 static inline int
-futex_atomic_cmpxchg_inatomic(int __user *uaddr, int oldval, int newval)
+futex_atomic_cmpxchg_inatomic(int *uval, int __user *uaddr,
+                              int oldval, int newval)
 {
         if (!access_ok(VERIFY_WRITE, uaddr, sizeof(int)))
                 return -EFAULT;
 
         {
-                register unsigned long r8 __asm ("r8");
+                register unsigned long r8 __asm ("r8") = 0;
+                unsigned long prev;
                 __asm__ __volatile__(
                         "       mf;;                                    \n"
                         "       mov ar.ccv=%3;;                         \n"
                         "[1:]   cmpxchg4.acq %0=[%1],%2,ar.ccv          \n"
                         "       .xdata4 \"__ex_table\", 1b-., 2f-.      \n"
                         "[2:]"
-                        : "=r" (r8)
+                        : "=r" (prev)
                         : "r" (uaddr), "r" (newval),
                           "rO" ((long) (unsigned) oldval)
                         : "memory");
+                *uval = prev;
                 return r8;
         }
 }