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, 13 insertions, 11 deletions

diff --git a/arch/microblaze/include/asm/futex.h b/arch/microblaze/include/asm/futex.h
index ad3fd61b2fe..fa019ed65df 100644
--- a/arch/microblaze/include/asm/futex.h
+++ b/arch/microblaze/include/asm/futex.h
@@ -94,31 +94,33 @@ 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)
 {
-        int prev, cmp;
+        int ret = 0, prev, cmp;
 
         if (!access_ok(VERIFY_WRITE, uaddr, sizeof(int)))
                 return -EFAULT;
 
-        __asm__ __volatile__ ("1:       lwx     %0, %2, r0;             \
-                                        cmp     %1, %0, %3;             \
-                                        beqi    %1, 3f;                 \
-                                2:      swx     %4, %2, r0;             \
-                                        addic   %1, r0, 0;              \
-                                        bnei    %1, 1b;                 \
+        __asm__ __volatile__ ("1:       lwx     %1, %3, r0;             \
+                                        cmp     %2, %1, %4;             \
+                                        beqi    %2, 3f;                 \
+                                2:      swx     %5, %3, r0;             \
+                                        addic   %2, r0, 0;              \
+                                        bnei    %2, 1b;                 \
                                 3:                                      \
                                 .section .fixup,\"ax\";                 \
                                 4:      brid    3b;                     \
-                                        addik   %0, r0, %5;             \
+                                        addik   %0, r0, %6;             \
                                 .previous;                              \
                                 .section __ex_table,\"a\";              \
                                 .word   1b,4b,2b,4b;                    \
                                 .previous;"                             \
-                : "=&r" (prev), "=&r"(cmp)                              \
+                : "+r" (ret), "=&r" (prev), "=&r"(cmp)  \
                 : "r" (uaddr), "r" (oldval), "r" (newval), "i" (-EFAULT));
 
-        return prev;
+        *uval = prev;
+        return ret;
 }
 
 #endif /* __KERNEL__ */