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, 10 insertions, 6 deletions

diff --git a/arch/sparc/include/asm/futex_64.h b/arch/sparc/include/asm/futex_64.h
index 47f95839dc69..e0862200d6a1 100644
--- a/arch/sparc/include/asm/futex_64.h
+++ b/arch/sparc/include/asm/futex_64.h
@@ -85,26 +85,30 @@ static inline int 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 ret = 0;
+
         __asm__ __volatile__(
-        "\n1:   casa    [%3] %%asi, %2, %0\n"
+        "\n1:   casa    [%4] %%asi, %3, %1\n"
         "2:\n"
         "       .section .fixup,#alloc,#execinstr\n"
         "       .align  4\n"
         "3:     sethi   %%hi(2b), %0\n"
         "       jmpl    %0 + %%lo(2b), %%g0\n"
-        "        mov    %4, %0\n"
+        "       mov     %5, %0\n"
         "       .previous\n"
         "       .section __ex_table,\"a\"\n"
         "       .align  4\n"
         "       .word   1b, 3b\n"
         "       .previous\n"
-        : "=r" (newval)
-        : "0" (newval), "r" (oldval), "r" (uaddr), "i" (-EFAULT)
+        : "+r" (ret), "=r" (newval)
+        : "1" (newval), "r" (oldval), "r" (uaddr), "i" (-EFAULT)
         : "memory");
 
-        return newval;
+        *uval = newval;
+        return ret;
 }
 
 #endif /* !(_SPARC64_FUTEX_H) */