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, 17 insertions, 15 deletions

diff --git a/arch/mips/include/asm/futex.h b/arch/mips/include/asm/futex.h
index b9cce90346cf..692a24bd83b7 100644
--- a/arch/mips/include/asm/futex.h
+++ b/arch/mips/include/asm/futex.h
@@ -132,9 +132,10 @@ 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 retval;
+        int ret = 0, val;
 
         if (!access_ok(VERIFY_WRITE, uaddr, sizeof(int)))
                 return -EFAULT;
@@ -145,25 +146,25 @@ futex_atomic_cmpxchg_inatomic(int __user *uaddr, int oldval, int newval)
                 "       .set    push                                    \n"
                 "       .set    noat                                    \n"
                 "       .set    mips3                                   \n"
-                "1:     ll      %0, %2                                  \n"
-                "       bne     %0, %z3, 3f                             \n"
+                "1:     ll      %1, %3                                  \n"
+                "       bne     %1, %z4, 3f                             \n"
                 "       .set    mips0                                   \n"
-                "       move    $1, %z4                                 \n"
+                "       move    $1, %z5                                 \n"
                 "       .set    mips3                                   \n"
-                "2:     sc      $1, %1                                  \n"
+                "2:     sc      $1, %2                                  \n"
                 "       beqzl   $1, 1b                                  \n"
                 __WEAK_LLSC_MB
                 "3:                                                     \n"
                 "       .set    pop                                     \n"
                 "       .section .fixup,\"ax\"                          \n"
-                "4:     li      %0, %5                                  \n"
+                "4:     li      %0, %6                                  \n"
                 "       j       3b                                      \n"
                 "       .previous                                       \n"
                 "       .section __ex_table,\"a\"                       \n"
                 "       "__UA_ADDR "\t1b, 4b                            \n"
                 "       "__UA_ADDR "\t2b, 4b                            \n"
                 "       .previous                                       \n"
-                : "=&r" (retval), "=R" (*uaddr)
+                : "+r" (ret), "=&r" (val), "=R" (*uaddr)
                 : "R" (*uaddr), "Jr" (oldval), "Jr" (newval), "i" (-EFAULT)
                 : "memory");
         } else if (cpu_has_llsc) {
@@ -172,31 +173,32 @@ futex_atomic_cmpxchg_inatomic(int __user *uaddr, int oldval, int newval)
                 "       .set    push                                    \n"
                 "       .set    noat                                    \n"
                 "       .set    mips3                                   \n"
-                "1:     ll      %0, %2                                  \n"
-                "       bne     %0, %z3, 3f                             \n"
+                "1:     ll      %1, %3                                  \n"
+                "       bne     %1, %z4, 3f                             \n"
                 "       .set    mips0                                   \n"
-                "       move    $1, %z4                                 \n"
+                "       move    $1, %z5                                 \n"
                 "       .set    mips3                                   \n"
-                "2:     sc      $1, %1                                  \n"
+                "2:     sc      $1, %2                                  \n"
                 "       beqz    $1, 1b                                  \n"
                 __WEAK_LLSC_MB
                 "3:                                                     \n"
                 "       .set    pop                                     \n"
                 "       .section .fixup,\"ax\"                          \n"
-                "4:     li      %0, %5                                  \n"
+                "4:     li      %0, %6                                  \n"
                 "       j       3b                                      \n"
                 "       .previous                                       \n"
                 "       .section __ex_table,\"a\"                       \n"
                 "       "__UA_ADDR "\t1b, 4b                            \n"
                 "       "__UA_ADDR "\t2b, 4b                            \n"
                 "       .previous                                       \n"
-                : "=&r" (retval), "=R" (*uaddr)
+                : "+r" (ret), "=&r" (val), "=R" (*uaddr)
                 : "R" (*uaddr), "Jr" (oldval), "Jr" (newval), "i" (-EFAULT)
                 : "memory");
         } else
                 return -ENOSYS;
 
-        return retval;
+        *uval = val;
+        return ret;
 }
 
 #endif