futex: Clarify FUTEX2 flags

sys_futex_waitv() is part of the futex2 series (the first and only so
far) of syscalls and has a flags field per futex (as opposed to flags
being encoded in the futex op).

This new flags field has a new namespace, which unfortunately isn't
super explicit. Notably it currently takes FUTEX_32 and
FUTEX_PRIVATE_FLAG.

Introduce the FUTEX2 namespace to clarify this

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: André Almeida <andrealmeid@igalia.com>
Link: https://lore.kernel.org/r/20230921105247.507327749@noisy.programming.kicks-ass.net

diff --git a/include/uapi/linux/futex.h b/include/uapi/linux/futex.h
index 71a5df8d26898b03c0ee03c1f9c87899bb74fe63..21d4eff411626062cbb38b2c13555cbd63ce5a17 100644 (file)
--- a/include/uapi/linux/futex.h
+++ b/include/uapi/linux/futex.h
@@ -44,10 +44,20 @@
                                         FUTEX_PRIVATE_FLAG)
 
 /*
- * Flags to specify the bit length of the futex word for futex2 syscalls.
- * Currently, only 32 is supported.
+ * Flags for futex2 syscalls.
  */
-#define FUTEX_32               2
+                       /*      0x00 */
+                       /*      0x01 */
+#define FUTEX2_SIZE_U32                0x02
+                       /*      0x04 */
+                       /*      0x08 */
+                       /*      0x10 */
+                       /*      0x20 */
+                       /*      0x40 */
+#define FUTEX2_PRIVATE         FUTEX_PRIVATE_FLAG
+
+/* do not use */
+#define FUTEX_32               FUTEX2_SIZE_U32 /* historical accident :-( */
 
 /*
  * Max numbers of elements in a futex_waitv array

diff --git a/kernel/futex/syscalls.c b/kernel/futex/syscalls.c
index a8074079b09e87ca867c2cfc84be021076198a65..ff696b0e2e5ce8f3fa5fdd4abdfeb9229da9f564 100644 (file)
--- a/kernel/futex/syscalls.c
+++ b/kernel/futex/syscalls.c
@@ -183,8 +183,7 @@ SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
        return do_futex(uaddr, op, val, tp, uaddr2, (unsigned long)utime, val3);
 }
 
-/* Mask of available flags for each futex in futex_waitv list */
-#define FUTEXV_WAITER_MASK (FUTEX_32 | FUTEX_PRIVATE_FLAG)
+#define FUTEX2_VALID_MASK (FUTEX2_SIZE_U32 | FUTEX2_PRIVATE)
 
 /**
  * futex_parse_waitv - Parse a waitv array from userspace
@@ -205,10 +204,10 @@ static int futex_parse_waitv(struct futex_vector *futexv,
                if (copy_from_user(&aux, &uwaitv[i], sizeof(aux)))
                        return -EFAULT;
 
-               if ((aux.flags & ~FUTEXV_WAITER_MASK) || aux.__reserved)
+               if ((aux.flags & ~FUTEX2_VALID_MASK) || aux.__reserved)
                        return -EINVAL;
 
-               if (!(aux.flags & FUTEX_32))
+               if (!(aux.flags & FUTEX2_SIZE_U32))
                        return -EINVAL;
 
                futexv[i].w.flags = aux.flags;