[tip: locking/core] futex: Clarify FUTEX2 flags

[tip-bot2 for peterz@xxxxxxxxxxxxx]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     4923954bbc4a760e0b2210e0cb5733726ac2e2e9
Gitweb:        https://git.kernel.org/tip/4923954bbc4a760e0b2210e0cb5733726ac2e2e9
Author:        peterz@xxxxxxxxxxxxx <peterz@xxxxxxxxxxxxx>
AuthorDate:    Thu, 21 Sep 2023 12:45:06 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Thu, 21 Sep 2023 19:22:04 +02:00

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@xxxxxxxxxxxxx>
Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: André Almeida <andrealmeid@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20230921105247.507327749@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
---
 include/uapi/linux/futex.h | 16 +++++++++++++---
 kernel/futex/syscalls.c    |  7 +++----
 2 files changed, 16 insertions(+), 7 deletions(-)

diff --git a/include/uapi/linux/futex.h b/include/uapi/linux/futex.h
index 71a5df8..21d4eff 100644
--- 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 a807407..ff696b0 100644
--- 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;