linux-kernel - [RFC][PATCH 01/10] futex: Clarify FUTEX2 flags

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Date:   Fri, 14 Jul 2023 15:39:00 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     tglx@...utronix.de, axboe@...nel.dk
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        mingo@...hat.com, dvhart@...radead.org, dave@...olabs.net,
        andrealmeid@...lia.com, Andrew Morton <akpm@...ux-foundation.org>,
        urezki@...il.com, hch@...radead.org, lstoakes@...il.com,
        Arnd Bergmann <arnd@...db.de>, linux-api@...r.kernel.org,
        linux-mm@...ck.org, linux-arch@...r.kernel.org,
        malteskarupke@....de
Subject: [RFC][PATCH 01/10] 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@...radead.org>
---
 include/uapi/linux/futex.h |   16 +++++++++++++---
 kernel/futex/syscalls.c    |    7 +++----
 2 files changed, 16 insertions(+), 7 deletions(-)

--- 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_32		0x02
+			/*	0x04 */
+			/*	0x08 */
+			/*	0x10 */
+			/*	0x20 */
+			/*	0x40 */
+#define FUTEX2_PRIVATE		FUTEX_PRIVATE_FLAG
+
+/* do not use */
+#define FUTEX_32		FUTEX2_32 /* historical accident :-( */
 
 /*
  * Max numbers of elements in a futex_waitv array
--- a/kernel/futex/syscalls.c
+++ b/kernel/futex/syscalls.c
@@ -183,8 +183,7 @@ SYSCALL_DEFINE6(futex, u32 __user *, uad
 	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_MASK (FUTEX2_32 | FUTEX2_PRIVATE)
 
 /**
  * futex_parse_waitv - Parse a waitv array from userspace
@@ -205,10 +204,10 @@ static int futex_parse_waitv(struct fute
 		if (copy_from_user(&aux, &uwaitv[i], sizeof(aux)))
 			return -EFAULT;
 
-		if ((aux.flags & ~FUTEXV_WAITER_MASK) || aux.__reserved)
+		if ((aux.flags & ~FUTEX2_MASK) || aux.__reserved)
 			return -EINVAL;
 
-		if (!(aux.flags & FUTEX_32))
+		if (!(aux.flags & FUTEX2_32))
 			return -EINVAL;
 
 		futexv[i].w.flags = aux.flags;