[PATCH 01/10] futex: Clarify FUTEX2 flags

Jens Axboe
2023-07-20T22:19:12Z

From: Peter Zijlstra <peterz@infradead.org>

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>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
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 71a5df8d2689..0c5abb6aa8f8 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_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
diff --git a/kernel/futex/syscalls.c b/kernel/futex/syscalls.c
index a8074079b09e..42b6c2fac7db 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_MASK (FUTEX2_32 | 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_MASK) || aux.__reserved)
return -EINVAL;

- if (!(aux.flags & FUTEX_32))
+ if (!(aux.flags & FUTEX2_32))
return -EINVAL;

futexv[i].w.flags = aux.flags;
--
2.40.1