linux-kernel - Re: [PATCH v1 02/14] futex: Extend the FUTEX2 flags

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Message-ID: <20230731192012.GA11704@hirez.programming.kicks-ass.net>
Date:   Mon, 31 Jul 2023 21:20:12 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     axboe@...nel.dk, linux-kernel@...r.kernel.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: Re: [PATCH v1 02/14] futex: Extend the FUTEX2 flags

On Mon, Jul 31, 2023 at 07:42:11PM +0200, Thomas Gleixner wrote:

> Aargh. This is really nasty to make FUTEX2_64 0x3 and abuse it to test
> the flags for validity. Intuitive and obvious is something else.

Like so then?

--- a/include/uapi/linux/futex.h
+++ b/include/uapi/linux/futex.h
@@ -57,6 +57,8 @@
 			/*	0x40 */
 #define FUTEX2_PRIVATE		FUTEX_PRIVATE_FLAG
 
+#define FUTEX2_SIZE_MASK	0x03
+
 /* do not use */
 #define FUTEX_32		FUTEX2_32 /* historical accident :-( */
 
--- a/kernel/futex/syscalls.c
+++ b/kernel/futex/syscalls.c
@@ -183,7 +183,7 @@ SYSCALL_DEFINE6(futex, u32 __user *, uad
 	return do_futex(uaddr, op, val, tp, uaddr2, (unsigned long)utime, val3);
 }
 
-#define FUTEX2_MASK (FUTEX2_64 | FUTEX2_PRIVATE)
+#define FUTEX2_MASK (FUTEX2_SIZE_MASK | FUTEX2_PRIVATE)
 
 /**
  * futex_parse_waitv - Parse a waitv array from userspace
@@ -208,11 +208,11 @@ static int futex_parse_waitv(struct fute
 			return -EINVAL;
 
 		if (!IS_ENABLED(CONFIG_64BIT) || in_compat_syscall()) {
-			if ((aux.flags & FUTEX2_64) == FUTEX2_64)
+			if ((aux.flags & FUTEX2_SIZE_MASK) == FUTEX2_64)
 				return -EINVAL;
 		}
 
-		if ((aux.flags & FUTEX2_64) != FUTEX2_32)
+		if ((aux.flags & FUTEX2_SIZE_MASK) != FUTEX2_32)
 			return -EINVAL;
 
 		futexv[i].w.flags = aux.flags;