linux-kernel - [patch V2 4/6] futex: Convert to scoped masked user access

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Message-ID: <20250916163252.227660125@linutronix.de>
Date: Tue, 16 Sep 2025 18:33:15 +0200 (CEST)
From: Thomas Gleixner <tglx@...utronix.de>
To: LKML <linux-kernel@...r.kernel.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
 Peter Zijlstra <peterz@...radead.org>,
 Darren Hart <dvhart@...radead.org>,
 Davidlohr Bueso <dave@...olabs.net>,
 André Almeida <andrealmeid@...lia.com>,
 kernel test robot <lkp@...el.com>,
 Russell King <linux@...linux.org.uk>,
 linux-arm-kernel@...ts.infradead.org,
 Nathan Chancellor <nathan@...nel.org>,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 x86@...nel.org,
 Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>,
 Jan Kara <jack@...e.cz>,
 linux-fsdevel@...r.kernel.org
Subject: [patch V2 4/6] futex: Convert to scoped masked user access

From: Thomas Gleixner <tglx@...utronix.de>

Replace the open coded implementation with the scoped masked user access
mechanism.

No functional change intended

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Darren Hart <dvhart@...radead.org>
Cc: Davidlohr Bueso <dave@...olabs.net>
Cc: "André Almeida" <andrealmeid@...lia.com>
---
V2: Convert to scoped variant
---
 kernel/futex/futex.h |   37 ++++---------------------------------
 1 file changed, 4 insertions(+), 33 deletions(-)
---
--- a/kernel/futex/futex.h
+++ b/kernel/futex/futex.h
@@ -285,48 +285,19 @@ static inline int futex_cmpxchg_value_lo
  * This does a plain atomic user space read, and the user pointer has
  * already been verified earlier by get_futex_key() to be both aligned
  * and actually in user space, just like futex_atomic_cmpxchg_inatomic().
- *
- * We still want to avoid any speculation, and while __get_user() is
- * the traditional model for this, it's actually slower than doing
- * this manually these days.
- *
- * We could just have a per-architecture special function for it,
- * the same way we do futex_atomic_cmpxchg_inatomic(), but rather
- * than force everybody to do that, write it out long-hand using
- * the low-level user-access infrastructure.
- *
- * This looks a bit overkill, but generally just results in a couple
- * of instructions.
  */
 static __always_inline int futex_get_value(u32 *dest, u32 __user *from)
 {
-	u32 val;
-
-	if (can_do_masked_user_access())
-		from = masked_user_access_begin(from);
-	else if (!user_read_access_begin(from, sizeof(*from)))
-		return -EFAULT;
-	unsafe_get_user(val, from, Efault);
-	user_read_access_end();
-	*dest = val;
+	scoped_masked_user_read_access(from, return -EFAULT,
+	       scoped_get_user(*dest, from); );
 	return 0;
-Efault:
-	user_read_access_end();
-	return -EFAULT;
 }
 
 static __always_inline int futex_put_value(u32 val, u32 __user *to)
 {
-	if (can_do_masked_user_access())
-		to = masked_user_access_begin(to);
-	else if (!user_write_access_begin(to, sizeof(*to)))
-		return -EFAULT;
-	unsafe_put_user(val, to, Efault);
-	user_write_access_end();
+	scoped_masked_user_write_access(to, return -EFAULT,
+	       scoped_put_user(val, to); );
 	return 0;
-Efault:
-	user_write_access_end();
-	return -EFAULT;
 }
 
 static inline int futex_get_value_locked(u32 *dest, u32 __user *from)