linux-kernel - [PATCH] futex: Use user_write_access_begin() in futex_put

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Message-ID: <20250809212442.240540-1-longman@redhat.com>
Date: Sat,  9 Aug 2025 17:24:42 -0400
From: Waiman Long <longman@...hat.com>
To: Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Darren Hart <dvhart@...radead.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	André Almeida <andrealmeid@...lia.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: linux-kernel@...r.kernel.org,
	Waiman Long <longman@...hat.com>
Subject: [PATCH] futex: Use user_write_access_begin() in futex_put_value()

Commit cec199c5e39b ("futex: Implement FUTEX2_NUMA") introduces a new
futex_put_value() helper function to write a value to the given user
address. However, it uses user_read_access_begin() before the write.
For arches that differentiate between read and write accesses, like
powerpc, futex_put_value() fails with a -EFAULT return value.  Fix that
by using user_write_access_begin().

Fixes: cec199c5e39b ("futex: Implement FUTEX2_NUMA")
Signed-off-by: Waiman Long <longman@...hat.com>
---
 kernel/futex/futex.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/futex/futex.h b/kernel/futex/futex.h
index c74eac572acd..2b6ae6a2b2a2 100644
--- a/kernel/futex/futex.h
+++ b/kernel/futex/futex.h
@@ -319,7 +319,7 @@ 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_read_access_begin(to, sizeof(*to)))
+	else if (!user_write_access_begin(to, sizeof(*to)))
 		return -EFAULT;
 	unsafe_put_user(val, to, Efault);
 	user_read_access_end();
-- 
2.50.1