linux-kernel - [patch 11/11] rseq: Convert to masked user access where applicable

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Message-ID: <20250813162824.484590626@linutronix.de>
Date: Wed, 13 Aug 2025 18:29:39 +0200 (CEST)
From: Thomas Gleixner <tglx@...utronix.de>
To: LKML <linux-kernel@...r.kernel.org>
Cc: Michael Jeanson <mjeanson@...icios.com>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Peter Zijlstra <peterz@...radead.org>,
 "Paul E. McKenney" <paulmck@...nel.org>,
 Boqun Feng <boqun.feng@...il.com>,
 Wei Liu <wei.liu@...nel.org>,
 Jens Axboe <axboe@...nel.dk>
Subject: [patch 11/11] rseq: Convert to masked user access where applicable

Masked user access optimizes the Spectre-V1 speculation barrier on
architectures which support it. As rseq_handle_notify_resume() is
frequently invoked, the access to the critical section pointer in the rseq
ABI is a hotpath, which is worth to optimize.

Replace also the clearing with the optimized version.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Boqun Feng <boqun.feng@...il.com>
---
 kernel/rseq.c |   23 +++--------------------
 1 file changed, 3 insertions(+), 20 deletions(-)

--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -245,20 +245,9 @@ static int rseq_reset_rseq_cpu_node_id(s
 /*
  * Get the user-space pointer value stored in the 'rseq_cs' field.
  */
-static int rseq_get_rseq_cs_ptr_val(struct rseq __user *rseq, u64 *rseq_cs)
+static inline int rseq_get_rseq_cs_ptr_val(struct rseq __user *rseq, u64 *rseq_cs)
 {
-	if (!rseq_cs)
-		return -EFAULT;
-
-#ifdef CONFIG_64BIT
-	if (get_user(*rseq_cs, &rseq->rseq_cs))
-		return -EFAULT;
-#else
-	if (copy_from_user(rseq_cs, &rseq->rseq_cs, sizeof(*rseq_cs)))
-		return -EFAULT;
-#endif
-
-	return 0;
+	return get_user_masked_u64(rseq_cs, &rseq->rseq_cs);
 }

 /*
@@ -358,13 +347,7 @@ static int clear_rseq_cs(struct rseq __u
 	 *
 	 * Set rseq_cs to NULL.
 	 */
-#ifdef CONFIG_64BIT
-	return put_user(0UL, &rseq->rseq_cs);
-#else
-	if (clear_user(&rseq->rseq_cs, sizeof(rseq->rseq_cs)))
-		return -EFAULT;
-	return 0;
-#endif
+	return put_user_masked_u64(0UL, &rseq->rseq_cs);
 }

 /*