[tip: sched/urgent] rseq: Don't advertise time slice extensions if disabled

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     010b7723c0a3b9ad58f50b715dbe2e7781d29400
Gitweb:        https://git.kernel.org/tip/010b7723c0a3b9ad58f50b715dbe2e7781d29400
Author:        Thomas Gleixner <tglx@xxxxxxxxxx>
AuthorDate:    Tue, 28 Apr 2026 09:34:45 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Fri, 01 May 2026 21:32:20 +02:00

rseq: Don't advertise time slice extensions if disabled

If time slice extensions have been disabled on the kernel command line,
then advertising them in RSEQ flags is wrong.

Adjust the conditionals to reflect reality, fixup the misleading comments
about the gap of these flags and the rseq::flags field.

Fixes: d6200245c75e ("rseq: Allow registering RSEQ with slice extension")
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Tested-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Link: https://patch.msgid.link/20260428224427.437059375%40kernel.org
Cc: stable@xxxxxxxxxxxxxxx
---
 include/uapi/linux/rseq.h |  5 ++++-
 kernel/rseq.c             |  9 +++++----
 2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/include/uapi/linux/rseq.h b/include/uapi/linux/rseq.h
index f69344f..ca6fe1f 100644
--- a/include/uapi/linux/rseq.h
+++ b/include/uapi/linux/rseq.h
@@ -28,7 +28,7 @@ enum rseq_cs_flags_bit {
 	RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT_BIT	= 0,
 	RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL_BIT	= 1,
 	RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE_BIT	= 2,
-	/* (3) Intentional gap to put new bits into a separate byte */
+	/* (3) Intentional gap to keep new bits separate */
 
 	/* User read only feature flags */
 	RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE_BIT	= 4,
@@ -161,6 +161,9 @@ struct rseq {
 	 *	- RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT
 	 *	- RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL
 	 *	- RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE
+	 *
+	 * It is now used for feature status advertisement by the kernel.
+	 * See: enum rseq_cs_flags_bit for further information.
 	 */
 	__u32 flags;
 
diff --git a/kernel/rseq.c b/kernel/rseq.c
index b9f1193..586f58f 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -462,10 +462,11 @@ SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len, int, flags, u32
 		return -EFAULT;
 
 	if (IS_ENABLED(CONFIG_RSEQ_SLICE_EXTENSION)) {
-		rseqfl |= RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE;
-		if (rseq_slice_extension_enabled() &&
-		    (flags & RSEQ_FLAG_SLICE_EXT_DEFAULT_ON))
-			rseqfl |= RSEQ_CS_FLAG_SLICE_EXT_ENABLED;
+		if (rseq_slice_extension_enabled()) {
+			rseqfl |= RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE;
+			if (flags & RSEQ_FLAG_SLICE_EXT_DEFAULT_ON)
+				rseqfl |= RSEQ_CS_FLAG_SLICE_EXT_ENABLED;
+		}
 	}
 
 	scoped_user_write_access(rseq, efault) {