rseq: Don't advertise time slice extensions if disabled

commit 010b772 upstream.

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: d620024 ("rseq: Allow registering RSEQ with slice extension")
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Tested-by: Dmitry Vyukov <dvyukov@google.com>
Link: https://patch.msgid.link/20260428224427.437059375%40kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Thomas Gleixner

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;
 

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) {