[RFC PATCH 1/2] kfence: Allow re-enabling KFENCE after system startup

[Tianchen Ding]

If once KFENCE is disabled by:
echo 0 > /sys/module/kfence/parameters/sample_interval
KFENCE could never be re-enabled until next rebooting.

Allow re-enabling it by writing a positive num to sample_interval.

Signed-off-by: Tianchen Ding <dtcccc@xxxxxxxxxxxxxxxxx>
---
 mm/kfence/core.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/mm/kfence/core.c b/mm/kfence/core.c
index 13128fa13062..19eb123c0bba 100644
--- a/mm/kfence/core.c
+++ b/mm/kfence/core.c
@@ -55,6 +55,7 @@ EXPORT_SYMBOL_GPL(kfence_sample_interval); /* Export for test modules. */
 #endif
 #define MODULE_PARAM_PREFIX "kfence."
 
+static int kfence_enable_late(void);
 static int param_set_sample_interval(const char *val, const struct kernel_param *kp)
 {
 	unsigned long num;
@@ -65,10 +66,11 @@ static int param_set_sample_interval(const char *val, const struct kernel_param
 
 	if (!num) /* Using 0 to indicate KFENCE is disabled. */
 		WRITE_ONCE(kfence_enabled, false);
-	else if (!READ_ONCE(kfence_enabled) && system_state != SYSTEM_BOOTING)
-		return -EINVAL; /* Cannot (re-)enable KFENCE on-the-fly. */
 
 	*((unsigned long *)kp->arg) = num;
+
+	if (num && !READ_ONCE(kfence_enabled) && system_state != SYSTEM_BOOTING)
+		return kfence_enable_late();
 	return 0;
 }
 
@@ -787,6 +789,16 @@ void __init kfence_init(void)
 		(void *)(__kfence_pool + KFENCE_POOL_SIZE));
 }
 
+static int kfence_enable_late(void)
+{
+	if (!__kfence_pool)
+		return -EINVAL;
+
+	WRITE_ONCE(kfence_enabled, true);
+	queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
+	return 0;
+}
+
 void kfence_shutdown_cache(struct kmem_cache *s)
 {
 	unsigned long flags;
-- 
2.27.0