rcu: kmemleak: Ignore kmemleak false positives when RCU-freeing objects

Since the actual slab freeing is deferred when calling kvfree_rcu(), so
is the kmemleak_free() callback informing kmemleak of the object
deletion. From the perspective of the kvfree_rcu() caller, the object is
freed and it may remove any references to it. Since kmemleak does not
scan RCU internal data storing the pointer, it will report such objects
as leaks during the grace period.

Tell kmemleak to ignore such objects on the kvfree_call_rcu() path. Note
that the tiny RCU implementation does not have such issue since the
objects can be tracked from the rcu_ctrlblk structure.

Signed-off-by: Catalin Marinas <[email protected]>
Reported-by: Christoph Paasch <[email protected]>
Closes: https://lore.kernel.org/all/[email protected]/
Cc: <[email protected]>
Tested-by: Christoph Paasch <[email protected]>
Reviewed-by: Paul E. McKenney <[email protected]>
Signed-off-by: Joel Fernandes (Google) <[email protected]>
Signed-off-by: Frederic Weisbecker <[email protected]>

Author: ctmarinas

kernel/rcu/tree.c

@@ -31,6 +31,7 @@
 #include <linux/bitops.h>
 #include <linux/export.h>
 #include <linux/completion.h>
+#include <linux/kmemleak.h>
 #include <linux/moduleparam.h>
 #include <linux/panic.h>
 #include <linux/panic_notifier.h>
@@ -3389,6 +3390,14 @@ void kvfree_call_rcu(struct rcu_head *head, void *ptr)
 		success = true;
 	}
 
+	/*
+	 * The kvfree_rcu() caller considers the pointer freed at this point
+	 * and likely removes any references to it. Since the actual slab
+	 * freeing (and kmemleak_free()) is deferred, tell kmemleak to ignore
+	 * this object (no scanning or false positives reporting).
+	 */
+	kmemleak_ignore(ptr);
+
 	// Set timer to drain after KFREE_DRAIN_JIFFIES.
 	if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING)
 		schedule_delayed_monitor_work(krcp);