eventfs: Free all of the eventfs_inode after RCU

The freeing of eventfs_inode via a kfree_rcu() callback. But the content
of the eventfs_inode was being freed after the last kref. This is
dangerous, as changes are being made that can access the content of an
eventfs_inode from an RCU loop.

Instead of using kfree_rcu() use call_rcu() that calls a function to do
all the freeing of the eventfs_inode after a RCU grace period has expired.

Link: https://lore.kernel.org/linux-trace-kernel/20240502200905.370261163@goodmis.org

Cc: stable@vger.kernel.org
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Fixes: 43aa6f97 ("eventfs: Get rid of dentry pointers without refcounts")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

fs/tracefs/event_inode.c

@@ -73,6 +73,21 @@ enum {
 
 #define EVENTFS_MODE_MASK	(EVENTFS_SAVE_MODE - 1)
 
+static void free_ei_rcu(struct rcu_head *rcu)
+{
+	struct eventfs_inode *ei = container_of(rcu, struct eventfs_inode, rcu);
+	struct eventfs_root_inode *rei;
+
+	kfree(ei->entry_attrs);
+	kfree_const(ei->name);
+	if (ei->is_events) {
+		rei = get_root_inode(ei);
+		kfree(rei);
+	} else {
+		kfree(ei);
+	}
+}
+
 /*
  * eventfs_inode reference count management.
  *
@@ -85,7 +100,6 @@ static void release_ei(struct kref *ref)
 {
 	struct eventfs_inode *ei = container_of(ref, struct eventfs_inode, kref);
 	const struct eventfs_entry *entry;
-	struct eventfs_root_inode *rei;
 
 	WARN_ON_ONCE(!ei->is_freed);
 
@@ -95,14 +109,7 @@ static void release_ei(struct kref *ref)
 			entry->release(entry->name, ei->data);
 	}
 
-	kfree(ei->entry_attrs);
-	kfree_const(ei->name);
-	if (ei->is_events) {
-		rei = get_root_inode(ei);
-		kfree_rcu(rei, ei.rcu);
-	} else {
-		kfree_rcu(ei, rcu);
-	}
+	call_rcu(&ei->rcu, free_ei_rcu);
 }
 
 static inline void put_ei(struct eventfs_inode *ei)