kmemleak: Report previously found leaks even after an error

If an error fatal to kmemleak (like memory allocation failure) happens,
kmemleak disables itself but it also removes the access to any
previously found memory leaks. This patch allows read-only access to the
kmemleak debugfs interface but disables any other action.
Reported-by: NNick Bowler <nbowler@elliptictech.com>
Signed-off-by: NCatalin Marinas <catalin.marinas@arm.com>

mm/kmemleak.c

@@ -1473,9 +1473,6 @@ static const struct seq_operations kmemleak_seq_ops = {
 
 static int kmemleak_open(struct inode *inode, struct file *file)
 {
-	if (!atomic_read(&kmemleak_enabled))
-		return -EBUSY;
-
 	return seq_open(file, &kmemleak_seq_ops);
 }
 
@@ -1549,6 +1546,9 @@ static ssize_t kmemleak_write(struct file *file, const char __user *user_buf,
 	int buf_size;
 	int ret;
 
+	if (!atomic_read(&kmemleak_enabled))
+		return -EBUSY;
+
 	buf_size = min(size, (sizeof(buf) - 1));
 	if (strncpy_from_user(buf, user_buf, buf_size) < 0)
 		return -EFAULT;
@@ -1608,20 +1608,24 @@ static const struct file_operations kmemleak_fops = {
 };
 
 /*
- * Perform the freeing of the kmemleak internal objects after waiting for any
- * current memory scan to complete.
+ * Stop the memory scanning thread and free the kmemleak internal objects if
+ * no previous scan thread (otherwise, kmemleak may still have some useful
+ * information on memory leaks).
  */
 static void kmemleak_do_cleanup(struct work_struct *work)
 {
 	struct kmemleak_object *object;
+	bool cleanup = scan_thread == NULL;
 
 	mutex_lock(&scan_mutex);
 	stop_scan_thread();
 
+	if (cleanup) {
 		rcu_read_lock();
 		list_for_each_entry_rcu(object, &object_list, object_list)
 			delete_object_full(object->pointer);
 		rcu_read_unlock();
+	}
 	mutex_unlock(&scan_mutex);
 }