[PATCH] ocfs2: fix crash issue if access released lockres in debugfs

[Rock Li]

Access locking_state of dlm debugfs may cause crash as scene below:

Proc A:                  Proc that access debuginfo:
add_lockres_tracking(lockresA)
...
                         ocfs2_dlm_seq_next():
                           //priv->p_iter_res points to next
                           //lockres e.g. B. priv->p_tmp_res hold
                           //copy of lockres A before leave
                         ocfs2_dlm_seq_show()
...
remove_lockres_tracking(lockres B):
  //free lockres B, l_debug_list in
  //priv->p_ter_res is updated but not
  //priv->p_tmp_res
...
                         ocfs2_dlm_seq_next():
			   //priv->p_tmp_res which holds a old copy of
                           //lockres A, the l_debug_list holds a
                           //out-of-date succeed pointer, which will
                           //cause crash as //access invalid memory
                           iter = v; //priv->p_tmp_res
                           iter = ocfs2_dlm_next_res(iter, priv)

The root cause of this issue is that private->p_iter_res acts as the
agent of accessing lockres and is protected by ocfs2_dlm_tracking_lock
while p_tmp_res is only a copy of the lockres and will be out-of-dated
after leave critial region of ocfs2_dlm_tracking_lock. We should use
priv->p_ter_res as the forward iterater instead.

Signed-off-by: Rock Li <lihongweizz@xxxxxxxxxx>
---
 fs/ocfs2/dlmglue.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ocfs2/dlmglue.c b/fs/ocfs2/dlmglue.c
index c28bc98..5d84350 100644
--- a/fs/ocfs2/dlmglue.c
+++ b/fs/ocfs2/dlmglue.c
@@ -3109,7 +3109,7 @@ static void *ocfs2_dlm_seq_next(struct seq_file *m, void *v, loff_t *pos)
 	struct ocfs2_lock_res *dummy = &priv->p_iter_res;
 
 	spin_lock(&ocfs2_dlm_tracking_lock);
-	iter = ocfs2_dlm_next_res(iter, priv);
+	iter = ocfs2_dlm_next_res(dummy, priv);
 	list_del_init(&dummy->l_debug_list);
 	if (iter) {
 		list_add(&dummy->l_debug_list, &iter->l_debug_list);
-- 
1.8.3.1