[PATCH net 3/3] l2tp: hold reference on tunnels printed in l2tp/tunnels debugfs file

[Guillaume Nault]

Use l2tp_tunnel_get_nth() instead of l2tp_tunnel_find_nth(), to be safe
against concurrent tunnel deletion.

Use the same mechanism as in l2tp_ppp.c for dropping the reference
taken by l2tp_tunnel_get_nth(). That is, drop the reference just
before looking up the next tunnel. In case of error, drop the last
accessed tunnel in l2tp_dfs_seq_stop().

That was the last use of l2tp_tunnel_find_nth().

Fixes: 0ad6614048cf ("l2tp: Add debugfs files for dumping l2tp debug info")
Signed-off-by: Guillaume Nault <g.na...@alphalink.fr>
---
 net/l2tp/l2tp_core.c    | 20 --------------------
 net/l2tp/l2tp_core.h    |  1 -
 net/l2tp/l2tp_debugfs.c | 15 +++++++++++++--
 3 files changed, 13 insertions(+), 23 deletions(-)

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index c8c4183f0f37..40261cb68e83 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -355,26 +355,6 @@ int l2tp_session_register(struct l2tp_session *session,
 }
 EXPORT_SYMBOL_GPL(l2tp_session_register);
 
-struct l2tp_tunnel *l2tp_tunnel_find_nth(const struct net *net, int nth)
-{
-       struct l2tp_net *pn = l2tp_pernet(net);
-       struct l2tp_tunnel *tunnel;
-       int count = 0;
-
-       rcu_read_lock_bh();
-       list_for_each_entry_rcu(tunnel, &pn->l2tp_tunnel_list, list) {
-               if (++count > nth) {
-                       rcu_read_unlock_bh();
-                       return tunnel;
-               }
-       }
-
-       rcu_read_unlock_bh();
-
-       return NULL;
-}
-EXPORT_SYMBOL_GPL(l2tp_tunnel_find_nth);
-
 /*****************************************************************************
  * Receive data handling
  *****************************************************************************/
diff --git a/net/l2tp/l2tp_core.h b/net/l2tp/l2tp_core.h
index e4896413b2b6..c199020f8a8a 100644
--- a/net/l2tp/l2tp_core.h
+++ b/net/l2tp/l2tp_core.h
@@ -222,7 +222,6 @@ struct l2tp_session *l2tp_session_get(const struct net *net,
 struct l2tp_session *l2tp_session_get_nth(struct l2tp_tunnel *tunnel, int nth);
 struct l2tp_session *l2tp_session_get_by_ifname(const struct net *net,
                                                const char *ifname);
-struct l2tp_tunnel *l2tp_tunnel_find_nth(const struct net *net, int nth);
 
 int l2tp_tunnel_create(struct net *net, int fd, int version, u32 tunnel_id,
                       u32 peer_tunnel_id, struct l2tp_tunnel_cfg *cfg,
diff --git a/net/l2tp/l2tp_debugfs.c b/net/l2tp/l2tp_debugfs.c
index 72e713da4733..b8f9d45bfeb1 100644
--- a/net/l2tp/l2tp_debugfs.c
+++ b/net/l2tp/l2tp_debugfs.c
@@ -47,7 +47,11 @@ struct l2tp_dfs_seq_data {
 
 static void l2tp_dfs_next_tunnel(struct l2tp_dfs_seq_data *pd)
 {
-       pd->tunnel = l2tp_tunnel_find_nth(pd->net, pd->tunnel_idx);
+       /* Drop reference taken during previous invocation */
+       if (pd->tunnel)
+               l2tp_tunnel_dec_refcount(pd->tunnel);
+
+       pd->tunnel = l2tp_tunnel_get_nth(pd->net, pd->tunnel_idx);
        pd->tunnel_idx++;
 }
 
@@ -96,7 +100,14 @@ static void *l2tp_dfs_seq_next(struct seq_file *m, void *v, 
loff_t *pos)
 
 static void l2tp_dfs_seq_stop(struct seq_file *p, void *v)
 {
-       /* nothing to do */
+       struct l2tp_dfs_seq_data *pd = v;
+
+       if (!pd || pd == SEQ_START_TOKEN)
+               return;
+
+       /* Drop reference taken by last invocation of l2tp_dfs_next_tunnel() */
+       if (pd->tunnel)
+               l2tp_tunnel_dec_refcount(pd->tunnel);
 }
 
 static void l2tp_dfs_seq_tunnel_show(struct seq_file *m, void *v)
-- 
2.17.0