[PATCH] ipv4: fix a RCU-list lock in fib_triestat_seq_show

[Qian Cai]

fib_triestat_seq_show() calls hlist_for_each_entry_rcu(tb, head,
tb_hlist) without rcu_read_lock() will trigger a warning,

 net/ipv4/fib_trie.c:2579 RCU-list traversed in non-reader section!!

 other info that might help us debug this:

 rcu_scheduler_active = 2, debug_locks = 1
 1 lock held by proc01/115277:
  #0: c0000014507acf00 (&p->lock){+.+.}-{3:3}, at: seq_read+0x58/0x670

 Call Trace:
  dump_stack+0xf4/0x164 (unreliable)
  lockdep_rcu_suspicious+0x140/0x164
  fib_triestat_seq_show+0x750/0x880
  seq_read+0x1a0/0x670
  proc_reg_read+0x10c/0x1b0
  __vfs_read+0x3c/0x70
  vfs_read+0xac/0x170
  ksys_read+0x7c/0x140
  system_call+0x5c/0x68

Signed-off-by: Qian Cai <cai@xxxxxx>
---
 net/ipv4/fib_trie.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c
index ff0c24371e33..73fa37476f03 100644
--- a/net/ipv4/fib_trie.c
+++ b/net/ipv4/fib_trie.c
@@ -2577,6 +2577,7 @@ static int fib_triestat_seq_show(struct seq_file *seq, void *v)
 		   " %zd bytes, size of tnode: %zd bytes.\n",
 		   LEAF_SIZE, TNODE_SIZE(0));
 
+	rcu_read_lock();
 	for (h = 0; h < FIB_TABLE_HASHSZ; h++) {
 		struct hlist_head *head = &net->ipv4.fib_table_hash[h];
 		struct fib_table *tb;
@@ -2597,6 +2598,7 @@ static int fib_triestat_seq_show(struct seq_file *seq, void *v)
 #endif
 		}
 	}
+	rcu_read_unlock();
 
 	return 0;
 }
-- 
2.21.0 (Apple Git-122.2)