packet: read num_members once in packet_rcv_fanout()

edumazet · davem330 · commit f98f4514d078 · 2015-06-21T09:23:22.000-07:00
We need to tell compiler it must not read f->num_members multiple times. Otherwise testing if num is not zero is flaky, and we could attempt an invalid divide by 0 in fanout_demux_cpu() Note bug was present in packet_rcv_fanout_hash() and packet_rcv_fanout_lb() but final 3.1 had a simple location after commit 95ec3eb ("packet: Add 'cpu' fanout policy.") Fixes: dc99f60 ("packet: Add fanout support.") Signed-off-by: Eric Dumazet <edumazet@google.com> Cc: Willem de Bruijn <willemb@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
@@ -1353,7 +1353,7 @@ static int packet_rcv_fanout(struct sk_buff *skb, struct net_device *dev,
 			     struct packet_type *pt, struct net_device *orig_dev)
 {
 	struct packet_fanout *f = pt->af_packet_priv;
-	unsigned int num = f->num_members;
+	unsigned int num = READ_ONCE(f->num_members);
 	struct packet_sock *po;
 	unsigned int idx;
 

Original file line number	Diff line number	Diff line change
`@@ -1353,7 +1353,7 @@ static int packet_rcv_fanout(struct sk_buff skb, struct net_device dev,`
`1353`	`1353`	`struct packet_type pt, struct net_device orig_dev)`
`1354`	`1354`	`{`
`1355`	`1355`	`struct packet_fanout *f = pt->af_packet_priv;`
`1356`		`- unsigned int num = f->num_members;`
	`1356`	`+ unsigned int num = READ_ONCE(f->num_members);`
`1357`	`1357`	`struct packet_sock *po;`
`1358`	`1358`	`unsigned int idx;`
`1359`	`1359`