[PATCH] packet: tpacket_snd(): fix signed/unsigned comparison

[Alexander Drozdov <al.drozdov@gmail.com>]

tpacket_fill_skb() can return a negative value (-errno) which
is stored in tp_len variable. In that case the following
condition will be (but shouldn't be) true:

tp_len > dev->mtu + dev->hard_header_len

as dev->mtu and dev->hard_header_len are both unsigned.

That may lead to just returning an incorrect EMSGSIZE errno
to the user.

Signed-off-by: Alexander Drozdov <al.drozdov@gmail.com>
---
 net/packet/af_packet.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index c9e8741..d1d3625 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -2403,7 +2403,8 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
 		}
 		tp_len = tpacket_fill_skb(po, skb, ph, dev, size_max, proto,
 					  addr, hlen);
-		if (tp_len > dev->mtu + dev->hard_header_len) {
+		if (likely(tp_len >= 0) &&
+		    tp_len > dev->mtu + dev->hard_header_len) {
 			struct ethhdr *ehdr;
 			/* Earlier code assumed this would be a VLAN pkt,
 			 * double-check this now that we have the actual
-- 
1.9.1