tcp: remove redundant checks

tcp_v4_rcv() checks the following before calling tcp_v4_do_rcv():

if (th->doff < sizeof(struct tcphdr) / 4)
    goto bad_packet;
if (!pskb_may_pull(skb, th->doff * 4))
    goto discard_it;

So following check in tcp_v4_do_rcv() is redundant
and "goto csum_err;" is wrong anyway.

if (skb->len < tcp_hdrlen(skb) || ...)
	goto csum_err;

A second check can be removed after no_tcp_socket label for same reason.

Same tests can be removed in tcp_v6_do_rcv()

Note : short tcp frames are not properly accounted in tcpInErrs MIB,
because pskb_may_pull() failure simply drops incoming skb, we might
fix this in a separate patch.

Signed-off-by: Eric Dumazet  <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_ipv4.c

@@ -1400,7 +1400,7 @@ int tcp_v4_do_rcv(struct sock *sk, struct sk_buff *skb)
 		return 0;
 	}
 
-	if (skb->len < tcp_hdrlen(skb) || tcp_checksum_complete(skb))
+	if (tcp_checksum_complete(skb))
 		goto csum_err;
 
 	if (sk->sk_state == TCP_LISTEN) {
@@ -1647,7 +1647,7 @@ no_tcp_socket:
 	if (!xfrm4_policy_check(NULL, XFRM_POLICY_IN, skb))
 		goto discard_it;
 
-	if (skb->len < (th->doff << 2) || tcp_checksum_complete(skb)) {
+	if (tcp_checksum_complete(skb)) {
 csum_error:
 		TCP_INC_STATS_BH(net, TCP_MIB_CSUMERRORS);
 bad_packet:

net/ipv6/tcp_ipv6.c

@@ -1250,7 +1250,7 @@ static int tcp_v6_do_rcv(struct sock *sk, struct sk_buff *skb)
 		return 0;
 	}
 
-	if (skb->len < tcp_hdrlen(skb) || tcp_checksum_complete(skb))
+	if (tcp_checksum_complete(skb))
 		goto csum_err;
 
 	if (sk->sk_state == TCP_LISTEN) {
@@ -1442,7 +1442,7 @@ no_tcp_socket:
 
 	tcp_v6_fill_cb(skb, hdr, th);
 
-	if (skb->len < (th->doff<<2) || tcp_checksum_complete(skb)) {
+	if (tcp_checksum_complete(skb)) {
 csum_error:
 		TCP_INC_STATS_BH(net, TCP_MIB_CSUMERRORS);
 bad_packet: