Subject: Re: [PATCH] tcp: harmonize tcp_vx_rcv header length assumptions

From: Eric Dumazet <[email-masked]>
Date: Tue, 12 Jan 2010 11:46:02 +0100

Le 12/01/2010 11:05, William Allen Simpson a écrit :
> Harmonize tcp_v4_rcv() and tcp_v6_rcv() -- better document tcp doff and
> header length assumptions.
> 
> Reduces multiply/shifts, marginally improving speed.
> 
> Retains redundant tcp header length checks before checksumming.
> 
> Stand-alone patch, originally developed for TCPCT.
> 
> Requires:
>   net: tcp_header_len_th and tcp_option_len_th
> 
> Signed-off-by: [email-masked]
> ---
>  net/ipv4/tcp_ipv4.c |   36 +++++++++++++++++++-------------
>  net/ipv6/tcp_ipv6.c |   56
> ++++++++++++++++++++++++++++----------------------
>  2 files changed, 52 insertions(+), 40 deletions(-)

Seems fine, but :

1) What means the "Transformed ?" you wrote several times ?

2) This part :

-	th = tcp_hdr(skb);
-
-	if (th->doff < sizeof(struct tcphdr)/4)
+	/* Check bad doff, compare doff directly to constant value */
+	tcp_header_len = tcp_hdr(skb)->doff;
+	if (tcp_header_len < (sizeof(struct tcphdr) / 4))
 		goto bad_packet;
-	if (!pskb_may_pull(skb, th->doff*4))
+
+	/* Check too short header and options */
+	tcp_header_len *= 4;
+	if (!pskb_may_pull(skb, tcp_header_len))
 		goto discard_it;

could be :  (no need for 4 multiplies/divides)

	tcp_header_len = tcp_header_len_th(tcp_hdr(skb));
	if (tcp_header_len < sizeof(struct tcphdr))
		goto bad_packet;
	/* Check too short header and options */
	if (!pskb_may_pull(skb, tcp_header_len))
		goto discard_it;