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

From: William Allen Simpson <[email-masked]>
Date: Tue, 12 Jan 2010 12:14:17 -0500

Eric Dumazet wrote:
> 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;
> 
Actually, tcp_header_len_th() has a multiply by 4 in it, too.

My code exactly parallels the existing code.  That is slightly faster
for bad packets, as it does the raw comparison first, saving the
multiply by 4 until it has passed that test.

My change just saves a store (and maybe a register load) over the
existing code.  This is supposed to be "fast path" after all....

Also adds comments that explain what we're doing.  As I mentioned
earlier in the thread:

                  ... back on Nov 10th, Ilpo brought to my attention --
   *hidden* inside the pskb_may_pull() -- the tcp header length is range
   checked for being too short (skb->len < th->doff * 4).

Anytime I find the code isn't obvious to me, I figure the next person
will benefit from some more comments....

(Of course, this patch also fixes existing comments that are not true!)