Messages in this thread | | | Date | Tue, 12 Jan 2010 12:14:17 -0500 | From | William Allen Simpson <> | Subject | Re: [PATCH] tcp: harmonize tcp_vx_rcv header length assumptions |
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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!)
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