Re: [ 074/167] [PATCH 14/26] tcp: fix tcp_trim_head()

Jonathan Nieder
2012-05-09T08:16:31Z

Ben Hutchings wrote:

> 3.2-stable review patch. If anyone has any objections, please let me know.
[...]
> [ Upstream commit 4fa48bf3c75069d636fc8830743c929a062e80dc ]
>
> commit f07d960df3 (tcp: avoid frag allocation for small frames)
> breaked assumption in tcp stack that skb is either linear (skb->data_len
> == 0), or fully fragged (skb->data_len == skb->len)
>
> tcp_trim_head() made this assumption, we must fix it.

Is this needed in the context of 3.2.y (which does not include
f07d960df3)?

Curious,
Jonathan

> Thanks to Vijay for providing a very detailed explanation.
>
> Reported-by: Vijay Subramanian <subramanian.vijay@gmail.com>
> Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
> Signed-off-by: David S. Miller <davem@davemloft.net>
> Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
> ---
> net/ipv4/tcp_output.c | 13 ++++++++-----
> 1 file changed, 8 insertions(+), 5 deletions(-)
>
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 097e0c7..7413437 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -1093,6 +1093,13 @@ static void __pskb_trim_head(struct sk_buff *skb, int len)
> {
> int i, k, eat;
>
> + eat = min_t(int, len, skb_headlen(skb));
> + if (eat) {
> + __skb_pull(skb, eat);
> + len -= eat;
> + if (!len)
> + return;
> + }
> eat = len;
> k = 0;
> for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
> @@ -1124,11 +1131,7 @@ int tcp_trim_head(struct sock *sk, struct sk_buff *skb, u32 len)
> if (skb_cloned(skb) && pskb_expand_head(skb, 0, 0, GFP_ATOMIC))
> return -ENOMEM;
>
> - /* If len == headlen, we avoid __skb_pull to preserve alignment. */
> - if (unlikely(len < skb_headlen(skb)))
> - __skb_pull(skb, len);
> - else
> - __pskb_trim_head(skb, len - skb_headlen(skb));
> + __pskb_trim_head(skb, len);
>
> TCP_SKB_CB(skb)->seq += len;
> skb->ip_summed = CHECKSUM_PARTIAL;
> --
> 1.7.10