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

[Jonathan Nieder]

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@xxxxxxxxx>
> Signed-off-by: Eric Dumazet <eric.dumazet@xxxxxxxxx>
> Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
> Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
> ---
>  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
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