netdev - Re: Bug in computing data_len in tcp

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Message-ID: <1322858880.2762.69.camel@edumazet-laptop>
Date:	Fri, 02 Dec 2011 21:48:00 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	subramanian.vijay@...il.com, therbert@...gle.com,
	netdev@...r.kernel.org
Subject: Re: Bug in computing data_len in tcp_sendmsg?

Le vendredi 02 décembre 2011 à 21:45 +0100, Eric Dumazet a écrit :
> Le vendredi 02 décembre 2011 à 15:24 -0500, David Miller a écrit :
> > From: Eric Dumazet <eric.dumazet@...il.com>
> > Date: Fri, 02 Dec 2011 21:22:49 +0100
> > 
> > > Le vendredi 02 décembre 2011 à 13:40 -0500, David Miller a écrit :
> > > 
> > >> Yes, for non-SG this always was technically possible.
> > >> 
> > > 
> > > Yes this can, I reproduced it very easily.
> > > 
> > > I find this hard to believe...
> > 
> > Grrr :-)
> > 
> > Ok, I'll think about this some more.
> 
> Maybe a quick fix would be to trim not len bytes but (len & ~3) bytes ?
> 
> This avoid reallocations and complex code for a 'should never happen in
> normal circumstances'...
> 
> Retransmits could transmits 3 bytes already ACKed, is it a big deal ?
> 
> patch on top of linux/net tree :
> 
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 63170e2..4e108c5 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -1126,7 +1126,7 @@ int tcp_trim_head(struct sock *sk, struct sk_buff *skb, u32 len)
>  
>  	/* If len == headlen, we avoid __skb_pull to preserve alignment. */
>  	if (unlikely(len < skb_headlen(skb)))
> -		__skb_pull(skb, len);
> +		__skb_pull(skb, len & ~3); /* preserve alignement of tcp/ip headers */
>  	else
>  		__pskb_trim_head(skb, len - skb_headlen(skb));
>  
> 


oops, thats the wrong version, here is it again :

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 63170e2..492b917 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1125,11 +1125,12 @@ int tcp_trim_head(struct sock *sk, struct sk_buff *skb, u32 len)
 		return -ENOMEM;
 
 	/* If len == headlen, we avoid __skb_pull to preserve alignment. */
-	if (unlikely(len < skb_headlen(skb)))
+	if (unlikely(len < skb_headlen(skb))) {
+		len &= ~3;
 		__skb_pull(skb, len);
-	else
+	} else {
 		__pskb_trim_head(skb, len - skb_headlen(skb));
-
+	}
 	TCP_SKB_CB(skb)->seq += len;
 	skb->ip_summed = CHECKSUM_PARTIAL;
 


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