netdev - Re: Bug in computing data_len in tcp

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Date:	Fri, 02 Dec 2011 12:59:48 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Vijay Subramanian <subramanian.vijay@...il.com>
Cc:	Tom Herbert <therbert@...gle.com>,
	Linux Netdev List <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>
Subject: Re: Bug in computing data_len in tcp_sendmsg?

Le jeudi 01 décembre 2011 à 22:18 -0800, Vijay Subramanian a écrit :
> >> I am looking at tcp_mtu_probe() and was wondering if this commit also
> >> impacts this function. Once the data are copied from skbs in the write
> >> queue to the probe skb, copied data are cleared from the original skbs
> >> in the write queue.
> >>
> >> It looks like the code assumes that the original skb will have data
> >> either in linear part or in paged part.  The call to
> >> __pskb_trim_head(skb, copy) for example does not clear linear part.
> >>
> >> Can someone more familiar with the code take a look? Apologies if I
> >> have read this wrong.
> >>
> >
> > tcp_mtu_probe() builds a linear skb, and populate it using
> > skb_copy_bits() [ this is frag aware, and aware of payload in header as
> > well ]
> >
> > I see no problem in it.
> >
> 
> Eric,
> I think you may have misunderstood me (I think my post was not very
> clear). Let me try again.
> 
> The MTU probe is built  correctly as a linear skb. As you point out,
> skb_copy_bits() is frag aware and copies data from both the header and
> pages.
> The issue is with the way the data is cleared from the write queue
> later in the function tcp_mtu_probe().
> 
> For example, if the MTU probe size was N bytes, then the probe is
> inserted at the front of the write_queue and N bytes are copied from
> the original write-queue skbs.
> As these N bytes are copied, if an skb is completely consumed, it is
> unlinked from the write_queue and freed. If an skb is only partially
> consumed, then the pointers are adjusted
> accordingly to erase the data. For a paged skb, fully consumed pages
> are unreferenced.
> 
> This is done as follows in tcp_mtu_probe()
> 
>            if (!skb_shinfo(skb)->nr_frags) {
>                skb_pull(skb, copy);
>                if (skb->ip_summed != CHECKSUM_PARTIAL)
>                    skb->csum = csum_partial(skb->data,
>                                 skb->len, 0);
>            } else {
>                __pskb_trim_head(skb, copy);
>                tcp_set_skb_tso_segs(sk, skb, mss_now);
>            }
> 
> It appears that the code assumes the data will either be in the linear
> part (the if condition) or in the paged part (else condition) but not
> both. Is this a correct assumption after the
> recent commit f07d960df33c5aef (tcp: avoid frag allocation for small frames)?
> 
> Since __pskb_trim_head() only only removes data from the non-linear
> part, the data in the linear part is never removed. Maybe for paged
> skbs, we need something like
> headlen = skb_headlen(skb);
> skb_pull(skb, headlen);
> __pskb_trim_head(skb, copy - headlen);
> 
> Thanks for your patience and hope this makes more sense than my previous post.
> 

Thanks for this detailed explanation !

And yes, you're probably right.

Are you willing to submit a patch to fix this ?

(If not, I can do it myself of course)


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