netdev - Re: Bug in computing data_len in tcp

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Date:	Thu, 1 Dec 2011 22:18:11 -0800
From:	Vijay Subramanian <subramanian.vijay@...il.com>
To:	Eric Dumazet <eric.dumazet@...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?

>> 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.

Regards,
Vijay Subramanian
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