netdev - Re: Bug in computing data_len in tcp

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Message-ID: <CA+mtBx_CUsuGunKONwBNA8qyVni16+RtrRL8w=THYY0mEDcvaA@mail.gmail.com>
Date:	Wed, 30 Nov 2011 20:06:09 -0800
From:	Tom Herbert <therbert@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Linux Netdev List <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>
Subject: Re: Bug in computing data_len in tcp_sendmsg?

On Wed, Nov 30, 2011 at 7:42 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Le mercredi 30 novembre 2011 à 17:48 -0800, Tom Herbert a écrit :
>> I believe that skb->data_len might no be computed correctly in
>> tcp_sendmsg.  Specifically, when skb_add_data_nocache (or
>> skb_add_data) is called skb->data_len is not updated (skb_put only
>> updates skb->len).  This results in the datalen in the head skbuf
>> being zero so any subsequent uses of the value lead to incorrect
>> results.  For instance, skb_headlen returns the length of the head
>> skbu data and not just that of the headers.  If I'm reading this
>> correctly, it's a pretty fundamental bug.
>>
>> I don't have a fix for this yet.
>>
>> Tom
>
> On which tree do you see a problem ?
>
> For example net-next seems fine to me :
>
>

On net-next.  Look at skb_add_data_nocache.  This doesn't touch
data_len, and really can't since skb_put wants skb linearized.

Actually, I think I am misinterpreting the meaning of data_len, looks
like it is intended to return the number of bytes in the page buffer
area (comment on the function would be have been nice ;-) ).  It would
seem that e1000e driver might be misinterpreting it also:

        segs = skb_shinfo(skb)->gso_segs ? : 1;
        /* multiply data chunks by size of headers */
        bytecount = ((segs - 1) * skb_headlen(skb)) + skb->len;

So maybe the bug is  that e1000e is misusing the function (and it
looks like some other drivers would be too)?.
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