From: Chris Leech <christopher.leech@intel.com>
To: netdev@oss.sgi.com, linux-kernel <linux-kernel@vger.kernel.org>
Subject: skb_padto and small fragmented transmits
Date: 05 Feb 2003 13:39:51 -0800 [thread overview]
Message-ID: <1044481190.9268.43.camel@localhost.localdomain> (raw)
While looking at the new software padding routines, something caught my
eye in skb_padto. It seemed that the fragmented portion of a packet
would actually be counted twice when checking to see if padding is
needed, as skb->len already includes the count of skb->data_len.
> unsigned int size = skb->len + skb->data_len;
I tested this by modifying e1000 to use skb_padto, disabling TCP
timestamps, and writing a small app to transmit 4 bytes using sendfile.
The resulting packet had 54 bytes of headers, and 4 bytes of data in a
separate fragment. Calling skb_padto(skb,60) should have linearized the
skb, and zeroed out the first 2 bytes of tailroom. Instead the length
was incorrectly calculated as 62 bytes, and the buffer was returned as
is.
Changing skb_padto to simply use size = skb->len fixed the padding, but
then I started seeing incorrect TCP checksums going out. I found this
comment in skb_copy_expand that seemed to explain things.
> BUG ALERT: ip_summed is not copied. Why does this work? Is it used
> only by netfilter in the cases when checksum is recalculated? --ANK
So after calling skb_copy_expand the checksum is not recalculated in
software, but the checksum offload information is discarded.
--
Chris Leech <christopher.leech@intel.com>
next reply other threads:[~2003-02-05 21:01 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-02-05 21:39 Chris Leech [this message]
2003-02-06 11:58 ` skb_padto and small fragmented transmits David S. Miller
[not found] <BD9B60A108C4D511AAA10002A50708F20BA2AAE3@orsmsx118.jf.intel.com>
2003-02-06 19:22 ` Chris Leech
2003-02-06 22:43 ` David S. Miller
2003-02-07 13:33 ` Alan Cox
2003-02-08 8:53 ` David S. Miller
[not found] <BD9B60A108C4D511AAA10002A50708F20BA2AAD1@orsmsx118.jf.intel.com>
2003-02-06 19:22 ` Chris Leech
2003-02-06 18:44 ` David S. Miller
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