From mboxrd@z Thu Jan 1 00:00:00 1970 From: Chris Leech Subject: skb_padto and small fragmented transmits Date: 05 Feb 2003 13:39:51 -0800 Sender: netdev-bounce@oss.sgi.com Message-ID: <1044481190.9268.43.camel@localhost.localdomain> Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Return-path: To: netdev@oss.sgi.com, linux-kernel Errors-to: netdev-bounce@oss.sgi.com List-Id: netdev.vger.kernel.org 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