Messages in this thread | | | Subject | skb_padto and small fragmented transmits | From | Chris Leech <> | Date | 05 Feb 2003 13:39:51 -0800 |
| |
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>
- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |