Re: [PATCH] net: asix: fix bad header length bug

[Emil Goode <emilgoode@gmail.com>]

Hello David,

Thank's for the review.

On Thu, Feb 06, 2014 at 01:37:12PM +0000, David Laight wrote:
> From: Emil Goode
> > The AX88772B occasionally send rx packets that cross urb boundaries
> > and the remaining partial packet is sent with no header.
> > When the buffer with a partial packet is of less number of octets
> > than the value of hard_header_len the buffer is discarded by the
> > usbnet module. This is causing dropped packages and error messages
> > in dmesg.
> > 
> > This can be reproduced by using ping with a packet size
> > between 1965-1976.
> 
> I think this can affect other USB ethernet drivers.
> Probably most of the ones that explicitly set rx_urb_len.
> 
> The ax88179_178a driver sets massive 20k receive urb.

The ax88179_178a has it's own bind function, so I believe it's
not affected by this change.

> I've seen over 10k of data in a single urb, dunno if it
> can actually generate more than 20k - possibly if the usb3 link
> is loaded with other traffic.
> It would be much more efficient for it to use an aligned 4k urb
> and then merge the fragment into skbs.
> 
> Once you've set:
> +	dev->net->hard_header_len = 0; /* Partial packets have no header */
> try setting the mtu to a multiple of 1k.

I tried setting the mtu to 2000 and when using ping with a large enough
packet size to fill the urb to rx_urb_size all packages are dropped.

> There is a very odd check in usbnet_change_mtu() that tries to stop the
> receive urb_length being a multiple of the usb packet size.

This is very odd indeed!

>
> This code looks as though it is hoping that the usb controller will discard
> any full length bulk messages after finding a short buffer.
> I suspect that might be just wishful thinking!
> 
> 	David
> 
> 
>