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

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Date:	Fri, 07 Feb 2014 10:38:04 +0100
From:	Bjørn Mork <bjorn@...k.no>
To:	Emil Goode <emilgoode@...il.com>
Cc:	David Laight <David.Laight@...LAB.COM>,
	"'Igor Gnatenko'" <i.gnatenko.brain@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Ming Lei <ming.lei@...onical.com>,
	Mark Brown <broonie@...aro.org>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	Glen Turner <gdt@....id.au>,
	"linux-usb\@vger.kernel.org" <linux-usb@...r.kernel.org>,
	"netdev\@vger.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] net: asix: fix bad header length bug

Emil Goode <emilgoode@...il.com> writes:
> On Thu, Feb 06, 2014 at 03:28:13PM +0000, David Laight wrote:
>> From: Igor Gnatenko
>> > On Thu, 2014-02-06 at 13:56 +0100, Emil Goode wrote:
>> > > 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.

> I will do some more digging in the code, but the test of skb->len
> against hard_header_len is done already in the completion callback
> function passed to usb_fill_bulk_urb so it seems that buffers of less
> than hard_header_len number of octets will be dropped regardless.

I am pretty sure you are right about this bug. And the exact same
solution is already used by the cx82310_eth minidriver, so I don't see
the problem.  Your fix is fine IMHO.  But you should apply it to all the
devices using asix_rx_fixup_common(), not just the ax88772 ones.

You could maybe make this a usbnet flag instead and create a generic
solution in usbnet, but frankly I believe the number of flags and their
meaning have exceeded drivers authors capabilities a long time ago.  At
least mine, which are quite limited ;-)

An example of that problem is another bloody obvious bug I noticed while
looking at this driver: The 'struct driver_info ax88178_info' points to
asix_rx_fixup_common without setting the FLAG_MULTI_PACKET.  This will
result in usbnet rx_process() calling usbnet_skb_return() on skbs which
are already consumed by the minidriver.  Not a big problem, but will
give some odd results.  But if you allow skbs shorter than ETH_HLEN to
slip through then it might go boom, so you should probably fix that as
well.

Bjørn
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