netdev - Re: [PATCH] usbnet: fix oops in usbnet_start

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Date:	Mon, 07 Nov 2011 18:05:22 +0400
From:	Konstantin Khlebnikov <khlebnikov@...nvz.org>
To:	Richard Cochran <richardcochran@...il.com>
CC:	Oliver Neukum <oneukum@...e.de>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	"devel@...nvz.org" <devel@...nvz.org>,
	Michael Riesch <michael@...sch.at>
Subject: Re: [PATCH] usbnet: fix oops in usbnet_start_xmit

Richard Cochran wrote:
> On Sun, Nov 06, 2011 at 10:33:37PM +0300, Konstantin Khlebnikov wrote:
>> This patch fixes the bug added in commit v3.1-rc7-1055-gf9b491e
>> SKB can be NULL at this point, at least for cdc-ncm.
>
> What? You mean .ndo_start_xmit is called with skb NULL?

no. cdc_ncm call usbnet_start_xmit with NULL skb from timer handler
and tx_fixup hook pickup skb from internal context. yeah, it really messy.

>
>> Let's call skb_tx_timestamp() after driver specific tx-fixup hacks.
>
> No, that won't work.
>
> That call is before the fixup on purpose, because some fixups add
> padding in front of the Ethernet payload, and this will spoil the PTP
> packet detection filter.
>
> I don't know why the skb can be NULL here. If that is really the case,
> then the correct fix is:
>
> 	if (skb)
> 		skb_tx_timestamp(skb);
>
> Thanks,
> Richard
>
>
>>
>> Signed-off-by: Konstantin Khlebnikov<khlebnikov@...nvz.org>
>> ---
>>   drivers/net/usb/usbnet.c |    4 ++--
>>   1 files changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
>> index 7d60821..485be70 100644
>> --- a/drivers/net/usb/usbnet.c
>> +++ b/drivers/net/usb/usbnet.c
>> @@ -1057,8 +1057,6 @@ netdev_tx_t usbnet_start_xmit (struct sk_buff *skb,
>>   	unsigned long		flags;
>>   	int retval;
>>
>> -	skb_tx_timestamp(skb);
>> -
>>   	// some devices want funky USB-level framing, for
>>   	// win32 driver (usually) and/or hardware quirks
>>   	if (info->tx_fixup) {
>> @@ -1075,6 +1073,8 @@ netdev_tx_t usbnet_start_xmit (struct sk_buff *skb,
>>   	}
>>   	length = skb->len;
>>
>> +	skb_tx_timestamp(skb);
>> +
>>   	if (!(urb = usb_alloc_urb (0, GFP_ATOMIC))) {
>>   		netif_dbg(dev, tx_err, dev->net, "no urb\n");
>>   		goto drop;
>>
>> --
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