netdev - Re: [PATCH] smsc95xx: Use skb_cow to deal with cloned skbs

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Date:   Tue, 18 Apr 2017 11:55:46 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     eric.dumazet@...il.com
Cc:     james.hughes@...pberrypi.org, netdev@...r.kernel.org,
        steve.glendinning@...well.net, UNGLinuxDriver@...rochip.com
Subject: Re: [PATCH] smsc95xx: Use skb_cow to deal with cloned skbs

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Tue, 18 Apr 2017 08:51:51 -0700

> On Tue, 2017-04-18 at 15:48 +0100, James Hughes wrote:
>> The driver was failing to check that the SKB wasn't cloned
>> before adding checksum data or adding header data.
>> Replace existing handling to extend the buffer with
>> skb_cow. Don't use skb_cow_head as the sw checksum
>> code modifies the data portion.
>> 
>> Signed-off-by: James Hughes <james.hughes@...pberrypi.org>
>> ---
>>  drivers/net/usb/smsc95xx.c | 10 +++-------
>>  1 file changed, 3 insertions(+), 7 deletions(-)
>> 
>> diff --git a/drivers/net/usb/smsc95xx.c b/drivers/net/usb/smsc95xx.c
>> index df60c98..04f6397 100644
>> --- a/drivers/net/usb/smsc95xx.c
>> +++ b/drivers/net/usb/smsc95xx.c
>> @@ -2067,13 +2067,9 @@ static struct sk_buff *smsc95xx_tx_fixup(struct usbnet *dev,
>>  	/* We do not advertise SG, so skbs should be already linearized */
>>  	BUG_ON(skb_shinfo(skb)->nr_frags);
>>  
>> -	if (skb_headroom(skb) < overhead) {
>> -		struct sk_buff *skb2 = skb_copy_expand(skb,
>> -			overhead, 0, flags);
>> -		dev_kfree_skb_any(skb);
>> -		skb = skb2;
>> -		if (!skb)
>> -			return NULL;
>> +	/* Make writable and expand space by overhead if required */
>> +	if (skb_cow(skb, overhead)) {
>> +		return NULL;
>>  	}
> 
> Note that this patch will probably force a copy of all locally generated
> TCP packets.
> 
> For them skb_cloned(skb) is true.
> 
> I do believe skb_cow_head() would be better, since TCP stack uses the
> __skb_header_release() helper to tell lower stacks they can write the
> header part, even on a clone.

Agreed.