netdev - Re: [PATCH v2] smsc95xx: Use skb_cow

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Date:   Wed, 19 Apr 2017 09:33:14 +0100
From:   James Hughes <james.hughes@...pberrypi.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Woojung.Huh@...rochip.com, netdev@...r.kernel.org,
        Steve Glendinning <steve.glendinning@...well.net>,
        Microchip Linux Driver Support <UNGLinuxDriver@...rochip.com>
Subject: Re: [PATCH v2] smsc95xx: Use skb_cow_head to deal with cloned skbs

On 18 April 2017 at 23:46, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Tue, 2017-04-18 at 22:09 +0000, Woojung.Huh@...rochip.com wrote:
>> > > @@ -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 header space by overhead if required
>> > */
>> > > + if (skb_cow_head(skb, overhead)) {
>> >
>> > I believe you still need to
>> >             dev_kfree_skb_any(skb);
>> >
>> I think caller of usbnet_start_xmit() takes care of free when return NULL.
>
> I do not think so. Here is the code in usbnet_start_xmit() :
>
> if (info->tx_fixup) {
>    skb = info->tx_fixup (dev, skb, GFP_ATOMIC);
>
>    if (!skb) { // Note that skb is NULL now
>
>        if (info->flags & FLAG_MULTI_PACKET)
>              goto not_drop;
>         netif_dbg(dev, tx_err, dev->net, "can't tx_fixup skb\n");
>         goto drop;
>
>
> If you really think about it (even before double checking the code in
> usbnet_start_xmit()), that would have been a very serious bug.
>
> Calling dev_kfree_skb_any(skb) in this fixup code, then later from
> usbnet_start_xmit() would have been a double free, or use after free.
>
>

So, I still need to return NULL (as per the code this is replacing) to
indicate failure, but need to free the skb prior to return, as per
fragment below.

/* Make writable and expand header space by overhead if required */
if (skb_cow_head(skb, overhead)) {
   dev_kfree_skb_any(skb);
   return NULL;
}

Once confirmed, I'll do a another patch.