Re: [PATCH net-next] mac80211: Check correct skb for shared states before freeing original

[Johannes Berg <johannes@sipsolutions.net>]

On Wed, 2014-09-10 at 16:06 -0400, Alexander Duyck wrote:
> The code for cloning the skb for an acknowledgement was checking to see if
> the cloned skb was shared and if it was it was then freeing the original
> skb.  Since a clone should never really be shared I suspect that the
> intention was to avoid freeing the clone if the original was shared.  As
> such I am updating the code so that if the original is shared we free the
> original and use the clone.  This avoids unnecessary work in the next
> section where we would be cloning the skb if the original is shared.

Thanks, yeah, I admit that this is clearly fishy.

> @@ -2087,7 +2087,7 @@ netdev_tx_t ieee80211_subif_start_xmit(struct sk_buff *skb,
>  			if (id >= 0) {
>  				info_id = id;
>  				info_flags |= IEEE80211_TX_CTL_REQ_TX_STATUS;

Luckily, we practically always go into this path.

> -			} else if (skb_shared(skb)) {
> +			} else if (skb_shared(orig_skb)) {
>  				kfree_skb(orig_skb);
>  			} else {
>  				kfree_skb(skb);

We have a clone already so we could just remove the whole "else if" I
think, but I'm guessing my intent was to keep it accounted to the socket
where possible rather than freeing the original in all cases.

So yeah, I think this makes sense. Maybe we should add a comment to the
if though to explain this?

johannes