Re: [PATCH v4 bluetooth] 6lowpan: fix incorrect return values in lowpan_rcv

[Alexander Aring <alex.aring@gmail.com>]

Hi Martin,

On Tue, Sep 16, 2014 at 03:44:43PM +0100, Martin Townsend wrote:
> I would like to keep freeing skb's out of process_data as process_data will become something like iphc_decompress_hdr and it would be good if that's all it did.  Otherwise I feel we are going to put a constraint on all future header decompression routines in that they must free the skb on error.  I think it would be better to defer this so on error you might want to try something else with the skb, maybe not but at least the option is there.
> So how about
> 
>         struct sk_buff * ret_skb;
>         switch (skb->data[0] & 0xe0) {
>         case LOWPAN_DISPATCH_IPHC:    /* ipv6 datagram */
>             ret_skb = process_data(skb, &hdr);
>             if (IS_ERR(ret_skb))
>                 goto drop_skb;
>             else
>                 skb = ret_skb;
>             break;
> 
> I know we currently have 3 calls to process_data so it will look fairly ugly in this patch but in my next patch to fix lowpan_rcv to handle uncompressed IPv6 packets that are fragmented there will only be one call to process_data so it won't look so bad.  You could even wrap it in a macro but I'm not a fan of this as they can obfuscate the code a bit.
> 
> Thoughts?
> 

sorry, I can't follow how this solve the issue if the "parameter skb" is
already consumed or not. If process_data returns a error before
parameter consume, then we should run kfree_skb(parameter_skb), if it's
afterwards we should do nothing. Point is we don't know that there. I
suppose if we do consume_skb and refcount reach 0 the parameter_skb
becomes a dangling pointer.

- Alex