Hi Alex,
On 16/09/14 18:38, Alexander Aring wrote:
> 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
process_data never consumes the skb, it may copy_expand and then consume
the old one so it will either return an error or an skb that contains
the uncompressed ipv6 header. By calling process_data using a different
sk_buff pointer (ret_skb) that the parameter we can check this for an
error and if so goto drop_skb which will kfree_skb(skb) which is fine as
skb is still valid. if ret_skb is good and we assign to skb and carry
on to the function that passes the skb up the stack,
lowpan_give_skb_to_devices, which deals with either consuming or kfreeing.
Or am I missing something?
- Martin.
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