Return-Path: Message-ID: <54188817.80707@xsilon.com> Date: Tue, 16 Sep 2014 19:57:27 +0100 From: Martin Townsend MIME-Version: 1.0 To: Alexander Aring CC: Jukka Rissanen , Martin Townsend , linux-zigbee-devel@lists.sourceforge.net, linux-bluetooth@vger.kernel.org, linux-wpan@vger.kernel.org, marcel@holtmann.org Subject: Re: [PATCH v4 bluetooth] 6lowpan: fix incorrect return values in lowpan_rcv References: <20140916121844.GA5349@omega> <54182C6B.6090801@xsilon.com> <20140916123421.GA5576@omega> <54182FB8.4080103@xsilon.com> <20140916124832.GB5576@omega> <1410873619.4860.20.camel@jrissane-mobl.ger.corp.intel.com> <20140916133206.GA6104@omega> <1410875570.4860.23.camel@jrissane-mobl.ger.corp.intel.com> <20140916140459.GB6104@omega> <54184CDB.9050200@xsilon.com> <20140916173806.GC6104@omega> In-Reply-To: <20140916173806.GC6104@omega> Content-Type: text/plain; charset=utf-8; format=flowed List-ID: 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.