Re: [PATCH v5 bluetooth-next] 6lowpan: Use pskb_expand_head in IPHC decompression.

[Martin Townsend <martin.townsend@xsilon.com>]

Hi Alex,

On 11/10/14 08:36, Alexander Aring wrote:
> Hi Martin,
>
> This is only a summarize what we should now to try to do here for next
> version.
>
> I know... this patch ends in a global debate on principles how we deal
> with the replacement of 6LoWPAN -> IPv6 header. Current behaviour is
> more a hack.
>
> On Thu, Oct 09, 2014 at 08:46:34AM +0100, Martin Townsend wrote:
>> Currently there are potentially 2 skb_copy_expand calls in IPHC
>> decompression.  This patch replaces this with one call to
>> pskb_expand_head.  It also checks to see if there is enough headroom
>> first to ensure it's only done if necessary.
>> As pskb_expand_head must only have one reference the calling code
>> now ensures this.
>>
>> Signed-off-by: Martin Townsend <martin.townsend@xsilon.com>
>> ---
>>  net/6lowpan/iphc.c      | 51 ++++++++++++++++++++++++-------------------------
>>  net/bluetooth/6lowpan.c |  7 +++++++
>>  2 files changed, 32 insertions(+), 26 deletions(-)
>>
>> diff --git a/net/6lowpan/iphc.c b/net/6lowpan/iphc.c
>> index 142eef5..853b4b8 100644
>> --- a/net/6lowpan/iphc.c
>> +++ b/net/6lowpan/iphc.c
>> @@ -174,30 +174,22 @@ static int uncompress_context_based_src_addr(struct sk_buff *skb,
>>  static int skb_deliver(struct sk_buff *skb, struct ipv6hdr *hdr,
>>  		       struct net_device *dev, skb_delivery_cb deliver_skb)
>>  {
>> -	struct sk_buff *new;
>>  	int stat;
>>  
>> -	new = skb_copy_expand(skb, sizeof(struct ipv6hdr), skb_tailroom(skb),
>> -			      GFP_ATOMIC);
>> -	kfree_skb(skb);
>> -
>> -	if (!new)
>> -		return -ENOMEM;
>> -
>> -	skb_push(new, sizeof(struct ipv6hdr));
>> -	skb_reset_network_header(new);
>> -	skb_copy_to_linear_data(new, hdr, sizeof(struct ipv6hdr));
>> +	skb_push(skb, sizeof(struct ipv6hdr));
>> +	skb_reset_network_header(skb);
>> +	skb_copy_to_linear_data(skb, hdr, sizeof(struct ipv6hdr));
>>  
>> -	new->protocol = htons(ETH_P_IPV6);
>> -	new->pkt_type = PACKET_HOST;
>> -	new->dev = dev;
>> +	skb->protocol = htons(ETH_P_IPV6);
>> +	skb->pkt_type = PACKET_HOST;
>> +	skb->dev = dev;
>>  
>>  	raw_dump_table(__func__, "raw skb data dump before receiving",
>> -		       new->data, new->len);
>> +		       skb->data, skb->len);
>>  
>> -	stat = deliver_skb(new, dev);
>> +	stat = deliver_skb(skb, dev);
>>  
>> -	kfree_skb(new);
>> +	consume_skb(skb);
>>  
>>  	return stat;
>>  }
>> @@ -460,7 +452,7 @@ int lowpan_process_data(struct sk_buff *skb, struct net_device *dev,
>>  	/* UDP data uncompression */
>>  	if (iphc0 & LOWPAN_IPHC_NH_C) {
>>  		struct udphdr uh;
>> -		struct sk_buff *new;
>> +		const int needed = sizeof(struct udphdr) + sizeof(hdr);
>>  
>>  		if (uncompress_udp_header(skb, &uh))
>>  			goto drop;
>> @@ -468,14 +460,13 @@ int lowpan_process_data(struct sk_buff *skb, struct net_device *dev,
>>  		/* replace the compressed UDP head by the uncompressed UDP
>>  		 * header
>>  		 */
>> -		new = skb_copy_expand(skb, sizeof(struct udphdr),
>> -				      skb_tailroom(skb), GFP_ATOMIC);
>> -		kfree_skb(skb);
>> -
>> -		if (!new)
>> -			return -ENOMEM;
>> -
>> -		skb = new;
>> +		if (skb_headroom(skb) < needed) {
>> +			err = pskb_expand_head(skb, needed, 0, GFP_ATOMIC);
>> +			if (unlikely(err)) {
>> +				kfree_skb(skb);
>> +				return err;
>> +			}
>> +		}
> use skb_cow_head here.
I don't know much about skb_cow_head but from the description it says that only the head is writable which I assume is the newly requested headroom?  I imagine this is for the usual case where you are adding headers and you go through the protocol layers.  For 6lowpan aren't we removing one header and replacing with a decompressed header which would suggest that we are going to modify the data (which would contain the 6lowpan header).

skb_cow seems a good fit to me though.
>
>>  
>>  		skb_push(skb, sizeof(struct udphdr));
>>  		skb_reset_transport_header(skb);
>> @@ -485,6 +476,14 @@ int lowpan_process_data(struct sk_buff *skb, struct net_device *dev,
>>  			       (u8 *)&uh, sizeof(uh));
>>  
>>  		hdr.nexthdr = UIP_PROTO_UDP;
>> +	} else {
>> +		if (skb_headroom(skb) < sizeof(hdr)) {
>> +			err = pskb_expand_head(skb, sizeof(hdr), 0, GFP_ATOMIC);
>> +			if (unlikely(err)) {
>> +				kfree_skb(skb);
>> +				return err;
>> +			}
>> +		}
> same here.
>
>>  	}
>>  
>>  	hdr.payload_len = htons(skb->len);
>> diff --git a/net/bluetooth/6lowpan.c b/net/bluetooth/6lowpan.c
>> index c2e0d14..6643a7c 100644
>> --- a/net/bluetooth/6lowpan.c
>> +++ b/net/bluetooth/6lowpan.c
>> @@ -343,6 +343,13 @@ static int recv_pkt(struct sk_buff *skb, struct net_device *dev,
>>  		kfree_skb(local_skb);
>>  		kfree_skb(skb);
>>  	} else {
>> +		/* Decompression may use pskb_expand_head so no shared skb's */
>> +		skb = skb_share_check(skb, GFP_ATOMIC);
>> +		if (!skb) {
>> +			dev->stats.rx_dropped++;
>> +			return NET_RX_DROP;
>> +		}
>> +
> I see now that case LOWPAN_DISPATCH_IPHC: runs a "local_skb =
> skb_clone(skb, GFP_ATOMIC);" and send the local_skb to process_data. So
> is the share_check really needed? In my opinion there are several things
> wrong.
>
> What bluetooth should do here is:
>
> Always run skb = skb_share_check(skb, GFP_ATOMIC); at first of this function.
>
> In if (skb->data[0] == LOWPAN_DISPATCH_IPV6):
>
>  - only run a skb_pull (doesn't change the data buffer, only skb attribute)
>    for the one byte dispatch.
>  - run other things like skb_reset_network_header, etc...
>
>  -> Currently this works because we run a complete skb_copy_expand here.
>     But this is not needed, we need a clone because we don't modify the
>     data buffer.
>
>  -> To Jukka: I don't see a skb_pull for the one byte dispatch value?
>     What's happend here? Also IPv6 dispatch is part of rfc4944 and btle
>     6LoWPAN is only realted to rfc6282 (IPHC). I think you don't need handling
>     for this here.
>
>
>
> In "case LOWPAN_DISPATCH_IPHC:" we should remove the skb_clone, because
> this is already handled by "skb_share_check" then.
Not necessarily, if the skb isn't shared, ie no-one has called skb_get on it then the clone will not happen.  I tried removing the skb_clone as in theory psk_expand_head should handle cloned skb's but I think it caused the oops that Jukka was seeing.  Because of this I only want to make changes absolutely necessary to get pskb_expand_head or skb_cow working as I can't test the bluetooth code and have to rely on Jukka.

If it's ok with everyone I'll change pskb_expand_head to skb_cow or skb_cow_head and leave the bluetooth code as it is with this v5 patch.

>
> - Alex
> --
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- Martin