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@xxxxxxxxxx>
>> ---
>> 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
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