Return-Path: Message-ID: <543B9B4C.1010303@xsilon.com> Date: Mon, 13 Oct 2014 10:28:44 +0100 From: Martin Townsend MIME-Version: 1.0 To: Alexander Aring CC: Martin Townsend , linux-bluetooth@vger.kernel.org, linux-wpan@vger.kernel.org, marcel@holtmann.org, jukka.rissanen@linux.intel.com Subject: Re: [PATCH v5 bluetooth-next] 6lowpan: Use pskb_expand_head in IPHC decompression. References: <1412840794-17283-1-git-send-email-martin.townsend@xsilon.com> <1412840794-17283-2-git-send-email-martin.townsend@xsilon.com> <20141011073620.GA28500@omega> <543B90FA.2060603@xsilon.com> <20141013085538.GA20544@omega> In-Reply-To: <20141013085538.GA20544@omega> Content-Type: text/plain; charset=utf-8 List-ID: Hi Alex, On 13/10/14 09:55, Alexander Aring wrote: > Hi Martin, > > On Mon, Oct 13, 2014 at 09:44:42AM +0100, Martin Townsend wrote: >> 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 >>>> --- >>>> 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. > okay, then use skb_cow here. I will remember that we could possible > maybe use "skb_cow_head" here. But yea I mean we running lot of pulls > and move the head pointer, then push now data on it, this would > overwrite the pulled data and other skb's may not pull here. If the data > is shared there, this would crash. > >>>> >>>> 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. > For me it's confusing, you do that on skb, but we never used skb > afterwards (maybe on freeing) but we use local_skb, the local_skb should > always be cloned because we running skb_clone before. skb_clone does ensure that the skb isn't shared anymore so I think it will be safe to remove the skb_share_check. Or I can move it to the start of the function like we do for 802.15.4? > > skb_share_check is a function which checks is the skb shared, then clone > it. But we don't do anything with skb afterwards. > > - Alex > -- > To unsubscribe from this list: send the line "unsubscribe linux-wpan" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -Martin.