Repository : ssh://git@open-mesh.org/batman-adv

On branches: maint,release-2017.0.1

commit 11a0c90b439190fd6a104caac5d4ef09e1f65a96 Author: Sven Eckelmann sven@narfation.org Date: Sat Mar 4 17:29:25 2017 +0100

batman-adv: Keep fragments equally sized

The batman-adv fragmentation packets have the design problem that they cannot be refragmented and cannot handle padding by the underlying link. The latter often leads to problems when networks are incorrectly configured and don't use a common MTU.

The sender could for example fragment a 1271 byte frame (plus external ethernet header (14) and batadv unicast header (10)) to fit in a 1280 bytes large MTU of the underlying link (max. 1294 byte frames). This would create a 1294 bytes large frame (fragment 2) and a 55 bytes large frame (fragment 1). The extra 54 bytes are the fragment header (20) added to each fragment and the external ethernet header (14) for the second fragment.

Let us assume that the next hop is then not able to transport 1294 bytes to its next hop. The 1294 byte large frame will be dropped but the 55 bytes large fragment will still be forwarded to its destination.

Or let us assume that the underlying hardware requires that each frame has a minimum size (e.g. 60 bytes). Then it will pad the 55 bytes frame to 60 bytes. The receiver of the 60 bytes frame will no longer be able to correctly assemble the two frames together because it is not aware that 5 bytes of the 60 bytes frame are padding and don't belong to the reassembled frame.

This can partly be avoided by splitting frames more equally. In this example, the 675 and 674 bytes large fragment frames could both potentially reach its destination without being too large or too small.

Reported-by: Martin Weinelt martin@darmstadt.freifunk.net Fixes: db56e4ecf5c2 ("batman-adv: Fragment and send skbs larger than mtu") Signed-off-by: Sven Eckelmann sven@narfation.org Acked-by: Linus Lüssing linus.luessing@c0d3.blue Signed-off-by: Simon Wunderlich sw@simonwunderlich.de

11a0c90b439190fd6a104caac5d4ef09e1f65a96 net/batman-adv/fragmentation.c | 20 +++++++++++++------- 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/net/batman-adv/fragmentation.c b/net/batman-adv/fragmentation.c index 11a23fd..8f964be 100644 --- a/net/batman-adv/fragmentation.c +++ b/net/batman-adv/fragmentation.c @@ -404,7 +404,7 @@ out: * batadv_frag_create - create a fragment from skb * @skb: skb to create fragment from * @frag_head: header to use in new fragment - * @mtu: size of new fragment + * @fragment_size: size of new fragment * * Split the passed skb into two fragments: A new one with size matching the * passed mtu and the old one with the rest. The new skb contains data from the @@ -414,11 +414,11 @@ out: */ static struct sk_buff *batadv_frag_create(struct sk_buff *skb, struct batadv_frag_packet *frag_head, - unsigned int mtu) + unsigned int fragment_size) { struct sk_buff *skb_fragment; unsigned int header_size = sizeof(*frag_head); - unsigned int fragment_size = mtu - header_size; + unsigned int mtu = fragment_size + header_size;

skb_fragment = netdev_alloc_skb(NULL, mtu + ETH_HLEN); if (!skb_fragment) @@ -456,7 +456,7 @@ int batadv_frag_send_packet(struct sk_buff *skb, struct sk_buff *skb_fragment; unsigned int mtu = neigh_node->if_incoming->net_dev->mtu; unsigned int header_size = sizeof(frag_header); - unsigned int max_fragment_size, max_packet_size; + unsigned int max_fragment_size, num_fragments; int ret;

/* To avoid merge and refragmentation at next-hops we never send @@ -464,10 +464,15 @@ int batadv_frag_send_packet(struct sk_buff *skb, */ mtu = min_t(unsigned int, mtu, BATADV_FRAG_MAX_FRAG_SIZE); max_fragment_size = mtu - header_size; - max_packet_size = max_fragment_size * BATADV_FRAG_MAX_FRAGMENTS; + + if (skb->len == 0 || max_fragment_size == 0) + return -EINVAL; + + num_fragments = (skb->len - 1) / max_fragment_size + 1; + max_fragment_size = (skb->len - 1) / num_fragments + 1;

/* Don't even try to fragment, if we need more than 16 fragments */ - if (skb->len > max_packet_size) { + if (num_fragments > BATADV_FRAG_MAX_FRAGMENTS) { ret = -EAGAIN; goto free_skb; } @@ -507,7 +512,8 @@ int batadv_frag_send_packet(struct sk_buff *skb, goto put_primary_if; }

- skb_fragment = batadv_frag_create(skb, &frag_header, mtu); + skb_fragment = batadv_frag_create(skb, &frag_header, + max_fragment_size); if (!skb_fragment) { ret = -ENOMEM; goto put_primary_if;