[linux-next] LinuxNextTracking branch, master, updated. next-20170320

batman at open-mesh.org batman at open-mesh.org
Tue Mar 21 00:15:39 CET 2017

The following commit has been merged in the master branch:
commit 1c2bcc766be44467809f1798cd4ceacafe20a852
Author: Sven Eckelmann <sven at narfation.org>
Date:   Wed Feb 22 17:25:42 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
    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 at darmstadt.freifunk.net>
    Fixes: ee75ed88879a ("batman-adv: Fragment and send skbs larger than mtu")
    Signed-off-by: Sven Eckelmann <sven at narfation.org>
    Acked-by: Linus L├╝ssing <linus.luessing at c0d3.blue>
    Signed-off-by: Simon Wunderlich <sw at simonwunderlich.de>

diff --git a/net/batman-adv/fragmentation.c b/net/batman-adv/fragmentation.c
index 11149e5..106bda5 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;


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