batman-adv: Calculate correct size for merged packets

The routing algorithm must be able to decide if a fragment can be merged with
the missing part and still be passed to a forwarding interface. The fragments
can only differ by one byte in case that the original payload had an uneven
length. In that situation the sender has to inform all possible receivers that
the tail is one byte longer using the flag UNI_FRAG_LARGETAIL.

The combination of UNI_FRAG_LARGETAIL and UNI_FRAG_HEAD flag makes it possible
to calculate the correct length for even and uneven sized payloads.

The original formula missed to add the unicast header at all and forgot to
remove the fragment header of the second fragment. This made the results highly
unreliable and only useful for machines with large differences between the
configured MTUs.

Reported-by: Russell Senior <russell@personaltelco.net>
Reported-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Sven Eckelmann <sven@narfation.org>

1 files changed, 6 insertions, 2 deletions

diff --git a/net/batman-adv/unicast.c b/net/batman-adv/unicast.c
index 811f7fc7932d..fc77079b18bb 100644
--- a/net/batman-adv/unicast.c
+++ b/net/batman-adv/unicast.c
@@ -225,6 +225,7 @@ int frag_send_skb(struct sk_buff *skb, struct bat_priv *bat_priv,
         int uc_hdr_len = sizeof(struct unicast_packet);
         int ucf_hdr_len = sizeof(struct unicast_frag_packet);
         int data_len = skb->len - uc_hdr_len;
+        int large_tail = 0;
 
         if (!bat_priv->primary_if)
                 goto dropped;
@@ -254,8 +255,11 @@ int frag_send_skb(struct sk_buff *skb, struct bat_priv *bat_priv,
         memcpy(frag1->orig, bat_priv->primary_if->net_dev->dev_addr, ETH_ALEN);
         memcpy(frag2, frag1, sizeof(struct unicast_frag_packet));
 
-        frag1->flags |= UNI_FRAG_HEAD;
-        frag2->flags &= ~UNI_FRAG_HEAD;
+        if (data_len & 1)
+                large_tail = UNI_FRAG_LARGETAIL;
+
+        frag1->flags = UNI_FRAG_HEAD | large_tail;
+        frag2->flags = large_tail;
 
         frag1->seqno = htons((uint16_t)atomic_inc_return(
                              &batman_if->frag_seqno));