On Sat, Apr 14, 2018 at 03:38:46AM +0200, Jason A. Donenfeld wrote:
> 2) When we pad the packet payload. In this case, we pad it to the
> nearest multiple of 16, but we don't let it exceed the device MTU.
> This is skb_padding in send.c. This behavior seems like the bug in
> your particular case, since what matters here is the route's MTU, not
> the device MTU. For full 1412 size packets, the payload is presumably
> being padded to 1424, since that's still less than the device MTU. In
> order to test this theory, try setting your route MTU, as you've
> described in your first email, to 1408 (which is a multiple of 16). If
> this works, let me know, as it will be good motivation for fixing
> skb_padding. If not, then it means there's a problem elsewhere to
> investigate too.
> 
> I'm CC'ing Luis on this email, as he was working on the MTU code a while back.

I'm still playing with this, but something like the following might fix
the issue, if you're interested in playing a bit.

=~=~=~=~=~=~=

diff --git a/src/device.c b/src/device.c
index 1614d61..3d18368 100644
--- a/src/device.c
+++ b/src/device.c
@@ -120,6 +120,7 @@ static netdev_tx_t xmit(struct sk_buff *skb, struct 
net_device *dev)
        struct sk_buff *next;
        struct sk_buff_head packets;
        sa_family_t family;
+       u32 mtu;
        int ret;

        if (unlikely(skb_examine_untrusted_ip_hdr(skb) != skb->protocol)) {
@@ -142,6 +143,8 @@ static netdev_tx_t xmit(struct sk_buff *skb, struct 
net_device *dev)
                goto err_peer;
        }

+       mtu = dst_mtu(skb_dst(skb)) ?: skb->dev->mtu;
+
        __skb_queue_head_init(&packets);
        if (!skb_is_gso(skb))
                skb->next = NULL;
@@ -168,6 +171,8 @@ static netdev_tx_t xmit(struct sk_buff *skb, struct 
net_device *dev)
                 */
                skb_dst_drop(skb);

+               PACKET_CB(skb)->mtu = mtu;
+
                __skb_queue_tail(&packets, skb);
        } while ((skb = next) != NULL);

diff --git a/src/queueing.h b/src/queueing.h
index d5948f3..c507536 100644
--- a/src/queueing.h
+++ b/src/queueing.h
@@ -46,6 +46,7 @@ struct packet_cb {
        u64 nonce;
        struct noise_keypair *keypair;
        atomic_t state;
+       u32 mtu;
        u8 ds;
 };
 #define PACKET_PEER(skb) (((struct packet_cb *)skb->cb)->keypair->entry.peer)
diff --git a/src/send.c b/src/send.c
index dddcc0b..e3b1ffd 100644
--- a/src/send.c
+++ b/src/send.c
@@ -116,11 +116,11 @@ static inline unsigned int skb_padding(struct sk_buff 
*skb)
         * isn't strictly neccessary, but it's better to be cautious here, 
especially
         * if that code ever changes.
         */
-       unsigned int last_unit = skb->len % skb->dev->mtu;
+       unsigned int last_unit = skb->len % PACKET_CB(skb)->mtu;
        unsigned int padded_size = (last_unit + MESSAGE_PADDING_MULTIPLE - 1) & 
~(MESSAGE_PADDING_MULTIPLE - 1);

-       if (padded_size > skb->dev->mtu)
-               padded_size = skb->dev->mtu;
+       if (padded_size > PACKET_CB(skb)->mtu)
+               padded_size = PACKET_CB(skb)->mtu;
        return padded_size - last_unit;
 }

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