0
down vote
favorite
I'm modifying the UDP protocol such that when connect() is called on a UDP socket, in addition to finding the route, a "Hello" packet is also sent to the destination.

From the UDP proto structure, I figured out that the function ip4_datagram_connect does the job of finding the route to the destination. Now at the end of this function, I need to send the Hello packet.

I don't think I can use udp_sendmsg() as it's used for copying and sending data from the userspace.
I think udp_send_skb() should be used to sent the hello. My problem is I don't know how to create an appropriate skbuff to store the Hello message (it should be a proper udp datagram) to be passed on to udp_send_skb(). I've tried this

int quic_connect(struct sock *sk, struct flowi4 *fl4){
struct sk_buff *hbuff;
char *hello;
int err = 0;

hbuff = dev_alloc_skb(2000);
if (!hbuff){
printk("xxxxx skbuff not alloted for QUIC connect\n");
goto out;
}
skb_reserve(hbuff, sizeof(struct udphdr));

security_skb_owned_by(hbuff, sk);

skb_push(hbuff, sizeof(struct udphdr));
skb_reset_transport_header(hbuff);
hbuff->sk = sk;

err = udp_send_skb(hbuff, fl4);


However, this gives me errors in the kernel log

BUG: unable to handle kernel NULL pointer dereference at 0000000000000018
IP: [<ffffffff81686555>] __ip_local_out+0x45/0x80
PGD 4f4dd067 PUD 4f4df067 PMD 0
Oops: 0000 [#1] SMP

which was quite expected as I guessed that the new skbuff I created wasn't filled up properly.

So my question is, what else do I need to fill in my skbuff before it can be handled properly by udp_send_skb. Better yet, is there a function which can do this for me?

The Oops is because you are trying to free a fixed string. I don't think your first C program should be kernel code.

This was just a dirty way of sending a recognizable packet to the other end. Later on, the string will be replaced with a struct. For now, I'm more focused on creating and sending off a a UDP packet.
Thanks for pointing it out though.

I edited the original post to ensure that I'm using the memory properly

Nope. Now you are copying 30 bytes out of a 24-byte string.

I see. I'll remove that line for the moment then and just send an empty datagram as that's my real problem. Thank you