How to send a UDP packet from inside linux kernel

Question

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.

1. I don't think I can use udp_sendmsg() as it's used for copying and sending data from the userspace.

2. 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 rtable *rt){
   struct sk_buff *skb;
   char *hello;
   int err = 0, exthdrlen, hh_len, datalen, trailerlen;
   char *data;
   
   hh_len = LL_RESERVED_SPACE(rt->dst.dev);
   exthdrlen = rt->dst.header_len;
   trailerlen = rt->dst.trailer_len;
   datalen = 200;
   
   //Create a buffer to be send without fragmentation
   skb = sock_alloc_send_skb(sk,
           exthdrlen + datalen + hh_len + trailerlen + 15,
           MSG_DONTWAIT, &err);
   if (skb == NULL)
       goto out;
   
   skb->ip_summed = CHECKSUM_PARTIAL;      // Use hardware checksum
   skb->csum = 0;
   skb_reserve(skb, hh_len);
   skb_shinfo(skb)->tx_flags = 1;          //Time stamp the packet 
   
   
   /*
    *  Find where to start putting bytes.
    */
   data = skb_put(skb, datalen + exthdrlen);
   skb_set_network_header(skb, exthdrlen);
   skb->transport_header = (skb->network_header +
                sizeof(struct iphdr));
   
   err = udp_send_skb(skb, 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
Modules linked in:
CPU: 0 PID: 3019 Comm: client Not tainted 3.13.11-ckt39-test006 #28
Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
task: ffff8800598df6b0 ti: ffff880047022000 task.ti: ffff880047022000
RIP: 0010:[<ffffffff81686555>]  [<ffffffff81686555>] __ip_local_out+0x45/0x80
RSP: 0018:ffff880047023d78  EFLAGS: 00010287
RAX: 0000000000000001 RBX: ffff880047008a00 RCX: 0000000020000000
RDX: 0000000000000000 RSI: ffff880047008a00 RDI: ffff8800666fde40
RBP: ffff880047023d88 R08: 0000000000003200 R09: 0000000000000001
R10: 0000000000000000 R11: 00000000000001f9 R12: ffff880047008a00
R13: ffff8800666fde80 R14: ffff880059aec380 R15: ffff880059aec690
FS:  00007f5508b04740(0000) GS:ffff88007fc00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: 0000000000000018 CR3: 000000004f561000 CR4: 00000000000406f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:
ffff880047023d80 ffff880047008a00 ffff880047023da0 ffffffff8168659d
ffffffff81c8f8c0 ffff880047023db8 ffffffff81687810 0000000000000000
ffff880047023df8 ffffffff816ac6be 0000000000000020 ffff880047008a00
Call Trace:
[<ffffffff8168659d>] ip_local_out+0xd/0x30
[<ffffffff81687810>] ip_send_skb+0x10/0x40
[<ffffffff816ac6be>] udp_send_skb+0x14e/0x3d0
[<ffffffff816b0e9e>] quic_connect+0x6e/0x80
[<ffffffff816aa3ff>] __ip4_datagram_connect+0x2bf/0x2d0
[<ffffffff816aa437>] ip4_datagram_connect+0x27/0x40
[<ffffffff816b8748>] inet_dgram_connect+0x38/0x80
[<ffffffff8161fd97>] SYSC_connect+0xc7/0x100
[<ffffffff817ed471>] ? __schedule+0x341/0x8c0
[<ffffffff816206e9>] SyS_connect+0x9/0x10
[<ffffffff817f8d42>] system_call_fastpath+0x16/0x1b
Code: c8 00 00 00 66 c1 c0 08 66 89 47 02 e8 d5 e0 ff ff 48 8b 53 58 b8 01 00 00 00 48 83 e2 fe 48 81 3d 9d 0e 64 00 f0 73 cc 81 74 26 <4c> 8b 42 18 49 c7 c1 f0 45 68 81 c7 04 24 00 00 00 80 31 c9 48
RIP  [<ffffffff81686555>] __ip_local_out+0x45/0x80
RSP <ffff880047023d78>
CR2: 0000000000000018
---[ end trace 474c5db1b9b19a03 ]---

So my question is, what else do I need to fill in my skbuff before it can be handled properly by udp_send_skb. Or am I missing something else here?

Answer 1

There is a bug in your code.

if (skb_tailroom(hbuff) > 30) {
    printk("     Enough room for QUIC connect message\n");
    hello = kmalloc(30, GFP_ATOMIC);      //You allocate slub memory
    hello = "Hello from QUIC connect";   //You let 'hello' point to a string, 
                                         //which is stored somewhere else. 
                                         //At this point, your slub memory 
                                         //allocated is lost.

    memcpy(__skb_put(hbuff, 30), hello, 30);
    kfree(hello);                 //You try to free the memory pointed by
                                  //hello as slub memory, I think this is
                                  // why you get mm/slub.c bug message.
} else

You can change your code like this:

if (skb_tailroom(hbuff) > 30) {
    printk("     Enough room for QUIC connect message\n");

    memcpy(__skb_put(hbuff, 30), "Hello from QUIC connect", 30);
} else

Answer 2

I just used the functions ip_make_skb followed by ip_send_skb. Since ip_make_skb is used to copy data from user space and I didn't need to do that, I just used a dummy pointer and provided the length to be copied as Zero (+sizeof udphdr, as required by this function). It's a dirty way to do it, but it works for now. My code:

int quic_connect(struct sock *sk, struct flowi4 *fl4, struct rtable *rt, int oif){
    struct sk_buff *skb;
    struct ipcm_cookie ipc;
    void *hello = "Hello";
    int err = 0;

    ipc.opt = NULL;
    ipc.tx_flags = 1;
    ipc.ttl = 0;
    ipc.tos = -1;
    ipc.oif = oif;
    ipc.addr = fl4->daddr;
    skb = ip_make_skb(sk, fl4, ip_generic_getfrag, hello, sizeof(struct udphdr),
              sizeof(struct udphdr), &ipc, &rt,
              0);
    err = PTR_ERR(skb);
    if (!IS_ERR_OR_NULL(skb)){
        err = udp_send_skb(skb, fl4);
    }

    return err;                      

}