eBPF: modifying UDP payload and responding to client - c

Question

I'm trying to modify the UDP payload I receive from the client to clone and redirect the packet in order to respond to it. Swapping the MAC and IP addresses is already done, as well as the cloning, but I don't know how to modify the UDP payload.
I want to send a payload with the following 3 bytes: 0x1a 0x31 0x0f back to the client, either by creating a new payload (better) and replacing it in the packet or by replacement. How can I do that?
Here is my code so far:
UPDATED BASED ON COMMENTS (Now only needs checksum recalculation):
int pingpong(struct __sk_buff *skb)
{
void *data = (void *)(long)skb->data;
void *data_end = (void *)(long)skb->data_end;
struct ethhdr *eth = data;
struct iphdr *ip;
struct udphdr *udpdata;
if ((void *)eth + sizeof(*eth) > data_end) {
return TC_ACT_UNSPEC;
}
ip = data + sizeof(*eth);
if ((void *)ip + sizeof(*ip) > data_end) {
return TC_ACT_UNSPEC;
}
udpdata = (void *)ip + sizeof(*ip);
if ((void *)udpdata + sizeof(*udpdata) > data_end) {
return TC_ACT_UNSPEC;
}
if (eth->h_proto != htons(ETH_P_IP)) {
return TC_ACT_UNSPEC;
}
if (ip->protocol != IPPROTO_UDP) {
return TC_ACT_UNSPEC;
}
unsigned int payload_size;
unsigned char *payload;
payload_size = ntohs(udpdata->len) - sizeof(*udpdata);
payload = (unsigned char *)udpdata + sizeof(*udpdata);
if ((void *)payload + payload_size > data_end) {
return TC_ACT_UNSPEC;
}
// 1. Swap the MACs
__u8 tmp_mac[ETH_ALEN];
memcpy(tmp_mac, eth->h_dest, ETH_ALEN);
memcpy(eth->h_dest, eth->h_source, ETH_ALEN);
memcpy(eth->h_source, tmp_mac, ETH_ALEN);
// 2. Swap the IPs
if (eth->h_proto == htons(ETH_P_IP)) {
__u32 tmp_ip = ip->saddr;
ip->saddr = ip->daddr;
ip->daddr = tmp_ip;
}
// 3. Swap the ports
udpdata->source = port;
udpdata->dest = srcport;
// 4. Change the payload to be 0x1a 0x31 0x0f
bpf_skb_adjust_room(skb, -1, BPF_ADJ_ROOM_NET, 0);
uint8_t byte1 = 0x1a;
uint8_t byte2 = 0x31;
uint8_t byte3 = 0x0f;
int ret = bpf_skb_store_bytes(skb, payload_offset, &byte1, sizeof(byte1), 0);
ret = bpf_skb_store_bytes(skb, payload_offset+1, &byte2, sizeof(byte2), 0);
ret = bpf_skb_store_bytes(skb, payload_offset+2, &byte3, sizeof(byte3), 0);
// Re-calculate the checksum
bpf_l4_csum_replace(skb, L4_CSUM_OFF, datap[0], byte1, 0);
bpf_l4_csum_replace(skb, L4_CSUM_OFF, datap[1], byte2, 0);
bpf_l4_csum_replace(skb, L4_CSUM_OFF, datap[2], byte3, 0);
// Not working!
// Final: Redirect to be sent
bpf_clone_redirect(skb, skb->ifindex, 0);
}
If I just change the payload without removing the 1 byte using the adjust_room function, it is sent, but the last byte is 00. I want to remove that.
Any tips please? Thanks!

Answer 1

As suggested by #Qeole, you can use the following to change the size and content of your UDP payload:
// Remove 1 byte after IP header.
bpf_skb_adjust_room(ctx, -1, BPF_ADJ_ROOM_NET, 0);
... re-check packet pointers or verifier will complain ...
// Need to rewrite the UDP header as the extra space was added before it.
*new_udphdr = *old_udphdr;
// Write payload.
udp_payload[0] = 0x1a
udp_payload[1] = 0x31
udp_payload[2] = 0x0f