Why am I not receiving my messages from kernel? - c

Question

I'm trying to send a message from kernel to user space using generic netlink and libnl, the part of my code which does this is implemented as follow:
int struct my_callback(struct sk_buff *skb, struct genl_info *info)
{
struct sk_buff *obuff;
void *msg_head;
if ((obuff = genlmsg_new(0, GFP_KERNEL)) == NULL) { // I've tried to change the len to NLMSG_GOODSIZE but not worked
pr_err("Failed allocating message to an reply\n");
return 0;
}
if ((msg_head = genlmsg_put_reply(obuff, info, &lunatik_family, 0, LIST_STATES)) == NULL) {
pr_err("Failed to put generic netlink header\n");
return 0;
}
//I've tried to put a genlmsg_end(obuff, msg_head); but didn't work as well
if (genlmsg_reply(obuff, info) < 0) {
pr_err("Failed to send message to user space\n");
return 0;
}
pr_info("Message sent to user-space\n");
return 0;
}
P.s: LIST_STATES is a member of enum and have the value of 3
And my user space code is basically:
static int req_handler(struct nl_msg *msg, void *arg)
{
struct nlmsghdr *nlhdr;
struct genlmsghdr *genlhdr;
nlhdr = nlmsg_hdr(msg);
genlhdr = genlmsg_hdr(nlhdr);
printf("Received a message from kernel: %d\n", genlhdr->cmd);
return NL_OK;
}
int socket_init(struct nl_sock *sock)
{
int err = -1;
if ((sock = nl_socket_alloc()) == NULL)
return err;
if ((err = genl_connect(sock)))
return err;
if ((err = genl_ctrl_resolve(sock, LUNATIK_FAMILY)) < 0)
return err;
// I've tried to use NL_CB_VALID, but when I use it I receive no message at all
nl_socket_modify_cb(sock, NL_CB_MSG_IN, NL_CB_CUSTOM, req_handler, NULL);
return 0;
}
My output on dmesg is:
Message sent to user-space
And my output on user space is:
Received a message from kernel: 0
I should receive 3 instead of 0, I noticed that I'm receiving only the ACK message, but not the message that I'm sending, I would like to know why this is happening and what I'm doing wrong.

Answer 1

The result of genl_ctrl_resolve() is twofold:
If < 0, it's an error code.
If >= 0, it's the family identication number.
You're throwing away your family identication number. Instead of
if ((err = genl_ctrl_resolve(sock, LUNATIK_FAMILY)) < 0)
return err;
, do
if ((lunatik_family = genl_ctrl_resolve(sock, LUNATIK_FAMILY)) < 0)
return lunatik_family;
Later, when you're setting up the Netlink Header, make sure to use it:
if (!genlmsg_put(..., ..., ..., lunatik_family, ..., ..., LIST_STATES, ...))
/* handle error */
And one more thing: nl_socket_modify_cb() also returns an error code. Instead of
nl_socket_modify_cb(sock, NL_CB_MSG_IN, NL_CB_CUSTOM, req_handler, NULL);
return 0;
do
return nl_socket_modify_cb(sock, NL_CB_MSG_IN, NL_CB_CUSTOM, req_handler, NULL);