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/* SPDX-License-Identifier: GPL-2.0 */
/*
* Generic Netlink example module
*
* An example on how to use the Generic Netlink API with ops and multicast
* groups.
*
* Copyright (c) 2022 Yaroslav de la Peña Smirnov <yps@yaroslavps.com>
*/
#define pr_fmt(fmt) "genltest: " fmt
#include "genltest.h"
#include <linux/module.h>
#include <net/genetlink.h>
/*
* Some materials that were used as source for this example/tutorial/guide:
*
* - https://wiki.linuxfoundation.org/networking/generic_netlink_howto
* - https://kernel.org/doc/html/next/userspace-api/netlink/intro.html
*/
#define MSG_MAX_LEN 1024
/* Forward declaration */
static struct genl_family genl_fam;
/* Handler for GENLTEST_CMD_ECHO messages received */
static int echo_doit(struct sk_buff *skb, struct genl_info *info)
{
int ret = 0;
void *hdr;
struct sk_buff *msg;
/* Check if the attribute is present and print it */
if (info->attrs[GENLTEST_A_MSG]) {
char *str = nla_data(info->attrs[GENLTEST_A_MSG]);
pr_info("message received: %s\n", str);
} else {
pr_info("empty message received\n");
}
/* Allocate a new buffer for the reply */
msg = nlmsg_new(NLMSG_DEFAULT_SIZE, GFP_KERNEL);
if (!msg) {
pr_err("failed to allocate message buffer\n");
return -ENOMEM;
}
/* Put the Generic Netlink header */
hdr = genlmsg_put(msg, info->snd_portid, info->snd_seq, &genl_fam, 0,
GENLTEST_CMD_ECHO);
if (!hdr) {
pr_err("failed to create genetlink header\n");
nlmsg_free(msg);
return -EMSGSIZE;
}
/* And the message */
if ((ret = nla_put_string(msg, GENLTEST_A_MSG,
"Hello from Kernel Space, Netlink!"))) {
pr_err("failed to create message string\n");
genlmsg_cancel(msg, hdr);
nlmsg_free(msg);
goto out;
}
/* Finalize the message and send it */
genlmsg_end(msg, hdr);
ret = genlmsg_reply(msg, info);
pr_info("reply sent\n");
out:
return ret;
}
/* Attribute validation policy for our echo command */
static struct nla_policy echo_pol[GENLTEST_A_MAX + 1] = {
[GENLTEST_A_MSG] = { .type = NLA_NUL_STRING },
};
/* Operations for our Generic Netlink family */
static struct genl_ops genl_ops[] = {
{
.cmd = GENLTEST_CMD_ECHO,
.policy = echo_pol,
.doit = echo_doit,
},
};
/* Multicast groups for our family */
static const struct genl_multicast_group genl_mcgrps[] = {
{ .name = GENLTEST_MC_GRP_NAME },
};
/* Generic Netlink family */
static struct genl_family genl_fam = {
.name = GENLTEST_GENL_NAME,
.version = GENLTEST_GENL_VERSION,
.maxattr = GENLTEST_A_MAX,
.ops = genl_ops,
.n_ops = ARRAY_SIZE(genl_ops),
.mcgrps = genl_mcgrps,
.n_mcgrps = ARRAY_SIZE(genl_mcgrps),
};
/* Multicast ping message to our genl multicast group */
static int echo_ping(const char *buf, size_t cnt)
{
int ret = 0;
void *hdr;
/* Allocate message buffer */
struct sk_buff *skb = genlmsg_new(NLMSG_DEFAULT_SIZE, GFP_KERNEL);
if (unlikely(!skb)) {
pr_err("failed to allocate memory for genl message\n");
return -ENOMEM;
}
/* Put the Generic Netlink header */
hdr = genlmsg_put(skb, 0, 0, &genl_fam, 0, GENLTEST_CMD_ECHO);
if (unlikely(!hdr)) {
pr_err("failed to allocate memory for genl header\n");
nlmsg_free(skb);
return -ENOMEM;
}
/* And the message */
if ((ret = nla_put_string(skb, GENLTEST_A_MSG, buf))) {
pr_err("unable to create message string\n");
genlmsg_cancel(skb, hdr);
nlmsg_free(skb);
return ret;
}
/* Finalize the message */
genlmsg_end(skb, hdr);
/* Send it over multicast to the 0-th mc group in our array. */
ret = genlmsg_multicast(&genl_fam, skb, 0, 0, GFP_KERNEL);
if (ret == -ESRCH) {
pr_warn("multicast message sent, but nobody was listening...\n");
} else if (ret) {
pr_err("failed to send multicast genl message\n");
} else {
pr_info("multicast message sent\n");
}
return ret;
}
/*
* Test sysfs attr to send multicast messages. The string in the buffer will be
* echoed to the multicast group.
*/
static ssize_t ping_store(struct kobject *kobj, struct kobj_attribute *attr,
const char *buf, size_t cnt)
{
int max = cnt > MSG_MAX_LEN ? MSG_MAX_LEN : cnt;
echo_ping(buf, max);
return max;
}
static struct kobject *kobj;
static struct kobj_attribute ping_attr = __ATTR_WO(ping);
static int __init init_genltest(void)
{
int ret = 0;
pr_info("init start\n");
kobj = kobject_create_and_add("genltest", kobj);
if (unlikely(!kobj)) {
pr_err("unable to create kobject\n");
return -ENOMEM;
}
ret = sysfs_create_file(kobj, &ping_attr.attr);
if (unlikely(ret)) {
pr_err("unable to create sysfs file\n");
kobject_put(kobj);
return ret;
}
ret = genl_register_family(&genl_fam);
if (unlikely(ret)) {
pr_crit("failed to register generic netlink family\n");
sysfs_remove_file(kobj, &ping_attr.attr);
kobject_put(kobj);
}
pr_info("init end\n");
return ret;
}
static void __exit exit_genltest(void)
{
if (unlikely(genl_unregister_family(&genl_fam))) {
pr_err("failed to unregister generic netlink family\n");
}
sysfs_remove_file(kobj, &ping_attr.attr);
kobject_put(kobj);
pr_info("exit\n");
}
MODULE_AUTHOR("Yaroslav de la Peña Smirnov");
MODULE_LICENSE("GPL");
module_init(init_genltest);
module_exit(exit_genltest);
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