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Using Netlink to post events from kernelspace to userspace

I have been working on an older version of Linux on which I wanted to use netlink as a way to post events from kernelspace to userspace. I wanted to avoid using Netlink Protocol Library Suite (libnl) so that I could delve deeper into netlink and have better control of what I am doing. I was already successful in using Generic Netlink Sockets to perform two way request-response communication between kernelspace and userspace. This time I referred ACPI daemon 1.0.10's source and used genlmsg_multicast() to post events from the kernel side. The implementation of netlink has evolved over time and the following code ought to work only on kernels versioned around 2.6.35 (I tested my code on Ubuntu 10.10)

Source Code Organization:


kern/genl_event.c:
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#include <net/genetlink.h>
#include <linux/module.h>
#include <linux/kernel.h>

#include "genl_event.h"

//Code based on http://people.ee.ethz.ch/~arkeller/linux/multi/kernel_user_space_howto-3.html

static struct genl_family genl_exmpl_event_family = {
 .id = GENL_ID_GENERATE,         //Genetlink should generate an id
 .hdrsize = 0,
 .name = GENL_EXMPL_EVENT_FAMILYNAME,        //The name of this family, used by userspace application
 .version = GENL_EXMPL_EVENT_VERSION,          //Version number  
 .maxattr = GENL_EXMPL_EVENT_A_MAX,
};

static struct genl_multicast_group genl_exmpl_event_group = {
 .name = GENL_EXMPL_EVENT_GROUPNAME,
};

uint8_t genl_exmpl_event_init(void) {
 int rc;

    //Step 01: Register the new family
 rc = genl_register_family(&genl_exmpl_event_family);
 if (rc != 0) {
  goto failure;
 }

 //Step 02: Register group for the new family
 //Referred: http://lxr.free-electrons.com/source/drivers/acpi/event.c?v=2.6.33#L254
 rc = genl_register_mc_group(&genl_exmpl_event_family, &genl_exmpl_event_group);
 if (rc != 0) {
  printk("Generic Netlink register group: %i",rc);
  genl_unregister_family(&genl_exmpl_event_family);
  goto failure;
 }

 return 0; 
failure:
 return -1;
}

void genl_exmpl_event_deinit(void) {
 int ret;

    //Unregister the family
 ret = genl_unregister_family(&genl_exmpl_event_family);
 if(ret !=0) {
  printk("Generic Netlink unregister family %i\n",ret);
 }
}

int genl_exmpl_event_send(u8 *msg, int len) {
 //Referred: http://lxr.free-electrons.com/source/drivers/acpi/event.c?v=2.6.33#L183
 struct sk_buff *skb;
 int rc;
 void *msg_head;

 //Send a message back ti userspace
    //Allocate some memory, since the size is not yet known use NLMSG_GOODSIZE
 skb = genlmsg_new(NLMSG_GOODSIZE, GFP_KERNEL);
 if (skb == NULL) {
  printk("Could not allocate skb\n");
  return 0;
 }

 //Create the message headers
    /* arguments of genlmsg_put: 
       struct sk_buff *, 
       int (sending) pid, 
       int sequence number, 
       struct genl_family *, 
       int flags, 
       u8 command index (why do we need this?)
    */
 msg_head = genlmsg_put(skb, 0, 0, &genl_exmpl_event_family, 0, GENL_EXMPL_EVENT_C_DO);
 if (msg_head == NULL) {
  rc = -ENOMEM;
  printk("genlmsg_put() returned error\n");
  return 0;
 }

 //Add a GENL_EXMPL_EVENT_A_MSG attribute (actual value to be sent)
 rc = nla_put(skb, GENL_EXMPL_EVENT_A_MSG, len, msg);
 if (rc != 0) {
  printk("nla_put() returned error\n");
  return 0;
 }
 
 //Finalize the message
 genlmsg_end(skb, msg_head);

    //Send the message
 rc = genlmsg_multicast(skb, 0,genl_exmpl_event_group.id,GFP_ATOMIC);
 if (rc != 0) {
  //printk("genlmsg_multicast() returned error (Group ID: %d)\n", genl_exmpl_event_group.id);
  return 0;
 } else {
  printk("genlmsg_multicast() event message sent (Group ID: %d): %s\n", genl_exmpl_event_group.id, msg);
 }
 return 0;
}

kern/genl_event.h:
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/* attributes (variables):
 * the index in this enum is used as a reference for the type,
 * userspace application has to indicate the corresponding type
 * the policy is used for security considerations 
 */
enum {
 GENL_EXMPL_EVENT_A_UNSPEC,
 GENL_EXMPL_EVENT_A_MSG,
    __GENL_EXMPL_EVENT_A_MAX,
};
#define GENL_EXMPL_EVENT_A_MAX (__GENL_EXMPL_EVENT_A_MAX - 1)

/* commands: enumeration of all commands (functions), 
 * used by userspace application to identify command to be executed
 */
enum {
 GENL_EXMPL_EVENT_C_UNSPEC,
 GENL_EXMPL_EVENT_C_DO,
 __GENL_EXMPL_EVENT_C_MAX,
};
#define GENL_EXMPL_EVENT_C_MAX (__GENL_EXMPL_EVENT_C_MAX - 1)

#define GENL_EXMPL_EVENT_VERSION 1
#define GENL_EXMPL_EVENT_FAMILYNAME "CONTROL_EXMPL"
#define GENL_EXMPL_EVENT_GROUPNAME "EXMPL_GRP"

uint8_t genl_exmpl_event_init(void);
void genl_exmpl_event_deinit(void);
int genl_exmpl_event_send(u8 *msg, int len);

kern/main.c:

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#include <linux/kernel.h>
#include <linux/module.h>
#include <net/genetlink.h>
#include <linux/ioport.h>

#include <linux/wait.h>
#include <linux/kthread.h>
#include <asm/io.h>
#include <linux/sched.h>
#include <linux/delay.h>

#include "genl_event.h"

struct task_struct *thr_ts1;

static char nl_string[50] = "Hello";


int thread_func(void *data) {
 while (!kthread_should_stop ()) {
  sprintf(nl_string, "Hello from kernel at jiffies %ld",jiffies);
  genl_exmpl_event_send(nl_string,50);
  set_current_state (TASK_INTERRUPTIBLE);
        schedule_timeout (cputime_to_jiffies(secs_to_cputime (5))); //Do not schedule this thread again for the specified number of seconds
 }
 return 0;
}

static int __init gnKernel_init(void) {
 printk("Generic Netlink Example Module inserted.\n");

 genl_exmpl_event_init();
 
 thr_ts1 = kthread_run(thread_func, NULL, "kthread1");

 return 0;
}

static void __exit gnKernel_exit(void) {
 kthread_stop(thr_ts1);
 genl_exmpl_event_deinit();

 printk("Generic Netlink Example Module unloaded.\n");
}

module_init(gnKernel_init);
module_exit(gnKernel_exit);
MODULE_LICENSE("GPL");

user/main.c:

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#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <unistd.h>
#include <poll.h>
#include <string.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/socket.h>
#include <signal.h>

#include <linux/genetlink.h>



/* Code based on
 * http://people.ee.ethz.ch/~arkeller/linux/multi/kernel_user_space_howto-3.html
 * and
 * http://sourcecodebrowser.com/acpid/1.0.10/acpid_2src_2drivers_2netlink_8c_source.html
 * and
 * http://sourcecodebrowser.com/acpid/1.0.10/acpid_2include_2acpid_2driver_2netlink_8h.html
 */



/* Generic macros for dealing with netlink sockets. Might be duplicated
 * elsewhere. It is recommended that commercial grade applications use
 * libnl or libnetlink and use the interfaces provided by the library
 */
#define NLMSG_TAIL(nlh) ((struct nlattr *) (((void *) (nlh)) + NLMSG_ALIGN((nlh)->nlmsg_len)))
#define NLA_OK(rta,len) ((len) >= (int)sizeof(struct nlattr) && \
                (rta)->nla_len >= sizeof(struct nlattr) && \
                (rta)->nla_len <= (len))
#define NLA_NEXT(rta,attrlen)      ((attrlen) -= NLA_ALIGN((rta)->nla_len), \
                               (struct nlattr*)(((char*)(rta)) + NLA_ALIGN((rta)->nla_len)))
#define NLA_LENGTH(len)     (NLA_ALIGN(sizeof(struct nlattr)) + (len))
#define NLA_SPACE(len)      NLA_ALIGN(NLA_LENGTH(len))
#define NLA_DATA(rta)   ((void*)(((char*)(rta)) + NLA_LENGTH(0)))
#define NLA_PAYLOAD(rta) ((int)((rta)->nla_len) - NLA_LENGTH(0))
#define GENLMSG_DATA(glh) ((void *)(NLMSG_DATA(glh) + GENL_HDRLEN))
#define GENLMSG_PAYLOAD(glh) (NLMSG_PAYLOAD(glh, 0) - GENL_HDRLEN)
#define GENL_MAX_FAM_OPS   256
#define GENL_MAX_FAM_GRPS   256


//Variables used for netlink
int nl_fd;  //netlink socket's file descriptor
struct sockaddr_nl nl_address;  //netlink socket address
int nl_family_id; //The family ID resolved by the netlink controller for this userspace program
int nl_group_id;
int nl_rxtx_length; //Number of bytes sent or received via send() or recv()
struct nlattr *nl_na;   //pointer to netlink attributes structure within the payload 
struct {    //memory for netlink request and response messages - headers are included
    struct nlmsghdr n;
    struct genlmsghdr g;
    char buf[256];
} nl_request_msg, nl_response_msg;
unsigned long nl_sequence_number = 0;
static const char *nl_family_name = "CONTROL_EXMPL";
static const char *nl_group_name = "EXMPL_GRP";
#define GENL_EXMPL_EVENT_VERSION 1



//Implementation of common netlink related methods: open, close, send, receive
int netlink_open(int * fd, unsigned long * seq_init, int protocol, int groups) {
    struct sockaddr_nl nladdr;  //netlink socket address
    socklen_t len;
    
    *fd = socket(AF_NETLINK, SOCK_RAW, protocol);
    if (*fd < 0) {
        return -1;
    }
    memset(&nladdr, 0, sizeof(nladdr));
    nladdr.nl_family = AF_NETLINK;
    nladdr.nl_groups = groups;


    if (bind(*fd, (struct sockaddr *)&nladdr, sizeof(nladdr)) < 0) {
        goto fail;
    }
    len = sizeof(nladdr);
    if (getsockname(*fd, (struct sockaddr *)&nladdr, &len) < 0) {
        goto fail;
    }

    *seq_init = time(NULL);

    return 0;
fail:
    close(*fd);
    return -1;
}

int netlink_send(int fd, unsigned long * seq_num, struct nlmsghdr *n, pid_t peer, int groups) {
    struct sockaddr_nl nladdr = { .nl_family = AF_NETLINK, .nl_pid = peer, .nl_groups = groups };
    n->nlmsg_seq = ++(*seq_num);
    return sendto(fd, n, n->nlmsg_len, 0, (struct sockaddr *)&nladdr, sizeof(nladdr));
}

int netlink_recv(int fd, struct nlmsghdr *n, pid_t *peer) {
    struct sockaddr_nl nladdr;
    socklen_t len = sizeof(nladdr);
    int ret = recvfrom(fd, n, n->nlmsg_len, 0, (struct sockaddr *)&nladdr, &len);
    *peer = nladdr.nl_pid;
    return ret;
}


int netlink_wait(int fd, unsigned long *seq_num, struct nlmsghdr *n, pid_t peer) {
    int len = n->nlmsg_len;

    for (;;) {
        pid_t sender;
        struct nlmsghdr * h;
        n->nlmsg_len = len;
        int ret = netlink_recv(fd, n, &sender);
        if (ret < 0)  {
            fprintf(stderr, "%s() | Error\n", __func__);
            continue;
        }
        if (sender != peer) {
            fprintf(stderr, "%s() | Error: Source PID mismatch\n", __func__);
            continue;
        }
        for (h = n; NLMSG_OK(h, ret); h = NLMSG_NEXT(h, ret)) {
            if (h->nlmsg_pid != getpid()) {
                fprintf(stderr, "%s() | Error: Destination PID mismatch\n", __func__);
                continue;
            }

            if (h->nlmsg_type == NLMSG_ERROR) {
                fprintf(stderr, "%s() | Error: Message receive error\n", __func__);
                return -1;
            }
            memcpy(n, h, h->nlmsg_len);
            return 0;
        }
    }
}

void netlink_close(int fd) {
    close(fd);
}

//Implementation of common netlink message parsing methods
int netlink_attr_attach(struct nlmsghdr *n, int max, int type, const void *data, int alen) {
    int len = NLA_LENGTH(alen);
    struct nlattr *nla;

    if (NLMSG_ALIGN(n->nlmsg_len) + NLA_ALIGN(len) > max) {
        return -1;
    }

    nla = NLMSG_TAIL(n);
    nla->nla_type = type;
    nla->nla_len = len;
    memcpy(NLA_DATA(nla), data, alen);
    n->nlmsg_len = NLMSG_ALIGN(n->nlmsg_len) + NLA_ALIGN(len);
    return 0;
}

int netlink_attr_parse(struct nlattr *table[], int max, struct nlattr *src, int len) {
    memset(table, 0, sizeof(struct nlattr *) * (max + 1));
    while (NLA_OK(src, len)) {
        if (src->nla_type <= max)
        table[src->nla_type] = src;
        src = NLA_NEXT(src, len);
    }
    return 0;
}

static int netlink_verify_group(struct nlattr * attr, int * group, const char * expected_group_name) {
    const char *name;
    if (attr == NULL) {
        return -1;
    }

    struct nlattr *attrs[CTRL_ATTR_MCAST_GRP_MAX + 1];
    netlink_attr_parse(attrs, CTRL_ATTR_MCAST_GRP_MAX, NLA_DATA(attr), attr->nla_len - NLA_HDRLEN);

    name = NLA_DATA(attrs[CTRL_ATTR_MCAST_GRP_NAME]);
    if (strcmp(name, expected_group_name)) {
        return -1;
    }

    *group = *(__u32 *) (NLA_DATA(attrs[CTRL_ATTR_MCAST_GRP_ID]));
    return 0;
}

int netlink_wait_for_event(int fd, char * received_msg, int max) {
    char buffer[256];
    int i;
    int ret;
    struct nlmsghdr *n = (struct nlmsghdr *) &buffer;
    struct nlmsghdr * h;
    pid_t sender;

    n->nlmsg_len = 256;

    
    ret = netlink_recv(fd, n, &sender);

    if (ret < 0) {
        return -1;
    }

    i = 0;
    for (h = n; NLMSG_OK(h, ret) && i < max; h = NLMSG_NEXT(h, ret), ++i) {
        if (h->nlmsg_type == NLMSG_ERROR) {
            return -1;
        }
        printf("Event received from Kernel: %s\n",(char *)NLA_DATA(((struct nlattr *) GENLMSG_DATA(h))));
    }

    return i;
}

int main(void) {
    int return_code;
//Step 1: Open & Bind the socket. Note that protocol = NETLINK_GENERIC
    return_code = netlink_open(&nl_fd, &nl_sequence_number, NETLINK_GENERIC,0);
    if (return_code < 0) {
        fprintf(stderr, "Error: Socket could not be created\n");
  return -1;
    }
    printf("%s() | Socket Opened\n", __func__);

//Step 2. Resolve the family ID corresponding to the string "EXMPL_EVENT"
    {
        char buffer[256];
        struct nlmsghdr *nlmsg = (struct nlmsghdr *)&buffer;
        struct nlattr *attrs[CTRL_ATTR_MAX + 1];

        nlmsg->nlmsg_len = NLMSG_LENGTH(GENL_HDRLEN);
        nlmsg->nlmsg_flags = NLM_F_REQUEST | NLM_F_ACK;
        nlmsg->nlmsg_type = GENL_ID_CTRL;
        nlmsg->nlmsg_pid = getpid();

        struct genlmsghdr *ghdr = NLMSG_DATA(nlmsg);
        ghdr->cmd = CTRL_CMD_GETFAMILY;

        netlink_attr_attach(nlmsg, 128, CTRL_ATTR_FAMILY_NAME, nl_family_name, strlen(nl_family_name) + 1);

        if (netlink_send(nl_fd, &nl_sequence_number, nlmsg, 0, 0) < 0) {
            fprintf(stderr, "%s() | Error: sending family ID request message\n", __func__);
            netlink_close(nl_fd);
            return -1;
        }
        printf("%s() | Family Request Sent\n", __func__);

        nlmsg->nlmsg_len = 256;
        //Wait for the response message
        if (netlink_wait(nl_fd, &nl_sequence_number, nlmsg, 0) < 0) {
            fprintf(stderr, "Error: receiving family ID request message\n", __func__);
            netlink_close(nl_fd);
            return -1;
        }

        //Validate response message
        if (nlmsg->nlmsg_type != GENL_ID_CTRL) {
            fprintf(stderr, "%s() | Error: family ID request - invalid message\n", __func__);
            netlink_close(nl_fd);
            return -1;
        }
        ghdr = NLMSG_DATA(nlmsg);
        if (ghdr->cmd != CTRL_CMD_NEWFAMILY) {
            fprintf(stderr, "%s() | Error: family ID request - invalid message\n", __func__);
            netlink_close(nl_fd);
            return -1;
        }
        if (nlmsg->nlmsg_len < NLMSG_LENGTH(GENL_HDRLEN)){
            fprintf(stderr, "%s() | Error: family ID request - invalid message\n", __func__);
            netlink_close(nl_fd);
            return -1;
        }


        netlink_attr_parse(attrs, CTRL_ATTR_MAX, NLMSG_DATA(nlmsg) + GENL_HDRLEN, NLMSG_PAYLOAD(nlmsg, GENL_HDRLEN));

        if (attrs[CTRL_ATTR_FAMILY_ID]) {
            nl_family_id = *(__u32 *) (NLA_DATA(attrs[CTRL_ATTR_FAMILY_ID]));
            printf("%s() | Family ID resolved for \"%s\": %d\n", __func__, nl_family_name, nl_family_id);
        }

        if (attrs[CTRL_ATTR_MCAST_GROUPS]) {
            int i;
            struct nlattr *attrs2[GENL_MAX_FAM_GRPS + 1];
            netlink_attr_parse(attrs2, GENL_MAX_FAM_GRPS, NLA_DATA(attrs[CTRL_ATTR_MCAST_GROUPS]), attrs[CTRL_ATTR_MCAST_GROUPS]->nla_len - NLA_HDRLEN);

            for (i = 0; i < GENL_MAX_FAM_GRPS; i++) {
                if (netlink_verify_group(attrs2[i], &nl_group_id, nl_group_name) == 0) {
                    printf("%s() | Group ID resolved for \"%s\": %d\n", __func__, nl_group_name, nl_group_id);
                }
            }
        }

//Step 3. Close and Reopen Socket for specified group
        netlink_close(nl_fd);
        if (netlink_open(&nl_fd, &nl_sequence_number, NETLINK_GENERIC, nl_group_id ? (1 << (nl_group_id - 1)) : 0)) {
            fprintf(stderr, "Error: Socket could not be re-created\n");
            return -1;
        }

        netlink_wait_for_event(nl_fd, NULL, 10);
    }
    netlink_close(nl_fd);
    printf("%s() | Socket Closed\n",__func__);
    return 0;
}

Makefile:

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EXTRA_CFLAGS += -I./kern

obj-m += nl_kern.o
nl_kern-objs := kern/main.o kern/genl_event.o

all: kernel-module-uninstall kernel-clean-ring-buffer kernel-build kernel-clean-temporary kernel-module-install user-build
 @tput setaf 3
 @echo "    done: all"
 @tput sgr0
clean: kernel-module-uninstall kernel-clean user-clean
 @tput setaf 3
 @echo "    done: clean"
 @tput sgr0
 
 
 
kernel-build:
 @tput setaf 1
 @echo "    kernel-build"
 @tput sgr0
 make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
kernel-clean:
 @tput setaf 1
 @echo "    kernel-clean"
 @tput sgr0
 make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean
kernel-clean-temporary:
 @tput setaf 1
 @echo "    kernel-clean-temporary"
 @tput sgr0
 -rm -rf *.o *~ core .depend .*.cmd *.mod.c .tmp_versions
 -rm -rf kern/*.o kern/*~ kern/core kern/.depend kern/.*.cmd kern/*.mod.c kern/.tmp_versions
 -rm -rf Module.symvers modules.order
kernel-module-install:
 @tput setaf 1
 @echo "    kernel-module-install"
 @tput sgr0
 -sudo insmod nl_kern.ko
kernel-module-uninstall:
 @tput setaf 1
 @echo "    kernel-module-uninstall"
 @tput sgr0
 -sudo rmmod nl_kern
kernel-clean-ring-buffer:
 @tput setaf 1
 @echo "    kernel-clean-ring-buffer"
 @tput sgr0
 sudo dmesg -c > /dev/null

 
 
user-build:
 @tput setaf 1
 @echo "    user-build"
 @tput sgr0
 gcc user/main.c -o nl_user.out
user-clean:
 @tput setaf 1
 @echo "    user-clean"
 @tput sgr0
 rm -rf *.out

Execution:
The makefile will load the kernel module automatically, so just execute nl_user.out to receive the events. The kernel module posts events every 5 seconds.

Running the code

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