Linux Kernel（11.1）- sysfs and hotplug

Linux提供兩種非同步的hotplug機制通知user-space裝置狀態的改變，一是usermode helper，另一個則是透過netlink。

usermode helper
每當kernel收到hotplug event便會執行"CONFIG_UEVENT_HELPER_PATH"（預設值是/sbin/hotplug，可以透過修改/proc/sys/kernel/hotplug修改預設值），在embedded system中，常用的是busybox，所以，通常會把UEVENT HELPER換成/sbin/mdev（找時間來寫寫busybox），執行UEVENT HELPER會攜帶一些環境變數，如ACTION/DEVPATH/SUBSYSTEM/HOME/PATH等等，透過這些環境變數，可以取得Device Name，MAJOR/MINOR number等等，我們先來看部份的程式碼，往後有機會在作深度研究。

int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,
         char *envp_ext[])
{
    ... 略...
    /* call uevent_helper, usually only enabled during early boot */
    if (uevent_helper[0]) {
        char *argv [3];

        argv [0] = uevent_helper;
        argv [1] = (char *)subsystem;
        argv [2] = NULL;
        retval = add_uevent_var(env, "HOME=/");
        if (retval)
            goto exit;
        retval = add_uevent_var(env,
                        "PATH=/sbin:/bin:/usr/sbin:/usr/bin");
        if (retval)
            goto exit;

        retval = call_usermodehelper(argv[0], argv,
                        env->envp, UMH_WAIT_EXEC);
    }
    ... 略...
}

我利用一個shell script來取代UEVENT HELPER，主要目的是要印出有哪些環境變數，和傳遞哪些參數給UEVENT HELPER。

#!/bin/sh
env >> /helper.log
echo "CMD:" $@ >> /helper.log
echo "----------     end     ----------" >> /helper.log

netlink
另外一條路徑就是netlink了，基本上也是傳遞相同的資訊。

int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,
         char *envp_ext[])
{
    ... 略...
#if defined(CONFIG_NET)
    /* send netlink message */
    if (uevent_sock) {
        struct sk_buff *skb;
        size_t len;

        /* allocate message with the maximum possible size */
        len = strlen(action_string) + strlen(devpath) + 2;
        skb = alloc_skb(len + env->buflen, GFP_KERNEL);
        if (skb) {
            char *scratch;

            /* add header */
            scratch = skb_put(skb, len);
            sprintf(scratch, "%s@%s", action_string, devpath);

            /* copy keys to our continuous event payload buffer */
            for (i = 0; i < env->envp_idx; i++) {
                len = strlen(env->envp[i]) + 1;
                scratch = skb_put(skb, len);
                strcpy(scratch, env->envp[i]);
            }

            NETLINK_CB(skb).dst_group = 1;
            retval = netlink_broadcast(uevent_sock, skb, 0, 1,
                            GFP_KERNEL);
            /* ENOBUFS should be handled in userspace */
            if (retval == -ENOBUFS) {
                retval = 0;
            }
        } else {
            retval = -ENOMEM;
        }
    }
#endif
    ... 略...
}

user-space的code如下：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <sys/poll.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>

#include <linux/types.h>
#include <linux/netlink.h>

int main(int argc, char *argv[])
{
    struct sockaddr_nl nls;
    struct pollfd pfd;
    char buf[512];

    memset(&nls, 0, sizeof(nls));
    nls.nl_family = AF_NETLINK;
    nls.nl_pid = getpid();
    nls.nl_groups = -1;

    pfd.events = POLLIN;
    pfd.fd = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_KOBJECT_UEVENT);
    if (pfd.fd == -1) {
        printf("Not root\n");
        exit(1);
    }

    if (bind(pfd.fd, (void*)&nls, sizeof(nls))) {
        printf("bind failed\n");
        exit(1);
    }
    while (-1 != poll(&pfd, 1, -1)) {
        int i, len = recv(pfd.fd, buf, sizeof(buf), MSG_DONTWAIT);
        if (len == -1) {
            printf("recv\n");
            exit(1);
        }
        i = 0;
        while (i < len) {
            printf("%s\n", buf + i);
            i += strlen(buf+i) + 1;
        }
    }
    printf("\n");
    return 0;
}