android uevent創建設備文件

linux kernel啓動過程中創建VFS，爲各驅動設備在sys/class, sys/devices, sys/block創建目錄和文件，分配kset和kobject，uevent文件， 然後發送uevent事件到用戶空間，這些目錄和文件創建需要用戶空間實際創建設備文件，即在/dev目錄下創建設備文件，供用戶空間使用。
具體過程參考uevent機制。
用戶空間還沒有啓動時，uevent事件存在緩存中，等到用戶空間啓動後，uevent進程獲取緩存中uevent事件進行處理。並且監聽新的uevent事件進行處理，比如熱插拔事件。
uevent事件使用netlink方式通信。

設備文件創建

int ueventd_main(int argc, char **argv)
{
    /*
     * init sets the umask to 077 for forked processes. We need to
     * create files with exact permissions, without modification by
     * the umask.
     */
    umask(000);

    /* Prevent fire-and-forget children from becoming zombies.
     * If we should need to wait() for some children in the future
     * (as opposed to none right now), double-forking here instead
     * of ignoring SIGCHLD may be the better solution.
     */
    signal(SIGCHLD, SIG_IGN);

    open_devnull_stdio();
    klog_init();
    klog_set_level(KLOG_NOTICE_LEVEL);

    NOTICE("ueventd started!\n");

    selinux_callback cb;
    cb.func_log = selinux_klog_callback;
    selinux_set_callback(SELINUX_CB_LOG, cb);

    char hardware[PROP_VALUE_MAX];
    property_get("ro.hardware", hardware);

    ueventd_parse_config_file("/ueventd.rc"); //解析編譯階段定義好的文件，定義了各種文件權限屬性等
    ueventd_parse_config_file(android::base::StringPrintf("/ueventd.%s.rc", hardware).c_str());

    device_init(); //處理kernel上報uevent事件，完成coldboot_done

    pollfd ufd;
    ufd.events = POLLIN;
    ufd.fd = get_device_fd();

    //繼續監聽uevent事件進行處理
    while (true) {
        ufd.revents = 0;
        int nr = poll(&ufd, 1, -1);
        if (nr <= 0) {
            continue;
        }
        if (ufd.revents & POLLIN) {
            handle_device_fd();
        }
    }

    return 0;
}

void device_init() {
    sehandle = NULL;
    if (is_selinux_enabled() > 0) {
        sehandle = selinux_android_file_context_handle();
        selinux_status_open(true);
    }

    /* is 256K enough? udev uses 16MB! */
    device_fd = uevent_open_socket(256*1024, true);
    if (device_fd == -1) {
        return;
    }
    fcntl(device_fd, F_SETFL, O_NONBLOCK);

    if (access(COLDBOOT_DONE, F_OK) == 0) {
        NOTICE("Skipping coldboot, already done!\n");
        return;
    }

    Timer t;
    coldboot("/sys/class");  // 處理sys/class 目錄下各種文件
    coldboot("/sys/block");// 處理sys/block目錄下各種文件
    coldboot("/sys/devices");// 處理sys/devices目錄下各種文件
    close(open(COLDBOOT_DONE, O_WRONLY|O_CREAT|O_CLOEXEC, 0000)); 創建"/dev/.coldboot_done"，表示coldboot完成
    NOTICE("Coldboot took %.2fs.\n", t.duration());
}

coldboot–>do_coldboot–

static void do_coldboot(DIR *d)
{
    struct dirent *de;
    int dfd, fd;

    dfd = dirfd(d);

    fd = openat(dfd, "uevent", O_WRONLY);//打開目錄下的uevent文件
    if(fd >= 0) {
        write(fd, "add\n", 4); //向uevent寫add，觸發kernel向用戶空間發送add事件
        close(fd);
        handle_device_fd(); //監聽處理add事件
    }

	//繼續處理子目錄，遞歸
    while((de = readdir(d))) {
        DIR *d2;

        if(de->d_type != DT_DIR || de->d_name[0] == '.')
            continue;

        fd = openat(dfd, de->d_name, O_RDONLY | O_DIRECTORY);
        if(fd < 0)
            continue;

        d2 = fdopendir(fd);
        if(d2 == 0)
            close(fd);
        else {
            do_coldboot(d2);
            closedir(d2);
        }
    }
}

#define UEVENT_MSG_LEN  2048
void handle_device_fd()
{
    char msg[UEVENT_MSG_LEN+2];
    int n;
    //監聽事件
    while ((n = uevent_kernel_multicast_recv(device_fd, msg, UEVENT_MSG_LEN)) > 0) {
        if(n >= UEVENT_MSG_LEN)   /* overflow -- discard */
            continue;

        msg[n] = '\0';
        msg[n+1] = '\0';

        struct uevent uevent;
        parse_event(msg, &uevent);

        if (sehandle && selinux_status_updated() > 0) {
            struct selabel_handle *sehandle2;
            sehandle2 = selinux_android_file_context_handle();
            if (sehandle2) {
                selabel_close(sehandle);
                sehandle = sehandle2;
            }
        }

        handle_device_event(&uevent); //事件處理
        handle_firmware_event(&uevent);
    }
}

static void handle_device_event(struct uevent *uevent)
{
    if (!strcmp(uevent->action,"add") || !strcmp(uevent->action, "change") || !strcmp(uevent->action, "online"))
        fixup_sys_perms(uevent->path);//設置權限

    if (!strncmp(uevent->subsystem, "block", 5)) {
        handle_block_device_event(uevent);
    } else if (!strncmp(uevent->subsystem, "platform", 8)) {
        handle_platform_device_event(uevent);
    } else {
        handle_generic_device_event(uevent);//創建文件
    }
}

創建各種設備文件, /dev目錄下

static void handle_generic_device_event(struct uevent *uevent)
{
    const char *base;
    const char *name;
    char devpath[DEVPATH_LEN] = {0};
    char **links = NULL;

    name = parse_device_name(uevent, 64);
    if (!name)
        return;

    struct ueventd_subsystem *subsystem =
            ueventd_subsystem_find_by_name(uevent->subsystem);

    if (subsystem) {
        const char *devname;

        switch (subsystem->devname_src) {
        case DEVNAME_UEVENT_DEVNAME:
            devname = uevent->device_name;
            break;

        case DEVNAME_UEVENT_DEVPATH:
            devname = name;
            break;

        default:
            ERROR("%s subsystem's devpath option is not set; ignoring event\n",
                    uevent->subsystem);
            return;
        }

        if (!assemble_devpath(devpath, subsystem->dirname, devname))
            return;
        mkdir_recursive_for_devpath(devpath);
    } else if (!strncmp(uevent->subsystem, "usb", 3)) {
         if (!strcmp(uevent->subsystem, "usb")) {
            if (uevent->device_name) {
                if (!assemble_devpath(devpath, "/dev", uevent->device_name))
                    return;
                mkdir_recursive_for_devpath(devpath);
             }
             else {
                 /* This imitates the file system that would be created
                  * if we were using devfs instead.
                  * Minors are broken up into groups of 128, starting at "001"
                  */
                 int bus_id = uevent->minor / 128 + 1;
                 int device_id = uevent->minor % 128 + 1;
                 /* build directories */
                 make_dir("/dev/bus", 0755);
                 make_dir("/dev/bus/usb", 0755);
                 snprintf(devpath, sizeof(devpath), "/dev/bus/usb/%03d", bus_id);
                 make_dir(devpath, 0755);
                 snprintf(devpath, sizeof(devpath), "/dev/bus/usb/%03d/%03d", bus_id, device_id);
             }
         } else {
             /* ignore other USB events */
             return;
         }
     } else if (!strncmp(uevent->subsystem, "graphics", 8)) {
         base = "/dev/graphics/";
         make_dir(base, 0755);
     } else if (!strncmp(uevent->subsystem, "drm", 3)) {
         base = "/dev/dri/";
         make_dir(base, 0755);
     } else if (!strncmp(uevent->subsystem, "oncrpc", 6)) {
         base = "/dev/oncrpc/";
         make_dir(base, 0755);
     } else if (!strncmp(uevent->subsystem, "adsp", 4)) {
         base = "/dev/adsp/";
         make_dir(base, 0755);
     } else if (!strncmp(uevent->subsystem, "msm_camera", 10)) {
         base = "/dev/msm_camera/";
         make_dir(base, 0755);
     } else if(!strncmp(uevent->subsystem, "input", 5)) {
         base = "/dev/input/";
         make_dir(base, 0755);
     } else if(!strncmp(uevent->subsystem, "mtd", 3)) {
         base = "/dev/mtd/";
         make_dir(base, 0755);
     } else if(!strncmp(uevent->subsystem, "sound", 5)) {
         base = "/dev/snd/";
         make_dir(base, 0755);
     } else if(!strncmp(uevent->subsystem, "misc", 4) &&
                 !strncmp(name, "log_", 4)) {
         INFO("kernel logger is deprecated\n");
         base = "/dev/log/";
         make_dir(base, 0755);
         name += 4;
     } else
         base = "/dev/";
     links = get_character_device_symlinks(uevent);

     if (!devpath[0])
         snprintf(devpath, sizeof(devpath), "%s%s", base, name);

     handle_device(uevent->action, devpath, uevent->path, 0,
             uevent->major, uevent->minor, links);
}

ueventd.rc中各種權設置

保存權限
ueventd_parse_config_file("/ueventd.rc");----》parse_config–》parse_line—》parse_line_device–》
set_device_permission–》add_dev_perms：
添加到dev_perms鏈表中。

前面接收處理uevent事件時，fixup_sys_perms中設置權限。