uevent message passing

Handling kernel uevent in userspace
        

⚫ ueventd
    ⚬ ueventd creates and 'poll' netlink socket.
        ueventd_main [init/ueventd.c]

    ⚬ read kernel event message -> parse -> handle
        handle_device_event [init/devices.c]
            -> device_changed [init/init.c]
                -> queue_device_triggers [init/init_parser.c]
                            |
            +---------------+
            |
            v
        : search action list to know what to do for this event.
          searching action which name is device-added/removed-[device node].
          [device node] is from 'DEVPATH' variable of uevent message.

    ⚬ action list : list declared at [init_parser.c]
        • builtin action (added by 'queue_builtin_action [init/init_parser.c]')
            ex. property_init, wait_for_coldboot_done etc.
        • from script (added by 'parse_action [init/init_parser.c]')
            section "on device-added-[device path] / on device-removed-[device path]"
            ex.
                [init.rc]
                on device-added-/dev/block/mmcblk0p18
                    exec /system/bin/sh /system/bin/hello_mmc.sh

⚫ other daemons (ex usbd)
    ⚬ create and 'poll' netlink socket. 

kobject uevent in kernel

   
⚫ Sendig kobject uevent to user space from kernel
    ⚬ Concept
        • Execute user-space binary with predefined + customized environment variables.
        • Send message to pre-defined netlink socket unit created by kernel.

    ⚬ From souce code (hotplug)
        • kernel node [kernel/sysctl.c] (ctl_table)
        : /proc/sys/kernel/hotplug

        • kobject_uevent_env() [kobject_uevent.c]
            default user process executable image path
                -> CONFIG_UEVENT_HELPER_PATH
        • kernel/sysctl.c
            'uevent_helper' value can be RWed through 'hotplug' node of sysctl.

    ⚬ From souce code (netlink)
        • kobject_uevent_env() [kobject_uevent.c]
            : #if defined(CONFIG_NET)
            write message to netlink socket unit (NETLINK_KOBJECT_UEVENT)
        • netlink.h
            netlink units
        • Creating in userspace
            socket(PF_NETLINK, SOCK_DGRAM, NETLINK_KOBJECT_UEVENT)