mdevd
Software
skarnet.org

The mdevd-netlink program

mdevd-netlink listens to the netlink interface for uevents (also called "hotplug" or "udev" events), and writes those uevents to its standard output, using a simple format.

mdevd expects uevents in the same format mdevd-netlink writes them. So the mdevd-netlink | mdevd command line will function as a daemon reading uevents from the netlink and handling them.

Interface

     mdevd-netlink [ -d notif ] [ -v verbosity ] [ -b kbufsz ]

• mdevd-netlink binds to the netlink interface and listens for hotplug events, as the udevd program does.
• It writes event information to its stdout. The output contains null characters, so a terminal will not display them correctly. To properly use mdevd-netlink, it should be piped into a handler program such as mdevd.
• mdevd-netlink is a long-lived program. When it receives a SIGTERM, it stops listening to hotplug events; it will exit as soon as it has flushed its event queue to stdout.

Options

• -d notif : when ready (actually listening to the netlink), write a newline to file descriptor notif then close it. This allows mdevd-netlink to use the s6 mechanism to notify readiness. notif cannot be lesser than 3. If this option is not given, no readiness notification is sent.
• -v verbosity : be more or less verbose. Default verbosity is 1. 0 will only print fatal error messages, 3 will print warnings every time the netlink interface sends something unexpected.
• -b kbufsz : try and reserve a kernel buffer of kbufsz bytes for the netlink queue. Too large a buffer wastes kernel memory; too small a buffer risks losing events. The default is 65536 (which is on the large side).

Protocol

• An event is a series of null-terminated strings as they are sent by the kernel to the netlink; mdevd-netlink adds a final empty string (i.e. an additional null character) to mark the end of the series.
• The first string is a short description of the event; it normally contains the string "@/". Other strings after the first are of the form "VARIABLE=value", and describe the environment which a hotplug helper for the event (registered in /proc/sys/kernel/hotplug) would be spawned with.
• Example (newlines added for clarity):

  add@/class/input/input9/mouse2\0
  ACTION=add\0
  DEVPATH=/class/input/input9/mouse2\0
  SUBSYSTEM=input\0
  SEQNUM=106\0
  PHYSDEVPATH=/devices/pci0000:00/0000:00:1d.1/usb2/2­2/2­2:1.0
  PHYSDEVBUS=usb\0
  PHYSDEVDRIVER=usbhid\0
  MAJOR=13\0
  MINOR=34\0
  \0 

Notes

• mdevd-netlink is a daemon; it should be run under a proper supervision system such as s6.
• If you are running mdevd-netlink, the program you pipe its output into should be the only program handling uevents; that means that /proc/sys/kernel/hotplug should be empty.
• The mdevd-netlink | mdevd command line is a quick way of running a uevent manager, but there are ways to improve it:
  • Using the execline scripting language, the pipeline { mdevd-netlink } mdevd command line will avoid leaving a shell around.
  • The best way to use mdevd-netlink and mdevd is with a supervision system: under s6 or s6-rc, write a service pipeline where mdevd-netlink is a producer and mdevd is a consumer. This setup has the advantage, among others, that you can restart the netlink listener and the event handler separately.