# Ueventd ------- Ueventd manages `/dev`, sets permissions for `/sys`, and handles firmware uevents. It has default behavior described below, along with a scripting language that allows customizing this behavior, built on the same parser as init. Ueventd has one generic customization parameter, the size of rcvbuf_size for the ueventd socket. It is customized by the `uevent_socket_rcvbuf_size` parameter, which takes the format of uevent_socket_rcvbuf_size For example uevent_socket_rcvbuf_size 16M Sets the uevent socket rcvbuf_size to 16 megabytes. ## /dev ---- Ueventd listens to the kernel uevent sockets and creates/deletes nodes in `/dev` based on the incoming add/remove uevents. It defaults to using `0600` mode and `root` user/group. It always creates the nodes with the SELabel from the current loaded SEPolicy. It has three default behaviors for the node path: 1. Block devices are created as `/dev/block/`. There are symlinks created to this node at `/dev/block///`, `/dev/block///by-name/`, and `/dev/block/by-name/` if the device is a boot device. 2. USB devices are created as `/dev/` if `DEVNAME` was specified for the uevent, otherwise as `/dev/bus/usb//` where `bus_id` is `uevent MINOR / 128 + 1` and `device_id` is `uevent MINOR % 128 + 1`. 3. All other devices are created as `/dev/` The permissions can be modified using a ueventd.rc script and a line that beings with `/dev`. These lines take the format of devname mode uid gid For example /dev/null 0666 root root When `/dev/null` is created, its mode will be set to `0666`, its user to `root` and its group to `root`. The path can be modified using a ueventd.rc script and a `subsystem` section. There are three to set for a subsystem: the subsystem name, which device name to use, and which directory to place the device in. The section takes the below format of subsystem devname uevent_devname|uevent_devpath [dirname ] `subsystem_name` is used to match uevent `SUBSYSTEM` value `devname` takes one of two options 1. `uevent_devname` specifies that the name of the node will be the uevent `DEVNAME` 2. `uevent_devpath` specified that the name of the node will be basename uevent `DEVPATH` `dirname` is an optional parameter that specifies a directory within `/dev` where the node will be created. For example subsystem sound devname uevent_devpath dirname /dev/snd Indicates that all uevents with `SUBSYSTEM=sound` will create nodes as `/dev/snd/`. ## /sys ---- Ueventd by default takes no action for `/sys`, however it can be instructed to set permissions for certain files in `/sys` when matching uevents are generated. This is done using a ueventd.rc script and a line that begins with `/sys`. These lines take the format of nodename attr mode uid gid For example /sys/devices/system/cpu/cpu* cpufreq/scaling_max_freq 0664 system system When a uevent that matches the pattern `/sys/devices/system/cpu/cpu*` is sent, the matching sysfs attribute, `cpufreq/scaling_max_freq`, will have its mode set to `0664`, its user to to `system` and its group set to `system`. Note that `*` matches as a wildcard and can be used anywhere in a path. ## Firmware loading ---------------- Ueventd by default serves firmware requests by searching through a list of firmware directories for a file matching the uevent `FIRMWARE`. It then forks a process to serve this firmware to the kernel. The list of firmware directories is customized by a `firmware_directories` line in a ueventd.rc file. This line takes the format of firmware_directories [ ]* For example firmware_directories /etc/firmware/ /odm/firmware/ /vendor/firmware/ /firmware/image/ Adds those 4 directories, in that order to the list of firmware directories that will be tried by ueventd. Note that this option always accumulates to the list; it is not possible to remove previous entries. Ueventd will wait until after `post-fs` in init, to keep retrying before believing the firmwares are not present. The exact firmware file to be served can be customized by running an external program by a `external_firmware_handler` line in a ueventd.rc file. This line takes the format of external_firmware_handler For example external_firmware_handler /devices/leds/red/firmware/coeffs.bin system /vendor/bin/led_coeffs.bin Will launch `/vendor/bin/led_coeffs.bin` as the system user instead of serving the default firmware for `/devices/leds/red/firmware/coeffs.bin`. Ueventd will provide the uevent `DEVPATH` and `FIRMWARE` to this external program on the environment via environment variables with the same names. Ueventd will use the string written to stdout as the new name of the firmware to load. It will still look for the new firmware in the list of firmware directories stated above. It will also reject file names with `..` in them, to prevent leaving these directories. If stdout cannot be read, or the program returns with any exit code other than `EXIT_SUCCESS`, or the program crashes, the default firmware from the uevent will be loaded. Ueventd will additionally log all messages sent to stderr from the external program to the serial console after the external program has exited. ## Coldboot -------- Ueventd must create devices in `/dev` for all devices that have already sent their uevents before ueventd has started. To do so, when ueventd is started it does what it calls a 'coldboot' on `/sys`, in which it writes 'add' to every 'uevent' file that it finds in `/sys/class`, `/sys/block`, and `/sys/devices`. This causes the kernel to regenerate the uevents for these paths, and thus for ueventd to create the nodes. For boot time purposes, this is done in parallel across a set of child processes. `ueventd.cpp` in this directory contains documentation on how the parallelization is done. There is an option to parallelize the restorecon function during cold boot as well. This should only be done for devices that do not use genfscon, which is the recommended method for labeling sysfs nodes. To enable this option, use the below line in a ueventd.rc script: parallel_restorecon enabled