platform_system_core/init/README.ueventd.md

Ueventd

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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 <size>

For example

uevent_socket_rcvbuf_size 16M

Sets the uevent socket rcvbuf_size to 16 megabytes.

/dev

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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/<basename uevent DEVPATH>. There are symlinks created to this node at /dev/block/<type>/<parent device>/<basename uevent DEVPATH>, /dev/block/<type>/<parent device>/by-name/<uevent PARTNAME>, and /dev/block/by-name/<uevent PARTNAME> if the device is a boot device.
2. USB devices are created as /dev/<uevent DEVNAME> if DEVNAME was specified for the uevent, otherwise as /dev/bus/usb/<bus_id>/<device_id> where bus_id is uevent MINOR / 128 + 1 and device_id is uevent MINOR % 128 + 1.
3. All other devices are created as /dev/<basename uevent DEVPATH>

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 <subsystem_name>
  devname uevent_devname|uevent_devpath
  [dirname <directory>]

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/<basename uevent DEVPATH>.

/sys

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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

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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.

/apex/*/firmware is also searched after a list of firmware directories.

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 <firmware_directory> [ <firmware_directory> ]*

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 <devpath> <user name to run as> <path to external program>

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

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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