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.
Ueventd reads /system/etc/ueventd.rc, all other files are imported via the import command, which takes the format of
import <path>
This command parses an ueventd config file, extending the current configuration. If path is a directory, each file in the directory is parsed as a config file. It is not recursive, nested directories will not be parsed. Imported files are parsed after the current file has been parsed.
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:
/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./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./dev/<basename uevent DEVPATH>Whether a device is considered a “boot device” is a bit complicated.
androidboot.boot_part_uuid which can be provided either via the kernel bootconfig or via the kernel commandline. As an example, you could set androidboot.boot_part_uuid=12345678-abcd-ef01-0234-6789abcdef01.boot_part_uuid is preferred, you can also specify the boot device via androidboot.boot_device or androidboot.boot_devices. These can be passed via the kernel bootconfig or the kernel command line. It is also possible to pass this via device tree by creating a boot_devices property in the Android firmware node. In most cases the boot_device is the sysfs path (without the /sys/devices or /sys/devices/platform prefix) to the closest parent of the block device that's on the “platform” bus. As an example, if the block device is /sys/devices/platform/soc@0/7c4000.mmc/mmc_host/mmc1/mmc1:0001/block/mmcblk1 then the boot_device is soc@0/7c4000.mmc since we strip off the /sys/devices/platform and nothing past the 7c4000.mmc directory represents a device on the “platform” bus. In the case that none of the parents are on the “platform” bus there are special rules for block devices under PCI and VBD (Virtual Block Device). NOTE: sysfs paths for block devices are not guaranteed to be stable between kernel versions, which is one of the reasons why it is suggested to use boot_part_uuid instead of boot_devices. ALSO NOTE: If more than one device matches (either because multiple boot_devices were listed or because there was more than one block device under the found sysfs directory) and these multiple matching devices provide some of the same named partitions then the behavior is unspecified.boot_devices has been required since Android 12 so this further fallback will not be described here.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 [options]
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 and/or driver section. There are three options to set for a subsystem or driver: the 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 the uevent SUBSYSTEM value.
devname takes one of three options:
uevent_devname specifies that the name of the node will be the uevent DEVNAMEuevent_devpath specifies that the name of the node will be basename uevent DEVPATHsys_name specifies that the name of the node will be the contents of /sys/DEVPATH/namedirname 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>.
The driver section has the exact same structure as a subsystem section, but will instead match the DRIVER value in a bind/unbind uevent. However, the driver section will be ignored for block devices.
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 [options]
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.
The path for a /dev or /sys entry can contain a * anywhere in the path.
* appears at the end of the string or if the options parameter is set to no_fnm_pathname, ueventd matches the entry by fnmatch(entry_path, incoming_path, 0)fnmatch(entry_path, incoming_path, FNM_PATHNAME)See the man page for fnmatch for more details.
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/*/etc/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 [group]> <path to external program>
The handler will be run as the given user, or if a group is provided, as the given user and group.
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.
The devpath argument may include asterisks (*) to match multiple paths. For example, the string /dev/*/red will match /dev/leds/red as well as /dev/lights/red. The pattern matching follows the rules of the fnmatch() function.
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.
If the kernel command-line argument firmware_class.path is set, this path will be used first by the kernel to search for the firmware files. If found, ueventd will not be called at all. See the kernel documentation for more details on this feature.
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. It is recommended that devices use genfscon for labeling sysfs nodes. However, some devices may benefit from enabling the parallelization option:
parallel_restorecon enabled
Do parallel restorecon to speed up boot process, subdirectories under /sys can be sliced by ueventd.rc, and run on multiple process. parallel_restorecon_dir
For example parallel_restorecon_dir /sys parallel_restorecon_dir /sys/devices parallel_restorecon_dir /sys/devices/platform parallel_restorecon_dir /sys/devices/platform/soc