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docs: Reword FACTORY_RESET.md for clarity
Primarily, what this does is disentangles the UEFI-specific parts of the factory reset support from the generic hooks.
This commit is contained in:
Adrian Vovk
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d85fc1189f
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cdc6710cf3
@ -11,72 +11,33 @@ In various scenarios it is important to be able to reset operating systems back
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into a "factory state", i.e. where all state, user data and configuration is
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reset so that it resembles the system state when it was originally shipped.
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systemd natively supports a concept of factory reset, that can both act as a
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specific implementation for UEFI based systems, as well as a series of hook
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points and a template for implementations on other systems.
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systemd natively supports a concept of factory reset, through a series of
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generic hook points that can be integrated with a factory reset mechanism.
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Factory reset always takes place during early boot, i.e. from a well-defined
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"clean" state. Factory reset operations may be requested from one boot to be
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"clean" state. Factory reset operations are requested from one boot to be
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executed on the next.
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Specifically, the following concepts are available:
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The mechanism works as follows:
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* The `factory-reset.target` unit may be used to request a factory reset
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operation and trigger a reboot in order to execute it. It by default executes
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three services: `systemd-factory-reset-request.service`,
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`systemd-tpm2-clear.service` and `systemd-factory-reset-reboot.service`.
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* The
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[`systemd-factory-reset-request.service`](https://www.freedesktop.org/software/systemd/man/latest/systemd-factory-reset-request.service.html)
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unit is typically invoked via `factory-reset.target`. It requests a factory
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reset operation for the next boot by setting the `FactoryResetRequest` EFI
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variable. The EFI variable contains information about the requesting OS, so
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that multi-boot scenarios are somewhat covered.
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* The
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[`systemd-tpm2-clear.service`](https://www.freedesktop.org/software/systemd/man/latest/systemd-tpm2-clear.service.html)
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unit can request a TPM2 clear operation from the firmware on the next
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boot. It is also invoked via `factory-reset.target`. UEFI firmwares that
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support TPMs will ask the user for confirmation and then reset the TPM,
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invalidating all prior keys associated with the security chip and generating
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a new seed key.
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* The
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* The `factory-reset.target` unit is used to request a factory reset operation
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and trigger a reboot in order to execute it. Services invoked via this target
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prepare the system such that a factory reset will be requested on the next
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boot. Once these services are done,
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[`systemd-factory-reset-reboot.service`](https://www.freedesktop.org/software/systemd/man/latest/systemd-factory-reset-reboot.service.html)
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unit automatically reboots the system as part of `factory-reset.target`. It
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is ordered after `systemd-tpm2-clear.service` and
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`systemd-factory-reset-request.service` in order to initiate the reboot that
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is supposed to execute the factory reset operations.
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is started, which triggers the reboot.
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* The `factory-reset-now.target` unit is started at boot whenever a factory
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reset is requested for the boot. A factory reset may be requested via a
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kernel command line option (`systemd.factory_reset=1`) or via the UEFI
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variable `FactoryResetRequest` (see above). The
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`systemd-factory-reset-generator` unit generator checks both these conditions
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and adds `factory-reset-now.target` to the boot transaction, already in the
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initial RAM disk (initrd).
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* On the next boot, `systemd-factory-reset-generator` checks whether or not a
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factory reset was requested. A factory reset may be requested via a kernel
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command line option (`systemd.factory_reset=1`) or via the UEFI variable
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`FactoryResetRequest` (see below). If either condition is met,
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`factory-reset-now.target` is added to the boot transaction.
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* The
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* `factory-reset-now.target` will be started at boot whenever a factory reset is
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requested. Services ordered before this target do the work of factory resetting
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the system. Once these services are done,
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[`systemd-factory-reset-complete.service`](https://www.freedesktop.org/software/systemd/man/latest/systemd-factory-reset-complete.service.html)
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unit is invoked after `factory-reset-now.target` and marks the factory reset
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operation as complete. The boot process then may continue.
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* The
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[`systemd-repart`](https://www.freedesktop.org/software/systemd/man/latest/systemd-repart.html)
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tool can take the factory reset logic into account. Either on explicit
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request via the `--factory-reset=` logic, or automatically derived from the
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aforementioned kernel command line switch and EFI variable. When invoked for
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factory reset it will securely erase all partitions marked for that via the
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`FactoryReset=` setting in its partition definition files. Once that is
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complete it will execute the usual setup operation, i.e. format new
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partitions again.
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* The
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[`systemd-logind.service(8)`](https://www.freedesktop.org/software/systemd/man/latest/systemd-logind.service.html)
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unit supports automatically binding factory reset to special keypresses
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(typically long presses), see the
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[`logind.conf(5)`](https://www.freedesktop.org/software/systemd/man/latest/logind.conf.html)
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man page.
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marks the factory reset operation as completed. The boot process may then
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continue.
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* The
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[`systemd-factory-reset`](https://www.freedesktop.org/software/systemd/man/latest/systemd-factory-reset.html)
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@ -93,27 +54,63 @@ Specifically, the following concepts are available:
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mentioned above. This may be used by various early boot services that
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potentially intent to reset system state during a factory reset operation.
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## Exposure in the UI
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* The
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[`systemd-logind.service(8)`](https://www.freedesktop.org/software/systemd/man/latest/systemd-logind.service.html)
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unit supports automatically binding factory reset to special keypresses
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(typically long presses). See the
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[`logind.conf(5)`](https://www.freedesktop.org/software/systemd/man/latest/logind.conf.html)
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man page.
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If a graphical UI shall expose a factory reset operation it should first check
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if requesting a factory reset is supported at all via the Varlink service
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mentioned above. Once a factory reset shall be executed it shall ask for
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activation of the `factory-reset.target` unit.
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## Implementation for UEFI systems
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Alternatively, `systemd-logind.service`'s hotkey support may be used, for
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example to request factory reset if the reboot button is pressed for a long
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time.
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systemd also provides an implementation of this mechanism for UEFI-based systems.
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This implementation can act completely standalone for distributions that rely on
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tools like `systemd-repart`, but it can also be extended to meet other needs.
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The UEFI support works as follows:
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* The
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[`systemd-factory-reset-request.service`](https://www.freedesktop.org/software/systemd/man/latest/systemd-factory-reset-request.service.html)
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unit is invoked via `factory-reset.target`. It requests a factory reset
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operation for the next boot by setting the `FactoryResetRequest` EFI
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variable. The EFI variable contains information about the requesting OS, so
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that multi-boot scenarios are somewhat covered.
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* The
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[`systemd-tpm2-clear.service`](https://www.freedesktop.org/software/systemd/man/latest/systemd-tpm2-clear.service.html)
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unit can request a TPM2 clear operation from the firmware on the next
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boot. It is also invoked via `factory-reset.target`. UEFI firmwares that
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support TPMs will ask the user for confirmation and then reset the TPM,
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invalidating all prior keys associated with the security chip and generating
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a new seed key.
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* The
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[`systemd-repart`](https://www.freedesktop.org/software/systemd/man/latest/systemd-repart.html)
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tool is one of the early-boot services that do the work of factory resetting
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the system. In normal operation, it starts on every boot. When invoked during
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a factory reset, it will erase all partitions marked for that via the
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`FactoryReset=` setting in its partition definition files. Once that is
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complete, it will resume its usual setup operation, i.e. reformatting the
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empty partition with a file system.
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## Support for non-UEFI Systems
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The above is a relatively bespoke solution for EFI systems. It uses EFI
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variables as stateful memory to request the factory reset on the next boot.
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On non-EFI systems, the `FactoryResetRequest` EFI variable cannot be used to
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communicate the factory reset request to the next boot. Instead, a service that
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somehow stores the request should be plugged into `factory-reset.target`. At
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boot, the request should then be fed back into the booted kernel via the
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`systemd.factory_reset=1` kernel command line option.
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On non-EFI systems, a different mechanism should be devised. A service
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requesting the factory request can then be plugged into
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`factory-reset.target`. At boot the request should then be fed back to the
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booted kernel via the `systemd.factory_reset=1` kernel command line option, in
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order to execute the reset operation.
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## Exposure in the UI
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If a graphical UI shall expose a factory reset operation, it should first check
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if requesting a factory reset is supported at all via the Varlink service
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mentioned above. Once the end-user triggers a factory reset, the UI can start
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the process by asking systemd to activate the `factory-reset.target` unit.
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Alternatively, `systemd-logind.service`'s hotkey support may be used. For
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example, it can be configured to request factory reset if the reboot button is
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pressed for a long time.
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## Support for Resetting other Resources than Partitions + TPM
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@ -122,8 +119,8 @@ partitions (via `systemd-repart`, see above) and reset the TPM (via
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`systemd-tpm2-clear.service`, see above).
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In some cases other resources shall be reset/erased too. To support that,
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define your own service and plug it into `factory-reset-now.target`, ensuring
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it is ordered before that.
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define your own service and plug it into `factory-reset-now.target` or the
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Varlink service. Ensure that your service is ordered before the target.
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## Factory Reset via Boot Menu
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