Add assesment docs

content/en/docs/Architecture/trustedboot.md

@@ -78,12 +78,17 @@ The keys are used to sign the UKI file, and to generate a PCR policy keypair req
 
 Check the relevant documentation on how to [extend the system with system extensions]({{%relref "/docs/advanced/sys-extensions" %}})
 
+### Trusted Boot - Boot Assessment
+
+See the [Trusted Boot - Boot Assessment]({{%relref "/docs/examples/boot_assessment_trusted_boot" %}}) documentation for more information on how to enable automatic boot assessment with Trusted Boot and how to make your own services participate in the boot assessment process.
+
 ### Considerations
 
 #### Booting command lines
 
 UKI file's signatures are including also the kernel command line, so any change to the kernel command line will require a new UKI file to be generated and the installer image to be rebuilt. This implies that you cannot change the booting options once the system is installed (and the system won't be able to access the encrypted data)
 
+
 ### References
 
 - [UEFI specification](https://uefi.org/sites/default/files/resources/UEFI_Spec_2_8_final.pdf)

content/en/docs/Examples/boot_assessment_trusted_boot.md

@@ -0,0 +1,118 @@
+---
+title: "Enabling Automatic Boot Assessment with Trusted Boot"
+linkTitle: "Enabling Automatic Boot Assessment with Trusted Boot"
+weight: 4
+description: This section describe examples on how to enable Automatic Boot Assessment with Trusted Boot in your own services.
+---
+
+
+# Enabling Automatic Boot Assessment with Trusted Boot
+
+In this tutorial, we will walk through how to configure Kairos to enable **automatic boot assessment**, where the boot loader can determine the health of a boot entry. Specifically, we'll configure systemd services to trigger the `boot-complete.target` to mark boot entries as *good* or *bad*. We'll also cover how to implement automatic reboots when a service fails, allowing retries of a boot entry until success or exhaustion of attempts.
+
+---
+
+
+## Overview of Trusted Boot Automatic Boot Assessment
+
+`systemd-boot` manages boot entries and provides a mechanism to automatically assess their success or failure. The key features are:
+
+1. **Boot Entry Marking**:
+    - A boot entry is marked as *good* when the `boot-complete.target` is reached during startup.
+    - If the system fails to reach `boot-complete.target`, the boot entry is marked as *bad*.
+
+2. **Retries**:
+    - By default, each boot entry has **3 retries**. A failure to reach `boot-complete.target` reduces the retry count. Once retries are exhausted, another boot entry is chosen if available.
+
+3. **Service-Level Controls**:
+    - Configure services to participate in boot entry assessment and trigger reboots on failure.
+
+---
+
+## Step 1: Configuring a Service to Trigger `boot-complete.target`
+
+To ensure a service's success or failure impacts the boot assessment, modify its service file to interact with `boot-complete.target`:
+
+1. **Edit the Service File**:  
+
+   Override the service configuration using:
+   ```bash
+   sudo systemctl edit <service-name>
+    ```
+
+2. **Add Dependencies and Order to the Service File**:
+
+   Append the following to the override file:
+    ```bash
+    [Unit]
+    # Ensure this unit starts after default system targets
+    After=default.target graphical.target multi-user.target
+    # Ensure this unit completes before boot-complete.target
+    Before=boot-complete.target
+
+    [Install]
+    # Make this service a hard dependency of boot-complete.target
+    RequiredBy=boot-complete.target
+    ```
+
+3. **Reload Systemd and Enable the Service:**:
+
+    ```bash
+    sudo systemctl daemon-reload
+    sudo systemctl enable <service-name>
+    ```
+
+4. **Explanation**:
+
+  - The service runs after critical system targets (e.g., default.target) to ensure the system is operational.
+  - The service must complete successfully to allow boot-complete.target to be reached.
+  - If the service fails, the boot entry is not marked as good.
+
+
+## Step 2: Adding Automatic Reboot to a Service
+
+To configure a service to automatically reboot the system upon failure:
+
+1.  Edit the Service File:
+
+    Override the service configuration using:
+    ```bash
+    sudo systemctl edit <service-name>
+    ```
+2. Add the Reboot Action:
+
+   In the [Unit] section, add:
+    ```bash
+    [Unit]
+    FailureAction=reboot
+    ```
+3. Reload Systemd:
+    ```bash
+    sudo systemctl daemon-reload
+    ```
+4. Explanation:
+   - On failure, FailureAction=reboot instructs systemd to reboot the system.
+   - This causes the boot entry to retry until success or retries are exhausted.
+
+
+## Step 3: Combining Both Approaches
+
+While the above configurations are independent, combining them can create a robust system:
+
+1. Trigger boot-complete.target:
+   Configure services as described in Step 1 to impact boot assessment.
+
+2. Enable Automatic Reboot:
+   Add `FailureAction=reboot` to relevant services as described in Step 2.
+
+3. Behavior:
+  - On a service failure, the system reboots (`FailureAction=reboot`).
+  - During the retry, if boot-complete.target is not reached, the boot entry is not marked as good, and retries continue.
+  - If retries are exhausted, the system attempts the next available boot entry.
+
+## Notes
+
+ - Services are started on both passive and active boot entries. So if a service is failing on active, and the failure is not due to the OS, it will also fail on passive. This can lead to the system rebooting on passive boot entries as well as active and end in the system booting to recovery.
+ - We recommend using this feature with caution, as it can lead to a boot loop if not configured correctly.
+ - Ideally, as the upgrade is done against the active images, we would recommend having 2 services, one for the active and one for the passive, to avoid the system rebooting on passive boot entries and having a safe fallback to the active boot entry. This can be achieved by using the `ConditionPathExists` directive in the service file to check if the service is running on the active or passive boot entry (marked by the files `/run/cos/active_mode` and `/run/cos/passive_mode`) so the service that auto reboots can be started only on the active boot entry.
+