tetragon: Add docs for the persistent enforcement

Adding docs for the persistent enforcement.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>

docs/content/en/docs/concepts/persistent-enforcement.md

@@ -0,0 +1,89 @@
+---
+title: "Persistent enforcement"
+weight: 2
+description: "How to configure persistent enforcement"
+---
+
+This page shows you how to configure persistent enforcement.
+
+## Concept
+
+The idea of persistent enforcement is to allow he enforcement policy to continue
+running even when its tetragon process is gone.
+
+This is configured with `--keep-sensors-on-exit` option.
+
+When the tetragon process exits the policy stays active, because it's pinned
+in sysfs bpf tree under `/sys/fs/bpf/tetragon` directory.
+
+When new tetragon process is started it performs following actions:
+
+- checks if there's existing `/sys/fs/bpf/tetragon` and moves it to `/sys/fs/bpf/tetragon_old` directory
+- setups configured policy
+- removes `/sys/fs/bpf/tetragon_old` directory.
+
+## Example
+
+This example shows how the persistent enforcement works on simple tracing policy.
+
+- Consider following enforcement tracing policy that kills any process that touches `/tmp/tetragon` file.
+
+```yaml
+apiVersion: cilium.io/v1alpha1
+kind: TracingPolicy
+metadata:
+  name: "enforcement"
+spec:
+  kprobes:
+  - call: "fd_install"
+    syscall: false
+    args:
+    - index: 0
+      type: int
+    - index: 1
+      type: "file"
+    selectors:
+    - matchArgs:
+      - index: 1
+        operator: "Equal"
+        values:
+        - "/tmp/tetragon"
+      matchActions:
+      - action: Sigkill
+```
+
+- Spawn tetragon with above policy and `--keep-sensors-on-exit` option.
+
+```
+# tetragon ... --keep-sensors-on-exit
+```
+
+- Verify the enforcement policy is in place.
+
+```
+$ cat /tmp/tetragon
+Killed
+```
+
+- Kill tetragon
+
+```
+time="2024-07-26T14:47:45Z" level=info msg="Perf ring buffer size (bytes)" percpu=68K total=272K
+time="2024-07-26T14:47:45Z" level=info msg="Perf ring buffer events queue size (events)" size=63K
+time="2024-07-26T14:47:45Z" level=info msg="Listening for events..."
+^C
+time="2024-07-26T14:50:50Z" level=info msg="Received signal interrupt, shutting down..."
+time="2024-07-26T14:50:50Z" level=info msg="Listening for events completed." error="context canceled"
+```
+
+- Verify the enforcement policy is **STILL** in place.
+
+```
+$ cat /tmp/tetragon
+Killed
+```
+
+##  Limitations
+
+At the moment we are not able to receive any events during the tetragon down time,
+it's just the enforcement that's in place.