fix(jailer): honor solitary --parent-cgroup option

If we specify `--parent-cgroup` without any `--cgroup` option, the
jailer do anything.

We could just error in that situation, and expect something else to
launch the jailer process in a cgroup, but we risk breaking anybody that
is already using it that way.

Instead, we move the process to the `--parent-cgroup` since it's the
most intuitive, although the specified `--parent-cgroup` is not a parent
in that case.

Link: firecracker-microvm#4287

Signed-off-by: Pablo Barbáchano <pablob@amazon.com>

CHANGELOG.md

@@ -48,6 +48,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- [#4309](https://github.com/firecracker-microvm/firecracker/pull/4309): The
+  jailer's option `--parent-cgroup` will move the process to that cgroup if no
+  `cgroup` options are provided.
 - Simplified and clarified the removal policy of deprecated API elements
   to follow semantic versioning 2.0.0. For more information, please refer to
   [this GitHub discussion](https://github.com/firecracker-microvm/firecracker/discussions/4135).

src/jailer/src/cgroup.rs

@@ -161,6 +161,16 @@ impl CgroupBuilder {
             }
         }
     }
+
+    // Returns the path to the root of the hierarchy
+    pub fn get_v2_hierarchy_path(&mut self) -> Result<&PathBuf, JailerError> {
+        match self.hierarchies.entry("unified".to_string()) {
+            Occupied(entry) => Ok(entry.into_mut()),
+            Vacant(_entry) => Err(JailerError::CgroupHierarchyMissing(
+                "cgroupsv2 hierarchy missing".to_string(),
+            )),
+        }
+    }
 }
 
 #[derive(Debug)]

src/jailer/src/env.rs

@@ -233,11 +233,23 @@ impl Env {
             .parse::<u8>()
             .map_err(|_| JailerError::CgroupInvalidVersion(cgroup_ver.to_string()))?;
 
-        let mut cgroup_builder = None;
+        let cgroups_args: &[String] = arguments.multiple_values("cgroup").unwrap_or_default();
+
+        // If the --parent-cgroup exists, and we have no other cgroups,
+        // then the intent is to move the process to that cgroup.
+        // Only applies to cgroupsv2 since it's a unified hierarchy
+        if cgroups_args.len() == 0 && cgroup_ver == 2 {
+            let mut builder = CgroupBuilder::new(cgroup_ver)?;
+            let cg_parent = builder.get_v2_hierarchy_path()?.join(parent_cgroup);
+            let cg_parent_procs = cg_parent.join("cgroup.procs");
+            if cg_parent.exists() {
+                fs::write(cg_parent_procs, std::process::id().to_string());
+            }
+        }
 
         // cgroup format: <cgroup_controller>.<cgroup_property>=<value>,...
         if let Some(cgroups_args) = arguments.multiple_values("cgroup") {
-            let builder = cgroup_builder.get_or_insert(CgroupBuilder::new(cgroup_ver)?);
+            let mut builder = CgroupBuilder::new(cgroup_ver)?;
             for cg in cgroups_args {
                 let aux: Vec<&str> = cg.split('=').collect();
                 if aux.len() != 2 || aux[1].is_empty() {