kata-types: add emptydir_mode configuration option

Add the emptydir_mode field to the Runtime configuration struct,
allowing runtime-rs to read the emptyDir handling mode from the
TOML config file. This is groundwork for trusted ephemeral data
storage support in runtime-rs (parity with the Go runtime).

Two modes are supported:
  - shared-fs (default): share emptyDir via virtio-fs/9p.
  - block-encrypted: plug a block device encrypted in-guest via
    CDH/LUKS2.

Empty values default to "shared-fs"; unknown values are rejected
during validation.

Signed-off-by: Fabiano Fidêncio <ffidencio@nvidia.com>
Assisted-by: Cursor <cursoragent@cursor.com>

src/libs/kata-types/src/config/runtime.rs

@@ -18,6 +18,12 @@ pub use shared_mount::SharedMount;
 /// Type of runtime VirtContainer.
 pub const RUNTIME_NAME_VIRTCONTAINER: &str = "virt_container";
 
+/// EmptyDir mode: share the emptyDir folder with the guest using shared-fs.
+pub const EMPTYDIR_MODE_SHARED_FS: &str = "shared-fs";
+
+/// EmptyDir mode: plug a block device to be encrypted in the guest.
+pub const EMPTYDIR_MODE_BLOCK_ENCRYPTED: &str = "block-encrypted";
+
 /// Kata runtime configuration information.
 #[derive(Debug, Default, Deserialize, Serialize)]
 pub struct Runtime {
@@ -143,6 +149,15 @@ pub struct Runtime {
     #[serde(default)]
     pub disable_guest_empty_dir: bool,
 
+    /// Specifies how Kubernetes emptyDir volumes are handled.
+    ///
+    /// Options:
+    /// - shared-fs (default): shares the emptyDir folder with the guest using the method
+    ///   given by the shared_fs setting.
+    /// - block-encrypted: plugs a block device to be encrypted in the guest via CDH/LUKS2.
+    #[serde(default)]
+    pub emptydir_mode: String,
+
     /// Determines how VFIO devices should be be presented to the container.
     ///
     /// Options:
@@ -222,6 +237,9 @@ impl ConfigOps for Runtime {
         if conf.runtime.internetworking_model.is_empty() {
             conf.runtime.internetworking_model = default::DEFAULT_INTERNETWORKING_MODEL.to_owned();
         }
+        if conf.runtime.emptydir_mode.is_empty() {
+            conf.runtime.emptydir_mode = EMPTYDIR_MODE_SHARED_FS.to_owned();
+        }
 
         for bind in conf.runtime.sandbox_bind_mounts.iter_mut() {
             // Split the bind mount, canonicalize the path and then append rw mode to it.
@@ -262,6 +280,15 @@ impl ConfigOps for Runtime {
             )));
         }
 
+        let emptydir_mode = &conf.runtime.emptydir_mode;
+        if emptydir_mode != EMPTYDIR_MODE_SHARED_FS
+            && emptydir_mode != EMPTYDIR_MODE_BLOCK_ENCRYPTED
+        {
+            return Err(std::io::Error::other(format!(
+                "Invalid emptydir_mode `{emptydir_mode}` in configuration file",
+            )));
+        }
+
         for shared_mount in &conf.runtime.shared_mounts {
             shared_mount.validate()?;
         }
@@ -360,6 +387,45 @@ vfio_mode = "guest_kernel"
         config.validate().unwrap_err();
     }
 
+    #[test]
+    fn test_invalid_emptydir_mode() {
+        let content = r#"
+[runtime]
+emptydir_mode = "invalid-value"
+"#;
+        let config: TomlConfig = TomlConfig::load(content).unwrap();
+        config.validate().unwrap_err();
+    }
+
+    #[test]
+    fn test_valid_emptydir_mode() {
+        let content = r#"
+[runtime]
+emptydir_mode = "shared-fs"
+"#;
+        let config: TomlConfig = TomlConfig::load(content).unwrap();
+        config.validate().unwrap();
+        assert_eq!(&config.runtime.emptydir_mode, "shared-fs");
+
+        let content = r#"
+[runtime]
+emptydir_mode = "block-encrypted"
+"#;
+        let config: TomlConfig = TomlConfig::load(content).unwrap();
+        config.validate().unwrap();
+        assert_eq!(&config.runtime.emptydir_mode, "block-encrypted");
+    }
+
+    #[test]
+    fn test_default_emptydir_mode() {
+        let content = r#"
+[runtime]
+"#;
+        let config: TomlConfig = TomlConfig::load(content).unwrap();
+        config.validate().unwrap();
+        assert_eq!(&config.runtime.emptydir_mode, "shared-fs");
+    }
+
     #[test]
     fn test_config() {
         let content = r#"