kata-types: emit agent.debug_console_shell on the guest cmdline

Add a debug_console_shell field to the [agent] config and, when it is set,
inject agent.debug_console_shell=<path> onto the guest kernel cmdline next to
the existing debug_console options. This is how the runtime-rs shims tell the
guest agent which shell the debug console should exec (the agent added support
for the option and its extension-prefix guard in a previous commit).

An empty value is not emitted, so default configurations are unchanged.

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

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

@@ -93,6 +93,12 @@ pub struct Agent {
     #[serde(default)]
     pub debug_console_enabled: bool,
 
+    /// Guest path of the shell the debug console execs; empty uses the agent's
+    /// built-in candidates (/bin/bash, /bin/sh). The agent only honors it if it
+    /// resolves under /run/kata-extensions/ (a guest extension mount).
+    #[serde(default)]
+    pub debug_console_shell: String,
+
     /// When enabled, the agent translates a container's VISIBLE_CDI_DEVICES
     /// environment variable into CDI GPU device requests (nvidia.com/gpu) so
     /// that the container sees the matching GPUs present in the VM.
@@ -186,6 +192,7 @@ impl std::default::Default for Agent {
             log_level: "info".to_string(),
             enable_tracing: false,
             debug_console_enabled: false,
+            debug_console_shell: String::new(),
             visible_cdi_devices: false,
             server_port: DEFAULT_AGENT_VSOCK_PORT,
             log_port: DEFAULT_AGENT_LOG_PORT,

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

@@ -53,6 +53,8 @@ pub const LOG_LEVEL_OPTION: &str = "agent.log";
 pub const LOG_LEVEL_DEBUG: &str = "debug";
 /// Option of which port will the debug console connect to
 pub const DEBUG_CONSOLE_VPORT_OPTION: &str = "agent.debug_console_vport";
+/// Option selecting the guest shell the debug console execs
+pub const DEBUG_CONSOLE_SHELL_OPTION: &str = "agent.debug_console_shell";
 /// Option of which port the agent's log will connect to
 pub const LOG_VPORT_OPTION: &str = "agent.log_vport";
 /// Option of setting the container's pipe size
@@ -248,6 +250,12 @@ impl TomlConfig {
                     DEFAULT_AGENT_DBG_CONSOLE_PORT.to_string(),
                 );
             }
+            if !cfg.debug_console_shell.is_empty() {
+                kv.insert(
+                    DEBUG_CONSOLE_SHELL_OPTION.to_string(),
+                    cfg.debug_console_shell.clone(),
+                );
+            }
             if cfg.visible_cdi_devices {
                 kv.insert(VISIBLE_CDI_DEVICES_OPTION.to_string(), "true".to_string());
             }
@@ -504,6 +512,7 @@ mod tests {
             enable_tracing: true,
             container_pipe_size: 20,
             debug_console_enabled: true,
+            debug_console_shell: "/run/kata-extensions/devkit/bin/devkit-sh".to_string(),
             launch_process_timeout: 60,
             visible_cdi_devices: true,
             ..Default::default()
@@ -518,7 +527,29 @@ mod tests {
         assert_eq!(kv.get("agent.container_pipe_size").unwrap(), "20");
         kv.get("agent.debug_console").unwrap();
         assert_eq!(kv.get("agent.debug_console_vport").unwrap(), "1026"); // 1026 is the default port
+        assert_eq!(
+            kv.get("agent.debug_console_shell").unwrap(),
+            "/run/kata-extensions/devkit/bin/devkit-sh"
+        );
         assert_eq!(kv.get("agent.launch_process_timeout").unwrap(), "60");
         assert_eq!(kv.get("agent.visible_cdi_devices").unwrap(), "true");
     }
+
+    #[test]
+    fn test_get_agent_kernel_params_no_debug_console_shell() {
+        let mut config = TomlConfig {
+            ..Default::default()
+        };
+        let agent_config = Agent {
+            debug_console_enabled: true,
+            ..Default::default()
+        };
+        let agent_name = "test_agent";
+        config.runtime.agent_name = agent_name.to_string();
+        config.agent.insert(agent_name.to_owned(), agent_config);
+
+        // An empty debug_console_shell must not be emitted on the cmdline.
+        let kv = config.get_agent_kernel_params().unwrap();
+        assert!(kv.get("agent.debug_console_shell").is_none());
+    }
 }