Add a flag to run a custom init instead of muvm-guest

Allow the user to replace muvm-guest with a custom init process. With
updated libkrun, it will even run as PID 1, making it possible to run
systemd.

Of course, this is not the suggested way to use muvm, but some use cases
necesitate the use of systemd. The next few commits will facilitate
running individual parts of muvm-guest as separate processes under a
custom service manager.

Signed-off-by: Val Packett <[email protected]>

Author: valpackett

crates/muvm/src/bin/muvm.rs

@@ -429,13 +429,21 @@ fn main() -> Result<ExitCode> {
         .to_str()
         .context("Temporary directory path contains invalid UTF-8")?
         .to_owned();
-    let muvm_guest_args = vec![
-        muvm_guest_path
-            .to_str()
-            .context("Failed to process `muvm-guest` path as it contains invalid UTF-8")?
-            .to_owned(),
-        muvm_config_path,
-    ];
+    let custom_init = options.custom_init_cmdline.is_some();
+    let muvm_guest_args = if let Some(cmdline) = options.custom_init_cmdline {
+        cmdline
+            .split_whitespace()
+            .map(|a| a.to_owned())
+            .collect::<Vec<String>>()
+    } else {
+        vec![
+            muvm_guest_path
+                .to_str()
+                .context("Failed to process `muvm-guest` path as it contains invalid UTF-8")?
+                .to_owned(),
+            muvm_config_path,
+        ]
+    };
 
     // And forward XAUTHORITY. This will be modified to fix the
     // display name in muvm-guest.
@@ -470,7 +478,11 @@ fn main() -> Result<ExitCode> {
 
     let krun_config_env = CString::new(format!("KRUN_CONFIG={}", config_file.path().display()))
         .context("Failed to process config_file var as it contains NUL character")?;
-    let env: Vec<*const c_char> = vec![krun_config_env.as_ptr(), std::ptr::null()];
+    let mut env: Vec<*const c_char> = vec![krun_config_env.as_ptr()];
+    if custom_init {
+        env.push(c"KRUN_INIT_PID1=1".as_ptr());
+    }
+    env.push(std::ptr::null());
 
     {
         // Sets environment variables to be configured in the context of the executable.

crates/muvm/src/cli_options.rs

@@ -47,6 +47,7 @@ pub struct Options {
     pub emulator: Option<Emulator>,
     pub init_commands: Vec<PathBuf>,
     pub user_init_commands: Vec<PathBuf>,
+    pub custom_init_cmdline: Option<String>,
     pub command: PathBuf,
     pub command_args: Vec<String>,
 }
@@ -179,6 +180,13 @@ pub fn options() -> OptionParser<Options> {
         )
         .argument("COMMAND")
         .many();
+    let custom_init_cmdline = long("custom-init-cmdline")
+        .help(
+            "Command and arguments to run as PID 1, replacing muvm's own init.
+            (Warning: this will break many muvm features, unless your init reimplements them.)",
+        )
+        .argument("CMDLINE")
+        .optional();
     let command = positional("COMMAND").help("the command you want to execute in the vm");
     let command_args = any::<String, _, _>("COMMAND_ARGS", |arg| {
         (!["--help", "-h"].contains(&&*arg)).then_some(arg)
@@ -202,6 +210,7 @@ pub fn options() -> OptionParser<Options> {
         emulator,
         init_commands,
         user_init_commands,
+        custom_init_cmdline,
         // positionals
         command,
         command_args,