muvm-guest: Support root and user init scripts

Introduce two scripts that can be placed in ~/.local/share/muvm

- init.sh: Executed as root, before dropping privileges
- user.sh: Executed as the user, after dropping privileges

If the files are also present in lower-priority XDG dirs (systemwide),
those are executed first.

This makes it possible to add custom VM configuration and setup or
debugging scripts without having to hack on the muvm code.

Signed-off-by: Asahi Lina <[email protected]>

crates/muvm/Cargo.toml

@@ -23,6 +23,7 @@ tempfile = { version = "3.10.1", default-features = false, features = [] }
 tokio = { version = "1.38.0", default-features = false, features = ["io-util", "macros", "net", "process", "rt-multi-thread", "sync"] }
 tokio-stream = { version = "0.1.15", default-features = false, features = ["net", "sync"] }
 uuid = { version = "1.10.0", default-features = false, features = ["std", "v7"] }
+xdg = "2.5.2"
 
 [features]
 default = []

crates/muvm/src/guest/bin/muvm-guest.rs

@@ -1,4 +1,5 @@
 use std::cmp;
+use std::env;
 use std::os::unix::process::CommandExt as _;
 use std::process::Command;
 
@@ -13,6 +14,7 @@ use muvm::guest::sommelier::exec_sommelier;
 use muvm::guest::user::setup_user;
 use muvm::guest::x11::setup_x11_forwarding;
 use muvm::utils::env::find_in_path;
+use nix::unistd::User;
 use rustix::process::{getrlimit, setrlimit, Resource};
 
 fn main() -> Result<()> {
@@ -40,6 +42,18 @@ fn main() -> Result<()> {
     }
     Command::new("/usr/lib/systemd/systemd-udevd").spawn()?;
 
+    let user = User::from_uid(options.uid)?.unwrap();
+
+    // Hack to fetch the XDG directories for our target user, before we
+    // change uid.
+    // SAFETY: Safe if and only if `muvm-guest` program is not multithreaded.
+    // See https://doc.rust-lang.org/std/env/fn.set_var.html#safety
+    env::set_var("HOME", user.dir);
+    let xdg_dirs = xdg::BaseDirectories::with_prefix("muvm")?;
+    // SAFETY: Safe if and only if `muvm-guest` program is not multithreaded.
+    // See https://doc.rust-lang.org/std/env/fn.set_var.html#safety
+    env::remove_var("HOME");
+
     setup_fex()?;
 
     configure_network()?;
@@ -50,6 +64,14 @@ fn main() -> Result<()> {
             .spawn()?;
     }
 
+    for init_file in xdg_dirs.find_data_files("init.sh") {
+        Command::new(init_file)
+            .arg(format!("{}", options.uid))
+            .arg(format!("{}", options.gid))
+            .status()
+            .context("Failed to execute init.sh")?;
+    }
+
     let run_path = match setup_user(options.username, options.uid, options.gid) {
         Ok(p) => p,
         Err(err) => return Err(err).context("Failed to set up user, bailing out"),
@@ -63,6 +85,13 @@ fn main() -> Result<()> {
 
     setup_x11_forwarding(run_path)?;
 
+    for init_file in xdg_dirs.find_data_files("user.sh") {
+        eprintln!("{:?}", init_file);
+        Command::new(init_file)
+            .status()
+            .context("Failed to execute user.sh")?;
+    }
+
     // Will not return if successful.
     exec_sommelier(&options.command, &options.command_args)
         .context("Failed to execute sommelier")?;