Pass through the current working directory

Aside from just being better UX, some commands are sensitive to it
and will not work correctly without it.

Co-authored-by: Nikodem Rabuliński <nikodem@rabulinski.com>
Signed-off-by: Nikodem Rabuliński <nikodem@rabulinski.com>

Author: WhatAmISupposedToPutHere

crates/muvm/src/bin/muvm.rs

@@ -89,11 +89,13 @@ fn main() -> Result<ExitCode> {
         unreachable!("`launch_vm` never returns");
     }
 
+    let cwd = env::current_dir()?;
     let inherit_env = !options.no_inherit_env;
-    let (lock_file, command, command_args, env) = match launch_or_lock(
+    let (lock_file, command, command_args, env, cwd) = match launch_or_lock(
         options.command,
         options.command_args,
         options.env,
+        cwd,
         options.no_tty,
         options.privileged,
         inherit_env,
@@ -108,7 +110,8 @@ fn main() -> Result<ExitCode> {
             command,
             command_args,
             env,
-        } => (lock_file, command, command_args, env),
+            cwd,
+        } => (lock_file, command, command_args, env, cwd),
     };
 
     // Make it lose CLOEXEC
@@ -121,6 +124,7 @@ fn main() -> Result<ExitCode> {
         command,
         command_args,
         env,
+        cwd,
         options.no_tty,
         options.privileged,
         inherit_env,

crates/muvm/src/guest/server_worker.rs

@@ -200,8 +200,9 @@ async fn handle_connection(mut stream: BufStream<UnixStream>) -> Result<ConnRequ
         vsock_port,
         tty,
         privileged,
+        cwd,
     } = read_request(&mut stream).await?;
-    debug!(command:?, command_args:?, env:?; "received launch request");
+    debug!(command:?, command_args:?, env:?, cwd:?; "received launch request");
 
     if command == Path::new("/muvmdropcaches") {
         // SAFETY: everything below should be async signal safe
@@ -274,7 +275,19 @@ async fn handle_connection(mut stream: BufStream<UnixStream>) -> Result<ConnRequ
         .envs(envs)
         .stdin(stdin)
         .stdout(stdout)
-        .stderr(stderr);
+        .stderr(stderr)
+        // The documentation says that if `command` is a relative path,
+        // it's ambiguous whether it should be interpreted relative
+        // to the parent's working directory or relative to `current_dir`.[0]
+        // Luckily for us, on UNIX in case of `Command::spawn`,
+        // it's implemented by calling `posix_spawn_file_actions_add_chdir`.[1]
+        // This is exactly what we want, as the user may want to run `muvm ./foo`
+        // in which case we need `./foo` to be interpreted relative to `current_dir`,
+        // not relative to cwd of the server worker.
+        //
+        // [0]: https://doc.rust-lang.org/stable/std/process/struct.Command.html#platform-specific-behavior-1
+        // [1]: https://github.com/rust-lang/rust/blob/cb50d4d8566b1ee97e9a5ef95a37a40936a62c30/library/std/src/sys/pal/unix/process/process_unix.rs#L705-L707
+        .current_dir(cwd);
     if tty {
         unsafe {
             cmd.pre_exec(|| {

crates/muvm/src/launch.rs

@@ -28,6 +28,7 @@ pub enum LaunchResult {
         command: PathBuf,
         command_args: Vec<String>,
         env: Vec<(String, Option<String>)>,
+        cwd: PathBuf,
     },
 }
 
@@ -89,6 +90,7 @@ fn wrapped_launch(
     command: PathBuf,
     command_args: Vec<String>,
     env: HashMap<String, String>,
+    cwd: PathBuf,
     no_tty: bool,
     privileged: bool,
 ) -> Result<ExitCode> {
@@ -100,7 +102,7 @@ fn wrapped_launch(
     _ = unlink(&path);
     let listener = UnixListener::bind(path).context("Failed to listen on vm socket")?;
     let tty = !no_tty && stdout_is_tty();
-    request_launch(command, command_args, env, vsock_port, tty, privileged)?;
+    request_launch(command, command_args, env, cwd, vsock_port, tty, privileged)?;
     let raw_tty = tty.then(|| RawTerminal::set().expect("Stdout should be a tty"));
     let code = run_io_host(listener, tty)?;
     drop(raw_tty);
@@ -111,6 +113,7 @@ pub fn launch_or_lock(
     command: PathBuf,
     command_args: Vec<String>,
     env: Vec<(String, Option<String>)>,
+    cwd: PathBuf,
     no_tty: bool,
     privileged: bool,
     inherit_env: bool,
@@ -122,6 +125,7 @@ pub fn launch_or_lock(
             command,
             command_args,
             env,
+            cwd,
         }),
         None => {
             let env = prepare_env_vars(env, inherit_env)?;
@@ -131,6 +135,7 @@ pub fn launch_or_lock(
                     command.clone(),
                     command_args.clone(),
                     env.clone(),
+                    cwd.clone(),
                     no_tty,
                     privileged,
                 ) {
@@ -182,6 +187,7 @@ pub fn request_launch(
     command: PathBuf,
     command_args: Vec<String>,
     env: HashMap<String, String>,
+    cwd: PathBuf,
     vsock_port: u32,
     tty: bool,
     privileged: bool,
@@ -195,6 +201,7 @@ pub fn request_launch(
         command,
         command_args,
         env,
+        cwd,
         vsock_port,
         tty,
         privileged,

crates/muvm/src/monitor.rs

@@ -1,4 +1,5 @@
 use std::collections::HashMap;
+use std::env::current_dir;
 use std::path::PathBuf;
 use std::thread;
 use std::time;
@@ -43,7 +44,8 @@ fn set_guest_pressure(pressure: GuestPressure) -> Result<()> {
         let command = PathBuf::from("/muvmdropcaches");
         let command_args = vec![];
         let env = HashMap::new();
-        request_launch(command, command_args, env, 0, false, true)?;
+        let cwd = current_dir()?;
+        request_launch(command, command_args, env, cwd, 0, false, true)?;
     }
 
     let wsf: u32 = pressure.into();
@@ -52,7 +54,8 @@ fn set_guest_pressure(pressure: GuestPressure) -> Result<()> {
     let command = PathBuf::from("/sbin/sysctl");
     let command_args = vec![format!("vm.watermark_scale_factor={}", wsf)];
     let env = HashMap::new();
-    request_launch(command, command_args, env, 0, false, true)
+    let cwd = current_dir()?;
+    request_launch(command, command_args, env, cwd, 0, false, true)
 }
 
 fn run() {

crates/muvm/src/utils/launch.rs

@@ -12,6 +12,7 @@ pub struct Launch {
     pub vsock_port: u32,
     pub tty: bool,
     pub privileged: bool,
+    pub cwd: PathBuf,
 }
 
 #[derive(Clone, Eq, PartialEq, Debug, Deserialize, Serialize)]