Add dynamic UNIX socket forwarding

Unconditionally forward a range of ports to sockets named
`$XDG_RUNTIME_DIR/krun/socket/port-$PORT`. This allows applications to
dynamically bind to sockets at those paths and have them forwarded to
the guest as vsock ports, after the krun VM is already running.

The port range is currently hardcoded as 50000..50200.

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

Author: hoshinolina

crates/muvm/src/bin/muvm.rs

@@ -297,6 +297,7 @@ fn main() -> Result<()> {
                 return Err(err).context("Failed to configure vsock for pulse socket");
             }
         }
+
         let hidpipe_path = Path::new(&run_path).join("hidpipe");
         spawn_hidpipe_server(hidpipe_path.clone()).context("Failed to spawn hidpipe thread")?;
         let hidpipe_path = CString::new(
@@ -312,6 +313,25 @@ fn main() -> Result<()> {
             let err = Errno::from_raw_os_error(-err);
             return Err(err).context("Failed to configure vsock for hidpipe socket");
         }
+
+        let socket_dir = Path::new(&run_path).join("krun/socket");
+        std::fs::create_dir_all(&socket_dir)?;
+        // Dynamic ports: Applications may listen on these sockets as neeeded.
+        for port in 50000..50200 {
+            let socket_path = socket_dir.join(format!("port-{}", port));
+            let socket_path = CString::new(
+                socket_path
+                    .to_str()
+                    .expect("socket_path should not contain invalid UTF-8"),
+            )
+            .context("Failed to process dynamic socket path as it contains NUL character")?;
+            // SAFETY: `socket_path` is a pointer to a `CString` with long enough lifetime.
+            let err = unsafe { krun_add_vsock_port(ctx_id, port, socket_path.as_ptr()) };
+            if err < 0 {
+                let err = Errno::from_raw_os_error(-err);
+                return Err(err).context("Failed to configure vsock for dynamic socket");
+            }
+        }
     }
 
     // Forward the native X11 display into the guest as a socket