brokerd: refuse / and a symbolic link as a socket's directory

brokerd makes a socket's directory 0700. With a socket directly in / it would
chmod /, and through a symbolic link it would change the link's target. Both
are now refused at start with #brokerd-start-failed. Without the fix the link
case started and served, with the shared directory made private.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/brokerd/src/serve.rs

@@ -99,9 +99,18 @@ pub fn start(
 /// Bind one socket: prepare its directory at 0700, make it private, remove any stale
 /// socket, bind, then make the socket itself private at 0600.
 fn listen(socket: &Path) -> Result<UnixListener, ServeError> {
+    // A socket needs a directory of its own: step 2 makes that directory 0700, which must never
+    // be `/` or a shared directory reached through a symbolic link.
     let dir = match socket.parent() {
-        Some(parent) => parent.to_path_buf(),
-        None => PathBuf::from("/"),
+        Some(parent) if parent.parent().is_some() && !parent.as_os_str().is_empty() => {
+            parent.to_path_buf()
+        }
+        _ => {
+            return Err(ServeError::Dir(
+                PathBuf::from(socket),
+                io::Error::other("a socket needs a directory of its own, not / or none"),
+            ));
+        }
     };
 
     // 1. The directory, at 0700.
@@ -110,6 +119,19 @@ fn listen(socket: &Path) -> Result<UnixListener, ServeError> {
         .mode(0o700)
         .create(&dir)
         .map_err(|e| ServeError::Dir(dir.clone(), e))?;
+    // `set_permissions` follows a symbolic link, so a link would make the directory it points to
+    // private instead: refuse it.
+    let kind = std::fs::symlink_metadata(&dir)
+        .map_err(|e| ServeError::Dir(dir.clone(), e))?
+        .file_type();
+    if !kind.is_dir() {
+        return Err(ServeError::Dir(
+            dir,
+            io::Error::other(
+                "is a symbolic link or not a directory; brokerd will not change its mode",
+            ),
+        ));
+    }
     // 2. Make it private, always, even when it was already there at 0755.
     std::fs::set_permissions(&dir, Permissions::from_mode(0o700))
         .map_err(|e| ServeError::Dir(dir.clone(), e))?;