Support mount bind a file

kernel/src/fs/path/mod.rs

@@ -267,6 +267,18 @@ impl Path {
     /// Returns `EINVAL` if either source or destination path is not in the
     /// current mount namespace.
     pub fn bind_mount_to(&self, dst_path: &Self, recursive: bool, ctx: &Context) -> Result<()> {
+        let can_bind = {
+            let src_is_dir = self.type_() == InodeType::Dir;
+            let dst_is_dir = dst_path.type_() == InodeType::Dir;
+            (src_is_dir && dst_is_dir) || (!src_is_dir && !dst_is_dir)
+        };
+        if !can_bind {
+            return_errno_with_message!(
+                Errno::ENOTDIR,
+                "the source and destination must both be directories or both be non-directories"
+            );
+        }
+
         let current_ns_proxy = ctx.thread_local.borrow_ns_proxy();
         let current_mnt_ns = current_ns_proxy.unwrap().mnt_ns();
         if !current_mnt_ns.owns(&self.mount) {

kernel/src/fs/path/mount.rs

@@ -277,10 +277,6 @@ impl Mount {
 
     /// Grafts the mount node tree to the mountpoint.
     pub(super) fn graft_mount_tree(&self, target_path: &Path) -> Result<()> {
-        if target_path.type_() != InodeType::Dir {
-            return_errno!(Errno::ENOTDIR);
-        }
-
         self.detach_from_parent();
         self.attach_to_path(target_path);
         Ok(())

kernel/src/syscall/mount.rs

@@ -88,10 +88,6 @@ fn do_bind_mount(src_name: CString, dst_path: Path, recursive: bool, ctx: &Conte
             .lookup(&fs_path)?
     };
 
-    if src_path.type_() != InodeType::Dir {
-        return_errno_with_message!(Errno::ENOTDIR, "src_name must be directory");
-    };
-
     src_path.bind_mount_to(&dst_path, recursive, ctx)?;
     Ok(())
 }