Update tmpfs root inode metadata from tar header.

When reading from a tar file, use the root dir metadata (UID, GID, Mode, MTime)
to update the tmpfs filesystem's root inode. Previously, the root inode was
always pre-populated in the path map without considering the tar header's
metadata.

PiperOrigin-RevId: 824690885

Author: ayushr2

pkg/sentry/fsimpl/tmpfs/tar.go

@@ -50,7 +50,6 @@ func (fs *filesystem) UntarUpperLayer(ctx context.Context, inFile *os.File) erro
 //   - filesystem.mu must be locked.
 func (fs *filesystem) readFromTar(ctx context.Context, tr *tar.Reader) error {
 	pathToInode := map[string]*inode{}
-	pathToInode["./"] = fs.root.inode
 	directoryToHeader := map[string]*tar.Header{}
 	fileToHeader := map[string]*tar.Header{}
 	symlinkToHeader := map[string]*tar.Header{}
@@ -127,6 +126,15 @@ func (fs *filesystem) mkdirFromTar(hdr *tar.Header, pathToInode map[string]*inod
 	if ino, ok := pathToInode[hdr.Name]; ok {
 		return ino, nil
 	}
+	if hdr.Name == "./" {
+		ino := fs.root.inode
+		ino.uid.Store(uint32(hdr.Uid))
+		ino.gid.Store(uint32(hdr.Gid))
+		ino.mode.Store(uint32(hdr.Mode) | linux.S_IFDIR)
+		ino.mtime.Store(hdr.ModTime.UnixNano())
+		pathToInode[hdr.Name] = ino
+		return ino, nil
+	}
 	dir, name := filepath.Split(strings.TrimSuffix(path, "/"))
 	parentInode, ok := pathToInode[dir]
 	if !ok {