Merge pull request #5285 from lifubang/followup-5275-maskpath

libct: Clean up and refactor maskPaths logic

libcontainer/rootfs_linux.go

@@ -419,7 +419,7 @@ func mountCgroupV2(m mountEntry, c *mountConfig) error {
 		// Mask `/sys/fs/cgroup` to ensure it is read-only, even when `/sys` is mounted
 		// with `rbind,ro` (`runc spec --rootless` produces `rbind,ro` for `/sys`).
 		err = utils.WithProcfdFile(m.dstFile, func(procfd string) error {
-			return maskPaths(c.root, []string{procfd}, c.label)
+			return maskDir(procfd, c.label)
 		})
 	}
 	return err
@@ -1328,12 +1328,20 @@ func verifyDevNull(f *os.File) error {
 	})
 }
 
+// maskDir mounts a read-only tmpfs on top of the specified path.
+func maskDir(path, mountLabel string) error {
+	return mount("tmpfs", path, "tmpfs", unix.MS_RDONLY, label.FormatMountLabel("nr_blocks=1,nr_inodes=1", mountLabel))
+}
+
 // maskPaths masks the top of the specified paths inside a container to avoid
 // security issues from processes reading information from non-namespace aware
 // mounts ( proc/kcore ).
 // For files, maskPath bind mounts /dev/null over the top of the specified path.
 // For directories, maskPath mounts read-only tmpfs over the top of the specified path.
-func maskPaths(rootFd *os.File, paths []string, mountLabel string) error {
+func maskPaths(rootFs string, paths []string, mountLabel string) error {
+	if len(paths) == 0 {
+		return nil
+	}
 	devNull, err := os.OpenFile("/dev/null", unix.O_PATH, 0)
 	if err != nil {
 		return fmt.Errorf("can't mask paths: %w", err)
@@ -1394,13 +1402,18 @@ func maskPaths(rootFd *os.File, paths []string, mountLabel string) error {
 				}
 			}
 			if bindFailed || sharedMaskSrc == nil {
-				err = mount("tmpfs", path, "tmpfs", unix.MS_RDONLY, label.FormatMountLabel("nr_blocks=1,nr_inodes=1", mountLabel))
+				err = maskDir(path, mountLabel)
 				if err == nil && !bindFailed && sharedMaskSrc == nil {
 					// Establish this mount as the reusable shared source. reopenAfterMount
 					// resolves the underlying inode via procfs and re-opens it through
 					// rootFd, so the resulting fd is anchored to the real path inside the
 					// container rootfs even if path was a /proc/self/fd/N alias.
+					rootFd, err := os.OpenFile(rootFs, unix.O_DIRECTORY|unix.O_CLOEXEC|unix.O_PATH, 0)
+					if err != nil {
+						return fmt.Errorf("open rootfs handle for masked paths: %w", err)
+					}
 					reopened, err := reopenAfterMount(rootFd, dstFh, unix.O_PATH|unix.O_CLOEXEC)
+					rootFd.Close()
 					if err != nil {
 						return fmt.Errorf("can't reopen shared directory mask: %w", err)
 					}

libcontainer/standard_init_linux.go

@@ -142,17 +142,10 @@ func (l *linuxStandardInit) Init() error {
 		}
 	}
 
-	if len(l.config.Config.MaskPaths) > 0 {
-		rootFd, err := os.OpenFile("/", unix.O_DIRECTORY|unix.O_CLOEXEC|unix.O_PATH, 0)
-		if err != nil {
-			return fmt.Errorf("open rootfs handle for masked paths: %w", err)
-		}
-		err = maskPaths(rootFd, l.config.Config.MaskPaths, l.config.Config.MountLabel)
-		rootFd.Close()
-		if err != nil {
-			return err
-		}
+	if err := maskPaths("/", l.config.Config.MaskPaths, l.config.Config.MountLabel); err != nil {
+		return err
 	}
+
 	pdeath, err := system.GetParentDeathSignal()
 	if err != nil {
 		return fmt.Errorf("can't get pdeath signal: %w", err)