cmd/snap-update-ns: detect and report read-only filesystems

This small patch adds detection of read-only filesystems to
secure-mkdir-all. This will be soon picked up by the hole-poking code.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

cmd/snap-update-ns/utils.go

@@ -47,6 +47,15 @@ var (
 	sysFchown  = syscall.Fchown
 )
 
+// ReadOnlyFsError is an error encapsulating encountered EROFS.
+type ReadOnlyFsError struct {
+	Path string
+}
+
+func (e *ReadOnlyFsError) Error() string {
+	return fmt.Sprintf("cannot operate on read-only filesystem at %s", e.Path)
+}
+
 // Create directories for all but the last segments and return the file
 // descriptor to the leaf directory. This function is a base for secure
 // variants of mkdir, touch and symlink.
@@ -102,6 +111,12 @@ func secureMkDir(fd int, segments []string, i int, perm os.FileMode, uid, gid in
 		switch err {
 		case syscall.EEXIST:
 			made = false
+		case syscall.EROFS:
+			// Treat EROFS specially: this is a hint that we have to poke a
+			// hole using tmpfs. The path below is the location where we
+			// need to poke the hole.
+			p := "/" + strings.Join(segments[:i], "/")
+			return -1, &ReadOnlyFsError{Path: p}
 		default:
 			return -1, fmt.Errorf("cannot mkdir path segment %q: %v", segment, err)
 		}

cmd/snap-update-ns/utils_test.go

@@ -115,6 +115,23 @@ func (s *utilsSuite) TestSecureMkdirAllLevel3(c *C) {
 	})
 }
 
+// Ensure that we can detect read only filesystems.
+func (s *utilsSuite) TestSecureMkdirAllROFS(c *C) {
+	s.sys.InsertFault(`mkdirat 3 "rofs" 0755`, syscall.EEXIST) // just realistic
+	s.sys.InsertFault(`mkdirat 4 "path" 0755`, syscall.EROFS)
+	err := update.SecureMkdirAll("/rofs/path", 0755, 123, 456)
+	c.Assert(err, ErrorMatches, `cannot operate on read-only filesystem at /rofs`)
+	c.Assert(err.(*update.ReadOnlyFsError).Path, Equals, "/rofs")
+	c.Assert(s.sys.Calls(), DeepEquals, []string{
+		`open "/" O_NOFOLLOW|O_CLOEXEC|O_DIRECTORY 0`,        // -> 3
+		`mkdirat 3 "rofs" 0755`,                              // -> EEXIST
+		`openat 3 "rofs" O_NOFOLLOW|O_CLOEXEC|O_DIRECTORY 0`, // -> 4
+		`close 3`,
+		`mkdirat 4 "path" 0755`, // -> EROFS
+		`close 4`,
+	})
+}
+
 // Ensure that we don't chown existing directories.
 func (s *utilsSuite) TestSecureMkdirAllExistingDirsDontChown(c *C) {
 	s.sys.InsertFault(`mkdirat 3 "abs" 0755`, syscall.EEXIST)