Towards safe mount

src/mount-support-test.c

@@ -68,9 +68,33 @@ static void test_get_nextpath__weird()
 	g_assert_cmpstr(result, ==, NULL);
 }
 
+static void test_is_subdir()
+{
+	// Sensible exaples are sensible
+	g_assert_true(is_subdir("/dir/subdir", "/dir/"));
+	g_assert_true(is_subdir("/dir/subdir", "/dir"));
+	g_assert_true(is_subdir("/dir/", "/dir"));
+	g_assert_true(is_subdir("/dir", "/dir"));
+	// Also without leading slash
+	g_assert_true(is_subdir("dir/subdir", "dir/"));
+	g_assert_true(is_subdir("dir/subdir", "dir"));
+	g_assert_true(is_subdir("dir/", "dir"));
+	g_assert_true(is_subdir("dir", "dir"));
+	// Some more ideas
+	g_assert_true(is_subdir("//", "/"));
+	g_assert_true(is_subdir("/", "/"));
+	g_assert_true(is_subdir("", ""));
+	// but this is not true
+	g_assert_false(is_subdir("/", "/dir"));
+	g_assert_false(is_subdir("/rid", "/dir"));
+	g_assert_false(is_subdir("/different/dir", "/dir"));
+	g_assert_false(is_subdir("/", ""));
+}
+
 static void __attribute__ ((constructor)) init()
 {
 	g_test_add_func("/mount/get_nextpath/typical",
 			test_get_nextpath__typical);
 	g_test_add_func("/mount/get_nextpath/weird", test_get_nextpath__weird);
+	g_test_add_func("/mount/is_subdir", test_is_subdir);
 }

src/mount-support.c

@@ -392,3 +392,35 @@ static char * __attribute__ ((used))
 	*offsetp = offset;
 	return (offset < fulllen) ? &path[offset] : NULL;
 }
+
+/**
+ * Check that @subdir is a subdir of @dir.
+**/
+static bool __attribute__ ((used))
+    is_subdir(const char *subdir, const char *dir)
+{
+	size_t dirlen = strlen(dir);
+	size_t subdirlen = strlen(subdir);
+
+	// @dir has to be at least as long as @subdir
+	if (subdirlen < dirlen)
+		return false;
+	// @dir has to be a prefix of @subdir
+	if (strncmp(subdir, dir, dirlen) != 0)
+		return false;
+	// @dir can look like "path/" (that is, end with the directory separator).
+	// When that is the case then given the test above we can be sure @subdir
+	// is a real subdirectory.
+	if (dirlen > 0 && dir[dirlen - 1] == '/')
+		return true;
+	// @subdir can look like "path/stuff" and when the directory separator
+	// is exactly at the spot where @dir ends (that is, it was not caught
+	// by the test above) then @subdir is a real subdirectory.
+	if (subdir[dirlen] == '/' && dirlen > 0)
+	 	return true;
+	// If both @dir and @subdir have identical length then given that the
+	// prefix check above @subdir is a real subdirectory.
+	if (subdirlen == dirlen)
+		return true;
+	return false;
+}