exports: Add recursion depth limit to flatpak_exports_path_get_mode

A symlink loop on the host filesystem would cause infinite recursion
and a stack overflow. Limit to 40 levels, matching the kernel's ELOOP
limit and the existing check in _exports_path_expose.

common/flatpak-exports.c

@@ -667,10 +667,10 @@ flatpak_exports_append_bwrap_args (FlatpakExports *exports,
     flatpak_bwrap_add_args (bwrap, "--ro-bind", "/usr/lib/os-release", "/run/host/os-release", NULL);
 }
 
-/* Returns FLATPAK_FILESYSTEM_MODE_NONE if not visible */
-FlatpakFilesystemMode
-flatpak_exports_path_get_mode (FlatpakExports *exports,
-                               const char     *path)
+static FlatpakFilesystemMode
+flatpak_exports_path_get_mode_full (FlatpakExports *exports,
+                                    const char     *path,
+                                    int             level)
 {
   guint n_keys;
   g_autofree const char **keys = (const char **) g_hash_table_get_keys_as_array (exports->hash, &n_keys);
@@ -681,6 +681,9 @@ flatpak_exports_path_get_mode (FlatpakExports *exports,
   g_autoptr(GString) path_builder = g_string_new ("");
   struct stat st;
 
+  if (level > 40) /* 40 is the current kernel ELOOP check */
+    return FLATPAK_FILESYSTEM_MODE_NONE;
+
   qsort (keys, n_keys, sizeof (char *), flatpak_strcmp0_ptr);
 
   /* Syntactic canonicalization only, no need to use host_fd */
@@ -739,7 +742,7 @@ flatpak_exports_path_get_mode (FlatpakExports *exports,
                   g_string_append (path2_builder, parts[j]);
                 }
 
-              return flatpak_exports_path_get_mode (exports, path2_builder->str);
+              return flatpak_exports_path_get_mode_full (exports, path2_builder->str, level + 1);
             }
         }
       else if (parts[i + 1] == NULL)
@@ -752,6 +755,14 @@ flatpak_exports_path_get_mode (FlatpakExports *exports,
   return FLATPAK_FILESYSTEM_MODE_READ_WRITE;
 }
 
+/* Returns FLATPAK_FILESYSTEM_MODE_NONE if not visible */
+FlatpakFilesystemMode
+flatpak_exports_path_get_mode (FlatpakExports *exports,
+                               const char     *path)
+{
+  return flatpak_exports_path_get_mode_full (exports, path, 0);
+}
+
 gboolean
 flatpak_exports_path_is_visible (FlatpakExports *exports,
                                  const char     *path)