ovl: add support for unique fsid per instance

The legacy behavior of ovl_statfs() reports the f_fsid filled by
underlying upper fs. This fsid is not unique among overlayfs instances
on the same upper fs.

With mount option uuid=on, generate a non-persistent uuid per overlayfs
instance and use it as the seed for f_fsid, similar to tmpfs.

This is useful for reporting fanotify events with fid info from different
instances of overlayfs over the same upper fs.

The old behavior of null uuid and upper fs fsid is retained with the
mount option uuid=null, which is the default.

The mount option uuid=off that disables uuid checks in underlying layers
also retains the legacy behavior.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>

1 files changed, 21 insertions, 4 deletions

diff --git a/fs/overlayfs/params.c b/fs/overlayfs/params.c
index 575a60b76a6c..1ff93467e793 100644
--- a/fs/overlayfs/params.c
+++ b/fs/overlayfs/params.c
@@ -65,6 +65,23 @@ static const struct constant_table ovl_parameter_bool[] = {
 	{}
 };
 
+static const struct constant_table ovl_parameter_uuid[] = {
+	{ "off",	OVL_UUID_OFF  },
+	{ "null",	OVL_UUID_NULL },
+	{ "on",		OVL_UUID_ON   },
+	{}
+};
+
+static const char *ovl_uuid_mode(struct ovl_config *config)
+{
+	return ovl_parameter_uuid[config->uuid].name;
+}
+
+static int ovl_uuid_def(void)
+{
+	return OVL_UUID_NULL;
+}
+
 static const struct constant_table ovl_parameter_xino[] = {
 	{ "off",	OVL_XINO_OFF  },
 	{ "auto",	OVL_XINO_AUTO },
@@ -129,7 +146,7 @@ const struct fs_parameter_spec ovl_parameter_spec[] = {
 	fsparam_flag("default_permissions", Opt_default_permissions),
 	fsparam_enum("redirect_dir",        Opt_redirect_dir, ovl_parameter_redirect_dir),
 	fsparam_enum("index",               Opt_index, ovl_parameter_bool),
-	fsparam_enum("uuid",                Opt_uuid, ovl_parameter_bool),
+	fsparam_enum("uuid",                Opt_uuid, ovl_parameter_uuid),
 	fsparam_enum("nfs_export",          Opt_nfs_export, ovl_parameter_bool),
 	fsparam_flag("userxattr",           Opt_userxattr),
 	fsparam_enum("xino",                Opt_xino, ovl_parameter_xino),
@@ -701,7 +718,7 @@ int ovl_init_fs_context(struct fs_context *fc)
 
 	ofs->config.redirect_mode	= ovl_redirect_mode_def();
 	ofs->config.index		= ovl_index_def;
-	ofs->config.uuid		= true;
+	ofs->config.uuid		= ovl_uuid_def();
 	ofs->config.nfs_export		= ovl_nfs_export_def;
 	ofs->config.xino		= ovl_xino_def();
 	ofs->config.metacopy		= ovl_metacopy_def;
@@ -947,8 +964,8 @@ int ovl_show_options(struct seq_file *m, struct dentry *dentry)
 			   ovl_redirect_mode(&ofs->config));
 	if (ofs->config.index != ovl_index_def)
 		seq_printf(m, ",index=%s", ofs->config.index ? "on" : "off");
-	if (!ofs->config.uuid)
-		seq_puts(m, ",uuid=off");
+	if (ofs->config.uuid != ovl_uuid_def())
+		seq_printf(m, ",uuid=%s", ovl_uuid_mode(&ofs->config));
 	if (ofs->config.nfs_export != ovl_nfs_export_def)
 		seq_printf(m, ",nfs_export=%s", ofs->config.nfs_export ?
 						"on" : "off");