aufs: optional support for FS_USERNS_MOUNT

Still I am not sure how this feature breaches the security. Some people
say it doesn't matter. But I don't know.
Anyway upon the request from the users, aufs implements it as a module
option.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

fs/aufs/module.c

@@ -138,6 +138,11 @@ int sysaufs_brs = 1;
 MODULE_PARM_DESC(brs, "use <sysfs>/fs/aufs/si_*/brN");
 module_param_named(brs, sysaufs_brs, int, 0444);
 
+/* this module parameter has no meaning when USER_NS is disabled */
+bool au_userns;
+MODULE_PARM_DESC(allow_userns, "allow unprivileged to mount under userns");
+module_param_named(allow_userns, au_userns, bool, 0444);
+
 /* ---------------------------------------------------------------------- */
 
 static char au_esc_chars[0x20 + 3]; /* 0x01-0x20, backslash, del, and NULL */
@@ -206,6 +211,7 @@ static int __init aufs_init(void)
 	if (unlikely(err))
 		goto out_sysrq;
 
+	aufs_fs_type.fs_flags |= au_userns ? FS_USERNS_MOUNT : 0;
 	err = register_filesystem(&aufs_fs_type);
 	if (unlikely(err))
 		goto out_cache;

fs/aufs/module.h

@@ -24,6 +24,7 @@ struct seq_file;
 
 /* module parameters */
 extern int sysaufs_brs;
+extern bool au_userns;
 
 /* ---------------------------------------------------------------------- */