user-util: add comments explaining what the user name size limits effectively mean

diff --git a/src/basic/user-util.c b/src/basic/user-util.c
index cee91541410fef4e138950ba1466f25bee28bc64..fa5dccfd169a3b110cc5ef6858a189d6b3365f8b 100644 (file)
--- a/src/basic/user-util.c
+++ b/src/basic/user-util.c
@@ -797,11 +797,11 @@ bool valid_user_group_name(const char *u, ValidUserFlags flags) {
                 sz = sysconf(_SC_LOGIN_NAME_MAX);
                 assert_se(sz > 0);
 
-                if (l > (size_t) sz)
+                if (l > (size_t) sz) /* glibc: 256 */
                         return false;
-                if (l > NAME_MAX) /* must fit in a filename */
+                if (l > NAME_MAX) /* must fit in a filename: 255 */
                         return false;
-                if (l > UT_NAMESIZE - 1)
+                if (l > UT_NAMESIZE - 1) /* must fit in utmp: 31 */
                         return false;
         }