user-util: Allow names starting with a digit

In 1a29610f5fa1bcb2eeb37d2c6b79d8d1a6dbb865 the change inadvertedly
disabled names with digit as the first character. This follow-up change
allows a digit as the first character in compat mode.

Fixes: #15141
(cherry picked from commit 93c23c9297e48e594785e0bb9c51504aae5fbe3e)

2 files changed, 19 insertions, 5 deletions

diff --git a/src/basic/user-util.c b/src/basic/user-util.c
index a491f5505e..e998a46e72 100644
--- a/src/basic/user-util.c
+++ b/src/basic/user-util.c
@@ -701,16 +701,18 @@ int take_etc_passwd_lock(const char *root) {
 bool valid_user_group_name_full(const char *u, bool strict) {
         const char *i;
         long sz;
+        bool warned = false;
 
         /* Checks if the specified name is a valid user/group name. Also see POSIX IEEE Std 1003.1-2008, 2016 Edition,
          * 3.437. We are a bit stricter here however. Specifically we deviate from POSIX rules:
          *
          * - We require that names fit into the appropriate utmp field
          * - We don't allow empty user names
-         * - No dots or digits in the first character
+         * - No dots in the first character
          *
          * If strict==true, additionally:
          * - We don't allow any dots (this conflicts with chown syntax which permits dots as user/group name separator)
+         * - We don't allow a digit as the first character
          *
          * Note that other systems are even more restrictive, and don't permit underscores or uppercase characters.
          */
@@ -720,17 +722,26 @@ bool valid_user_group_name_full(const char *u, bool strict) {
 
         if (!(u[0] >= 'a' && u[0] <= 'z') &&
             !(u[0] >= 'A' && u[0] <= 'Z') &&
+            !(u[0] >= '0' && u[0] <= '9' && !strict) &&
             u[0] != '_')
                 return false;
 
-        bool warned = false;
+        bool only_digits_seen = u[0] >= '0' && u[0] <= '9';
+
+        if (only_digits_seen) {
+                log_warning("User or group name \"%s\" starts with a digit, accepting for compatibility.", u);
+                warned = true;
+        }
 
         for (i = u+1; *i; i++) {
                 if (((*i >= 'a' && *i <= 'z') ||
                      (*i >= 'A' && *i <= 'Z') ||
                      (*i >= '0' && *i <= '9') ||
-                     IN_SET(*i, '_', '-')))
+                     IN_SET(*i, '_', '-'))) {
+                        if (!(*i >= '0' && *i <= '9'))
+                                only_digits_seen = false;
                         continue;
+                        }
 
                 if (*i == '.' && !strict) {
                         if (!warned) {
@@ -744,6 +755,9 @@ bool valid_user_group_name_full(const char *u, bool strict) {
                 return false;
         }
 
+        if (only_digits_seen)
+                return false;
+
         sz = sysconf(_SC_LOGIN_NAME_MAX);
         assert_se(sz > 0);
 
diff --git a/src/test/test-user-util.c b/src/test/test-user-util.c
index 084a584876..11c01e5189 100644
--- a/src/test/test-user-util.c
+++ b/src/test/test-user-util.c
@@ -96,7 +96,7 @@ static void test_valid_user_group_name_compat(void) {
         assert_se(valid_user_group_name_compat("eff."));
 
         assert_se(valid_user_group_name_compat("some5"));
-        assert_se(!valid_user_group_name_compat("5some"));
+        assert_se(valid_user_group_name_compat("5some"));
         assert_se(valid_user_group_name_compat("INNER5NUMBER"));
 }
 
@@ -166,7 +166,7 @@ static void test_valid_user_group_name_or_id_compat(void) {
         assert_se(valid_user_group_name_or_id_compat("kk-k"));
 
         assert_se(valid_user_group_name_or_id_compat("some5"));
-        assert_se(!valid_user_group_name_or_id_compat("5some"));
+        assert_se(valid_user_group_name_or_id_compat("5some"));
         assert_se(valid_user_group_name_or_id_compat("INNER5NUMBER"));
 }