user-sessions: do not remove /etc/nologin

pam_nologin looks for /etc/nologin and /run/nologin.
user-sessions creates (and removes) /run/nologin, but also removes
/etc/nologin. (This behaviour is unchanged since the introduction
of the binary in e92787416c691c3f34f47349e5eae3fa68eae856.)

By not removing pam_nologin we fully drop compatibility with PAM < 1.1.
This has the advantage that now /etc/nologin can be used by administrator to
disable user logins, e.g. for extended maintanance. We already specified
PAM >= 1.1.2 as dependency, so this was already covered.

The makes the code match the man page.

Fixes #26965.

2 files changed, 14 insertions, 11 deletions

diff --git a/src/shared/fileio-label.c b/src/shared/fileio-label.c
index d03b0548ec..572b8f62ad 100644
--- a/src/shared/fileio-label.c
+++ b/src/shared/fileio-label.c
@@ -23,9 +23,15 @@ int write_string_file_atomic_label_ts(const char *fn, const char *line, struct t
 int create_shutdown_run_nologin_or_warn(void) {
         int r;
 
-        /* This is used twice: once in systemd-user-sessions.service, in order to block logins when we actually go
-         * down, and once in systemd-logind.service when shutdowns are scheduled, and logins are to be turned off a bit
-         * in advance. We use the same wording of the message in both cases. */
+        /* This is used twice: once in systemd-user-sessions.service, in order to block logins when we
+         * actually go down, and once in systemd-logind.service when shutdowns are scheduled, and logins are
+         * to be turned off a bit in advance. We use the same wording of the message in both cases.
+         *
+         * Traditionally, there was only /etc/nologin, and we managed that. Then, in PAM 1.1
+         * support for /run/nologin was added as alternative
+         * (https://github.com/linux-pam/linux-pam/commit/e9e593f6ddeaf975b7fe8446d184e6bc387d450b).
+         * 13 years later we stopped managing /etc/nologin, leaving it for the administrator to manage.
+         */
 
         r = write_string_file_atomic_label("/run/nologin",
                                            "System is going down. Unprivileged users are not permitted to log in anymore. "
diff --git a/src/user-sessions/user-sessions.c b/src/user-sessions/user-sessions.c
index 6b7493fd88..37867ee3ed 100644
--- a/src/user-sessions/user-sessions.c
+++ b/src/user-sessions/user-sessions.c
@@ -15,7 +15,7 @@
 #include "string-util.h"
 
 static int run(int argc, char *argv[]) {
-        int r, k;
+        int r;
 
         if (argc != 2)
                 return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
@@ -29,14 +29,11 @@ static int run(int argc, char *argv[]) {
         if (r < 0)
                 return r;
 
-        if (streq(argv[1], "start")) {
-                r = unlink_or_warn("/run/nologin");
-                k = unlink_or_warn("/etc/nologin");
-                if (r < 0)
-                        return r;
-                return k;
+        /* We only touch /run/nologin. See create_shutdown_run_nologin_or_warn() for details. */
 
-        } else if (streq(argv[1], "stop"))
+        if (streq(argv[1], "start"))
+                return unlink_or_warn("/run/nologin");
+        if (streq(argv[1], "stop"))
                 return create_shutdown_run_nologin_or_warn();
 
         return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Unknown verb '%s'.", argv[1]);