log: never log into foreign fd #2 in PID 1 or its pre-execve() children

Fixes: #5401

src/basic/log.c

@@ -72,6 +72,7 @@ static bool show_color = false;
 static bool show_location = false;
 
 static bool upgrade_syslog_to_journal = false;
+static bool always_reopen_console = false;
 
 /* Akin to glibc's __abort_msg; which is private and we hence cannot
  * use here. */
@@ -95,7 +96,7 @@ static int log_open_console(void) {
         if (console_fd >= 0)
                 return 0;
 
-        if (getpid() == 1) {
+        if (always_reopen_console) {
                 console_fd = open_terminal("/dev/console", O_WRONLY|O_NOCTTY|O_CLOEXEC);
                 if (console_fd < 0)
                         return console_fd;
@@ -1171,3 +1172,7 @@ int log_syntax_internal(
                         unit_fmt, unit,
                         NULL);
 }
+
+void log_set_always_reopen_console(bool b) {
+        always_reopen_console = b;
+}

src/basic/log.h

@@ -220,6 +220,7 @@ LogTarget log_target_from_string(const char *s) _pure_;
 void log_received_signal(int level, const struct signalfd_siginfo *si);
 
 void log_set_upgrade_syslog_to_journal(bool b);
+void log_set_always_reopen_console(bool b);
 
 int log_syntax_internal(
                 const char *unit,

src/core/main.c

@@ -1414,10 +1414,17 @@ int main(int argc, char *argv[]) {
 
         log_set_upgrade_syslog_to_journal(true);
 
-        /* Disable the umask logic */
+        if (getpid() == 1) {
-        if (getpid() == 1)
+                /* Disable the umask logic */
                 umask(0);
 
+                /* Always reopen /dev/console when running as PID 1 or one of its pre-execve() children. This is
+                 * important so that we never end up logging to any foreign stderr, for example if we have to log in a
+                 * child process right before execve()'ing the actual binary, at a point in time where socket
+                 * activation stderr/stdout area already set up. */
+                log_set_always_reopen_console(true);
+        }
+
         if (getpid() == 1 && detect_container() <= 0) {
 
                 /* Running outside of a container as PID 1 */