um: kmsg_dump: only dump when no output console available

The initial intention of the UML kmsg_dumper is to dump the kernel
buffers to stdout if there is no console available to perform the
regular crash output.

However, if ttynull was registered as a console, no crash output was
seen. Commit e23fe90dec ("um: kmsg_dumper: always dump when not tty
console") tried to fix this by performing the kmsg_dump unless the
stdio console was behind /dev/console or enabled. But this allowed
kmsg dumping to occur even if other non-stdio consoles will output
the crash output. Also, a console being the driver behind
/dev/console has nothing to do with a crash scenario.

Restore the initial intention by dumping the kernel buffers to stdout
only if a non-ttynull console is registered and enabled. Also add
detailed comments so that it is clear why these rules are applied.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20221116162152.193147-8-john.ogness@linutronix.de

arch/um/kernel/kmsg_dump.c

@@ -17,13 +17,22 @@ static void kmsg_dumper_stdout(struct kmsg_dumper *dumper,
 	unsigned long flags;
 	size_t len = 0;
 
-	/* only dump kmsg when no console is available */
+	/*
+	 * If no consoles are available to output crash information, dump
+	 * the kmsg buffer to stdout.
+	 */
+
 	if (!console_trylock())
 		return;
 
 	for_each_console(con) {
-		if(strcmp(con->name, "tty") == 0 &&
-		   (con->flags & (CON_ENABLED | CON_CONSDEV)) != 0) {
+		/*
+		 * The ttynull console and disabled consoles are ignored
+		 * since they cannot output. All other consoles are
+		 * expected to output the crash information.
+		 */
+		if (strcmp(con->name, "ttynull") != 0 &&
+		    (con->flags & CON_ENABLED)) {
 			break;
 		}
 	}