printk: don't prefer unsuited consoles on registration

console election: If some console happens to be registered first which does
not provide a tty binding (!console->device), it prevents that more suited
consoles which are registered later on can enter the candidate pool for
console_device().  This is observable with KGDB's console which may already
be registered (and exploited!) during early debugger connections, that is
before any regular console registration.

This patch fixes the issue by postponing the final, automated
preferred_console selection until someone with a non-NULL device handler
comes around.

Signed-off-by: Jan Kiszka <jan.kiszka@web.de>
Cc: Jason Wessel <jason.wessel@windriver.com>
Cc: Gerd Hoffmann <kraxel@suse.de>
Cc: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/printk.c

@@ -1157,8 +1157,11 @@ void register_console(struct console *console)
 			console->index = 0;
 		if (console->setup == NULL ||
 		    console->setup(console, NULL) == 0) {
-			console->flags |= CON_ENABLED | CON_CONSDEV;
-			preferred_console = 0;
+			console->flags |= CON_ENABLED;
+			if (console->device) {
+				console->flags |= CON_CONSDEV;
+				preferred_console = 0;
+			}
 		}
 	}