[PATCH] don't prefer unsuited consoles on registration

[Jan Kiszka]

KGDB triggers a (probably so far theoretical) issue of the automatic
init 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@xxxxxx>

---
 kernel/printk.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

Index: b/kernel/printk.c
===================================================================
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -1126,8 +1126,11 @@ void register_console(struct console *co
 			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;
+			}
 		}
 	}
 
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