[RESEND][PATCH v2] register_console: prevent adding the same console twice

["Andreas Bießmann" <andreas@biessmann.de>]

This patch guards the console_drivers list to be corrupted. The
for_each_console() macro insist on a strictly forward list ended by NULL:

 con0->next->con1->next->NULL

Without this patch it may happen easily to destroy this list for example by
adding 'earlyprintk' twice, especially on embedded devices where the early
console is often a single static instance.  This will result in the following
list:

 con0->next->con0

This in turn will result in an endless loop in console_unlock() later on by
printing the first __log_buf line endlessly.

Signed-off-by: Andreas Bießmann <andreas@biessmann.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Kay Sievers <kay@vrfy.org>
Cc: Ben Hutchings <ben@decadent.org.uk>
Cc: stable@vger.kernel.org
---
since v1:
 * use WARN() as Ben suggested
 * print 'already registered' instead of 'prevent registering'

 kernel/printk/printk.c |    7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 5b5a708..b4e8500 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2226,6 +2226,13 @@ void register_console(struct console *newcon)
 	struct console *bcon = NULL;
 	struct console_cmdline *c;
 
+	if (console_drivers)
+		for_each_console(bcon)
+			if (WARN(bcon == newcon,
+					"console '%s%d' already registered\n",
+					bcon->name, bcon->index))
+				return;
+
 	/*
 	 * before we register a new CON_BOOT console, make sure we don't
 	 * already have a valid console
-- 
1.7.10.4