[RFC/PATCH] printk: Fix preferred console selection with multiple matches

[Benjamin Herrenschmidt]

In the following circumstances, the rule of selecting the console
corresponding to the last "console=" entry on the command line as
the preferred console (CON_CONSDEV, ie, /dev/console) fails. This
is a specific example, but it could happen with different consoles
that have a similar name aliasing mechanism.

 - The kernel command line has both console=tty0 and console=ttyS0
in that order (the latter with speed etc... arguments). This is common
with some cloud setups such as Amazon Linux.

 - add_preferred_console is called early to register "uart0". In
our case that happens from acpi_parse_spcr() on arm64 since the
"enable_console" argument is true on that architecture. This causes
"uart0" to become entry 0 of the console_cmdline array.

Now, because of the above, what happens is:

 - add_preferred_console is called by the cmdline parsing for tty0
and ttyS0 respectively, thus occupying entries 1 and 2 of the
console_cmdline array (since this happens after ACPI SPCR parsing).
At that point preferred_console is set to 2 as expected.

 - When the tty layer kicks in, it will call register_console for tty0.
This will match entry 1 in console_cmdline array. It isn't our preferred
console but because it's our only console at this point, it will end up
"first" in the consoles list.

 - When 8250 probes the actual serial port later on, it calls
register_console for ttyS0. At that point the loop in register_console
tries to match it with the entries in the console_cmdline array. Ideally
this should match ttyS0 in entry 2, which is preferred, causing it to
be inserted first and to replace tty0 as CONSDEV. However, 8250 provides
a "match" hook in its struct console, and that hook will match "uart"
as an alias to "ttyS". So we match uart0 at entry 0 in the array which
is not the preferred console and will not match entry 2 which is since
we break out of the loop on the first match. As a result, we don't set
CONSDEV and don't insert it first, but second in the console list.

As a result, we end up with tty0 remaining first in the array, and thus
/dev/console going there instead of the last user specified one which
is ttyS0.

This tentative fix changes the loop in register_console to continue
matching with the array until the match is actually the preferred
console.

Signed-off-by: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
---
 kernel/printk/printk.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 5aa96098c64d..d36b9901c0e0 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2646,8 +2646,8 @@ void register_console(struct console *newcon)
 		if (i == preferred_console) {
 			newcon->flags |= CON_CONSDEV;
 			has_preferred = true;
+			break;
 		}
-		break;
 	}
 
 	if (!(newcon->flags & CON_ENABLED))
-- 
2.17.1