printk: Fix preferred console selection with multiple matches

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 register_console() to scan first for consoles
specified on the command line, and only if none is found, to then
scan for consoles specified by the architecture.

Link: https://lore.kernel.org/r/20200213095133.23176-3-pmladek@suse.com
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/kernel/printk/console_cmdline.h b/kernel/printk/console_cmdline.h
index 11f19c466af5..3ca74ad391d6 100644
--- a/kernel/printk/console_cmdline.h
+++ b/kernel/printk/console_cmdline.h
@@ -6,6 +6,7 @@ struct console_cmdline
 {
 	char	name[16];			/* Name of the driver	    */
 	int	index;				/* Minor dev. to use	    */
+	bool	user_specified;			/* Specified by command line vs. platform */
 	char	*options;			/* Options for the driver   */
 #ifdef CONFIG_A11Y_BRAILLE_CONSOLE
 	char	*brl_options;			/* Options for braille driver */