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>

diff --git a/kernel/printk/console_cmdline.h b/kernel/printk/console_cmdline.h
index 11f19c4..3ca74ad 100644 (file)
--- 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 */

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 0ebcdf5..f76ef3f 100644 (file)
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2116,7 +2116,7 @@ asmlinkage __visible void early_printk(const char *fmt, ...)
 #endif
 
 static int __add_preferred_console(char *name, int idx, char *options,
-                                  char *brl_options)
+                                  char *brl_options, bool user_specified)
 {
        struct console_cmdline *c;
        int i;
@@ -2131,6 +2131,8 @@ static int __add_preferred_console(char *name, int idx, char *options,
                if (strcmp(c->name, name) == 0 && c->index == idx) {
                        if (!brl_options)
                                preferred_console = i;
+                       if (user_specified)
+                               c->user_specified = true;
                        return 0;
                }
        }
@@ -2140,6 +2142,7 @@ static int __add_preferred_console(char *name, int idx, char *options,
                preferred_console = i;
        strlcpy(c->name, name, sizeof(c->name));
        c->options = options;
+       c->user_specified = user_specified;
        braille_set_options(c, brl_options);
 
        c->index = idx;
@@ -2194,7 +2197,7 @@ static int __init console_setup(char *str)
        idx = simple_strtoul(s, NULL, 10);
        *s = 0;
 
-       __add_preferred_console(buf, idx, options, brl_options);
+       __add_preferred_console(buf, idx, options, brl_options, true);
        console_set_on_cmdline = 1;
        return 1;
 }
@@ -2215,7 +2218,7 @@ __setup("console=", console_setup);
  */
 int add_preferred_console(char *name, int idx, char *options)
 {
-       return __add_preferred_console(name, idx, options, NULL);
+       return __add_preferred_console(name, idx, options, NULL, false);
 }
 
 bool console_suspend_enabled = true;
@@ -2636,7 +2639,7 @@ early_param("keep_bootcon", keep_bootcon_setup);
  * Care need to be taken with consoles that are statically
  * enabled such as netconsole
  */
-static int try_enable_new_console(struct console *newcon)
+static int try_enable_new_console(struct console *newcon, bool user_specified)
 {
        struct console_cmdline *c;
        int i;
@@ -2644,6 +2647,8 @@ static int try_enable_new_console(struct console *newcon)
        for (i = 0, c = console_cmdline;
             i < MAX_CMDLINECONSOLES && c->name[0];
             i++, c++) {
+               if (c->user_specified != user_specified)
+                       continue;
                if (!newcon->match ||
                    newcon->match(newcon, c->name, c->index, c->options) != 0) {
                        /* default matching */
@@ -2673,9 +2678,10 @@ static int try_enable_new_console(struct console *newcon)
 
        /*
         * Some consoles, such as pstore and netconsole, can be enabled even
-        * without matching.
+        * without matching. Accept the pre-enabled consoles only when match()
+        * and setup() had a change to be called.
         */
-       if (newcon->flags & CON_ENABLED)
+       if (newcon->flags & CON_ENABLED && c->user_specified == user_specified)
                return 0;
 
        return -ENOENT;
@@ -2752,11 +2758,12 @@ void register_console(struct console *newcon)
                }
        }
 
-       /*
-        * See if this console matches one we selected on
-        * the command line or if it was statically enabled
-        */
-       err = try_enable_new_console(newcon);
+       /* See if this console matches one we selected on the command line */
+       err = try_enable_new_console(newcon, true);
+
+       /* If not, try to match against the platform default(s) */
+       if (err == -ENOENT)
+               err = try_enable_new_console(newcon, false);
 
        /* printk() messages are not printed to the Braille console. */
        if (err || newcon->flags & CON_BRL)