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Subject[PATCH v4 2/3] printk: Fix preferred console selection with multiple matches
Date
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>

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.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---
kernel/printk/console_cmdline.h | 1 +
kernel/printk/printk.c | 29 ++++++++++++++++++-----------
2 files changed, 19 insertions(+), 11 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 */
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 0ebcdf53e75d..f76ef3f0efca 100644
--- 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)
--
2.16.4
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