If an invalid option value is used with "printk.devkmsg=<value>",
it is silently ignored.
If a valid option value is used, it is honored but the wrong return
value (0) is used, indicating that the command line option had an
error and was not handled. This string is not added to init's
environment strings due to init/main.c::unknown_bootoption()
checking for a '.' in the boot option string and then considering
that string to be an "Unused module parameter".

Print a warning message if a bad option string is used.
Always return 1 from the __setup handler to indicate that the command
line option has been handled.

Fixes: 750afe7babd1 ("printk: add kernel parameter to control writes to /dev/kmsg")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: Igor Zhbanov <i.zhbanov@omprussia.ru>
Link: lore.kernel.org/r/64644a2f-4a20-bab3-1e15-3b2cdd0defe3@omprussia.ru
Cc: Borislav Petkov <bp@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: John Ogness <john.ogness@linutronix.de>
---
 kernel/printk/printk.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- linux-next-20220228.orig/kernel/printk/printk.c
+++ linux-next-20220228/kernel/printk/printk.c
@@ -152,8 +152,10 @@ static int __control_devkmsg(char *str)
 
 static int __init control_devkmsg(char *str)
 {
-	if (__control_devkmsg(str) < 0)
+	if (__control_devkmsg(str) < 0) {
+		pr_warn("printk.devkmsg: bad option string '%s'\n", str);
 		return 1;
+	}
 
 	/*
 	 * Set sysctl string accordingly:
@@ -172,7 +174,7 @@ static int __init control_devkmsg(char *
 	 */
 	devkmsg_log |= DEVKMSG_LOG_MASK_LOCK;
 
-	return 0;
+	return 1;
 }
 __setup("printk.devkmsg=", control_devkmsg);

On 2022-02-28, Randy Dunlap <rdunlap@infradead.org> wrote:
> If an invalid option value is used with "printk.devkmsg=<value>",
> it is silently ignored.
> If a valid option value is used, it is honored but the wrong return
> value (0) is used, indicating that the command line option had an
> error and was not handled. This string is not added to init's
> environment strings due to init/main.c::unknown_bootoption()
> checking for a '.' in the boot option string and then considering
> that string to be an "Unused module parameter".
>
> Print a warning message if a bad option string is used.
> Always return 1 from the __setup handler to indicate that the command
> line option has been handled.
>
> Fixes: 750afe7babd1 ("printk: add kernel parameter to control writes to /dev/kmsg")
> Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
> Reported-by: Igor Zhbanov <i.zhbanov@omprussia.ru>

Reviewed-by: John Ogness <john.ogness@linutronix.de>

> Link: lore.kernel.org/r/64644a2f-4a20-bab3-1e15-3b2cdd0defe3@omprussia.ru

The message at this link is very helpful in explaining the state of
declaring kernel parameters. Hopefully someday someone will document
and/or comment this stuff.

John Ogness

On 3/1/22 04:58, John Ogness wrote:
> On 2022-02-28, Randy Dunlap <rdunlap@infradead.org> wrote:
>> If an invalid option value is used with "printk.devkmsg=<value>",
>> it is silently ignored.
>> If a valid option value is used, it is honored but the wrong return
>> value (0) is used, indicating that the command line option had an
>> error and was not handled. This string is not added to init's
>> environment strings due to init/main.c::unknown_bootoption()
>> checking for a '.' in the boot option string and then considering
>> that string to be an "Unused module parameter".
>>
>> Print a warning message if a bad option string is used.
>> Always return 1 from the __setup handler to indicate that the command
>> line option has been handled.
>>
>> Fixes: 750afe7babd1 ("printk: add kernel parameter to control writes to /dev/kmsg")
>> Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
>> Reported-by: Igor Zhbanov <i.zhbanov@omprussia.ru>
> 
> Reviewed-by: John Ogness <john.ogness@linutronix.de>
> 
>> Link: lore.kernel.org/r/64644a2f-4a20-bab3-1e15-3b2cdd0defe3@omprussia.ru
> 
> The message at this link is very helpful in explaining the state of
> declaring kernel parameters. Hopefully someday someone will document
> and/or comment this stuff.

I have added some docs to include/linux/init.h.
Andrew has it in his mmotm queue.

-- 
~Randy

On Mon 2022-02-28 14:05:56, Randy Dunlap wrote:
> If an invalid option value is used with "printk.devkmsg=<value>",
> it is silently ignored.
> If a valid option value is used, it is honored but the wrong return
> value (0) is used, indicating that the command line option had an
> error and was not handled. This string is not added to init's
> environment strings due to init/main.c::unknown_bootoption()
> checking for a '.' in the boot option string and then considering
> that string to be an "Unused module parameter".
> 
> Print a warning message if a bad option string is used.
> Always return 1 from the __setup handler to indicate that the command
> line option has been handled.
> 
> Fixes: 750afe7babd1 ("printk: add kernel parameter to control writes to /dev/kmsg")
> Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
> Reported-by: Igor Zhbanov <i.zhbanov@omprussia.ru>
> Link: lore.kernel.org/r/64644a2f-4a20-bab3-1e15-3b2cdd0defe3@omprussia.ru
> Cc: Borislav Petkov <bp@suse.de>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Petr Mladek <pmladek@suse.com>
> Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: John Ogness <john.ogness@linutronix.de>

The patch has been committed into printk/linux.git, branch for-5.18.

Best Regards,
Petr

On (22/02/28 14:05), Randy Dunlap wrote:
> If an invalid option value is used with "printk.devkmsg=<value>",
> it is silently ignored.
> If a valid option value is used, it is honored but the wrong return
> value (0) is used, indicating that the command line option had an
> error and was not handled. This string is not added to init's
> environment strings due to init/main.c::unknown_bootoption()
> checking for a '.' in the boot option string and then considering
> that string to be an "Unused module parameter".
> 
> Print a warning message if a bad option string is used.
> Always return 1 from the __setup handler to indicate that the command
> line option has been handled.
> 
> Fixes: 750afe7babd1 ("printk: add kernel parameter to control writes to /dev/kmsg")
> Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
> Reported-by: Igor Zhbanov <i.zhbanov@omprussia.ru>
> Link: lore.kernel.org/r/64644a2f-4a20-bab3-1e15-3b2cdd0defe3@omprussia.ru
> Cc: Borislav Petkov <bp@suse.de>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Petr Mladek <pmladek@suse.com>
> Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: John Ogness <john.ogness@linutronix.de>

Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>

On Mon 2022-02-28 14:05:56, Randy Dunlap wrote:
> If an invalid option value is used with "printk.devkmsg=<value>",
> it is silently ignored.
> If a valid option value is used, it is honored but the wrong return
> value (0) is used, indicating that the command line option had an
> error and was not handled. This string is not added to init's
> environment strings due to init/main.c::unknown_bootoption()
> checking for a '.' in the boot option string and then considering
> that string to be an "Unused module parameter".
> 
> Print a warning message if a bad option string is used.
> Always return 1 from the __setup handler to indicate that the command
> line option has been handled.
> 
> Fixes: 750afe7babd1 ("printk: add kernel parameter to control writes to /dev/kmsg")
> Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
> Reported-by: Igor Zhbanov <i.zhbanov@omprussia.ru>
> Link: lore.kernel.org/r/64644a2f-4a20-bab3-1e15-3b2cdd0defe3@omprussia.ru

Good catch! I am learning something new every day :-)

Reviewed-by: Petr Mladek <pmladek@suse.com>

Best Regards,
Petr