printk/console: Allow to disable console output by using console="" or console=null
Subsystems (2)
Petr Mladek
maintainerPATCH 1/1 printk/console: Allow to disable console output by using console="" or console=null
The commit 48021f98130880dd74 ("printk: handle blank console arguments passed in.") prevented crash caused by empty console= parameter value.
Unfortunately, this value is widely used on Chromebooks to disable the console output. The above commit caused performance regression because the messages were pushed on slow console even though nobody was watching it.
Use ttynull driver explicitly for console="" and console=null parameters. It has been created for exactly this purpose.
It causes that preferred_console is set. As a result, ttySX and ttyX are not used as a fallback. And only ttynull console gets registered by default.
It still allows to register other consoles either by additional console= parameters or SPCR. It prevents regression because it worked this way even before. Also it is a sane semantic. Preventing output on all consoles should be done another way, for example, by introducing mute_console parameter.
---
This is backport of the commit 3cffa06aeef7ece30f6b5ac0e ("printk/console: Allow to disable console output by using console="" or console=null") for stable release:
+ 4.4, 4.9, 4.14, 4.19, 5.4
Please, use the original upstream commit for stable release:
+ 5.10
It should fix the problem reported at
https://www.spinics.net/lists/stable/msg509616.html
kernel/printk/printk.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)
kernel/printk/printk.c +8 -1
@@ -2032,8 +2032,15 @@ static int __init console_setup(char *str)char *s, *options, *brl_options =NULL ;int idx; -if (str[0 ] ==0 ) +/* +* console="" or console=null have been suggested as a way to +* disable console output. Use ttynull that has been created +* for exacly this purpose. +*/ +if (str[0 ] ==0 ||strcmp (str,"null" ) ==0 ) { +__add_preferred_console ("ttynull" ,0 ,NULL ,NULL );return 1 ; + }if (_braille_console_setup (&str, &brl_options))return 1 ;
Signature · 1 line
2.26.2
Quoted 34 lines · Petr Mladek
The commit
48021f98130880dd74("printk: handle blank console arguments passed in.") prevented crash caused by empty console= parameter value.Unfortunately, this value is widely used on Chromebooks to disable the console output. The above commit caused performance regression because the messages were pushed on slow console even though nobody was watching it.
Use ttynull driver explicitly for console="" and console=null parameters. It has been created for exactly this purpose.
It causes that
preferred_consoleis set. As a result, ttySX and ttyX are not used as a fallback. And only ttynull console gets registered by default.It still allows to register other consoles either by additional console= parameters or SPCR. It prevents regression because it worked this way even before. Also it is a sane semantic. Preventing output on all consoles should be done another way, for example, by introducing
mute_consoleparameter.Suggested-by Sergey Senozhatsky <[email protected]>Reviewed-by Guenter Roeck <[email protected]>Tested-by Guenter Roeck <[email protected]>Signed-off-by Petr Mladek <[email protected]>---
This is backport of the commit
3cffa06aeef7ece30f6b5ac0e("printk/console: Allow to disable console output by using console="" or console=null") for stable release:
Quoted 8 lines
+ 4.4, 4.9, 4.14, 4.19, 5.4
Please, use the original upstream commit for stable release:
+ 5.10
It should fix the problem reported at
https://www.spinics.net/lists/stable/msg509616.html
Thanks, now all queued up!
greg k-h
Quoted 8 lines · Petr Mladek
The commit
48021f98130880dd74("printk: handle blank console arguments passed in.") prevented crash caused by empty console= parameter value.Unfortunately, this value is widely used on Chromebooks to disable the console output. The above commit caused performance regression because the messages were pushed on slow console even though nobody was watching it.
We actually had to revert this patch on Chromebooks, so we'll have to revert it again from stable releases after it gets there. The problem is two-fold:
First, it is used in Chromebooks to disable the default console in production images; that default console may be set in a devicetree file, and this patch doesn't really disable it. In other words, Chromebooks use "console=" to implement "mute_console" as suggested below, and this patch does not address that use case.
Second, the patch causes some unexplained problems with dm-verity, which inexplicably fails on some Chromebooks when the patch is in place. We never tracked down the root cause because the patch doesn't work for us anyway.
Guenter
Quoted 60 lines
Use ttynull driver explicitly for console="" and console=null parameters. It has been created for exactly this purpose.
It causes that
preferred_consoleis set. As a result, ttySX and ttyX are not used as a fallback. And only ttynull console gets registered by default.It still allows to register other consoles either by additional console= parameters or SPCR. It prevents regression because it worked this way even before. Also it is a sane semantic. Preventing output on all consoles should be done another way, for example, by introducing
mute_consoleparameter.Suggested-by Sergey Senozhatsky <[email protected]>Reviewed-by Guenter Roeck <[email protected]>Tested-by Guenter Roeck <[email protected]>Signed-off-by Petr Mladek <[email protected]>---
This is backport of the commit
3cffa06aeef7ece30f6b5ac0e("printk/console: Allow to disable console output by using console="" or console=null") for stable release:+ 4.4, 4.9, 4.14, 4.19, 5.4
Please, use the original upstream commit for stable release:
+ 5.10
It should fix the problem reported at
https://www.spinics.net/lists/stable/msg509616.html
kernel/printk/printk.c| 9 ++++++++-1 file changed, 8 insertions(+), 1 deletion(-)kernel/printk/printk.c +8
-1@@ -2032,8 +2032,15 @@ static int __init console_setup(char *str)char *s, *options, *brl_options =NULL ;int idx; -if (str[0 ] ==0 ) +/* +* console="" or console=null have been suggested as a way to +* disable console output. Use ttynull that has been created +* for exacly this purpose. +*/ +if (str[0 ] ==0 ||strcmp (str,"null" ) ==0 ) { +__add_preferred_console ("ttynull" ,0 ,NULL ,NULL );return 1 ; + }if (_braille_console_setup (&str, &brl_options))return 1 ;
Petr Mladek
maintainerQuoted 4 lines · Guenter Roeck
Quoted 3 lines · Petr Mladek
The commit
48021f98130880dd74("printk: handle blank console arguments passed in.") prevented crash caused by empty console= parameter value.
Quoted 8 lines
Quoted 5 lines
Unfortunately, this value is widely used on Chromebooks to disable the console output. The above commit caused performance regression because the messages were pushed on slow console even though nobody was watching it.
We actually had to revert this patch on Chromebooks, so we'll have to revert it again from stable releases after it gets there.
What patch was or need to get reverted on Chromebooks, please?
1. commit 48021f98130880dd74 ("printk: handle blank console arguments passed in.")
or
2. commit commit 3cffa06aeef7ece30f6b5ac0e ("printk/console: Allow to disable console output by using console="" or console=null")
I know that the 1st patch caused problems on Chromebook. The 2nd one was supposed to fix the problem.
The 2nd patch is being backported here? Do you still see the problems with it, please?
Quoted 7 lines
The problem is two-fold:
First, it is used in Chromebooks to disable the default console in production images; that default console may be set in a devicetree file, and this patch doesn't really disable it. In other words, Chromebooks use "console=" to implement "
mute_console" as suggested below, and this patch does not address that use case.
I guess that you are talking about the 1st patch.
The 2nd patch should make it working basically the same way as when reverting the 1st patch. The difference is that it prefers the fake ttynull console driver instead of none. It should be better because it will provide a kind of null console for stdin/stdou/stderr of the init process. But it still should result into a none-driver when ttynull driver is not available.
Or do you use another extra patch for Chromebooks, please?
Quoted 4 lines
Second, the patch causes some unexplained problems with dm-verity, which inexplicably fails on some Chromebooks when the patch is in place. We never tracked down the root cause because the patch doesn't work for us anyway.
Interesting. I wonder what console was really registered when it complained.
Best Regards,
Petr
Quoted 16 lines · Petr Mladek
Quoted 4 lines · Guenter Roeck
Quoted 3 lines · Petr Mladek
The commit
48021f98130880dd74("printk: handle blank console arguments passed in.") prevented crash caused by empty console= parameter value.Quoted 8 lines
Quoted 5 lines
Unfortunately, this value is widely used on Chromebooks to disable the console output. The above commit caused performance regression because the messages were pushed on slow console even though nobody was watching it.
We actually had to revert this patch on Chromebooks, so we'll have to revert it again from stable releases after it gets there.
What patch was or need to get reverted on Chromebooks, please?
Quoted 8 lines
1. commit
48021f98130880dd74("printk: handle blank console arguments passed in.")or
2. commit commit
3cffa06aeef7ece30f6b5ac0e("printk/console: Allow to disable console output by using console="" or console=null")
Both.
Quoted 7 lines
I know that the 1st patch caused problems on Chromebook. The 2nd one was supposed to fix the problem.
The 2nd patch is being backported here? Do you still see the problems with it, please?
Yes.
Quoted 9 lines
Quoted 8 lines
The problem is two-fold:
First, it is used in Chromebooks to disable the default console in production images; that default console may be set in a devicetree file, and this patch doesn't really disable it. In other words, Chromebooks use "console=" to implement "
mute_console" as suggested below, and this patch does not address that use case.
Quoted 9 lines
I guess that you are talking about the 1st patch.
The 2nd patch should make it working basically the same way as when reverting the 1st patch. The difference is that it prefers the fake ttynull console driver instead of none. It should be better because it will provide a kind of null console for stdin/stdou/stderr of the init process. But it still should result into a none-driver when ttynull driver is not available.
Yes, that was what we initially assumed as well - only, as the patch states, it doesn't work as needed for Chromebooks if another console is registered by other means.
Quoted 2 lines
Or do you use another extra patch for Chromebooks, please?
Not that I know of. We just rely on the old behavior.
Quoted 7 lines
Quoted 4 lines
Second, the patch causes some unexplained problems with dm-verity, which inexplicably fails on some Chromebooks when the patch is in place. We never tracked down the root cause because the patch doesn't work for us anyway.
Interesting. I wonder what console was really registered when it complained.
It was with a Chromebook using devicetree, where the console is primarily passed via devicetree and not as boot parameter. I don't recall the exact model(s).
Guenter