[Buildroot] ARC no console output (was Re: [PATCH 1/2] init/console: Use ttynull as a fallback when there is no console)

[Buildroot] ARC no console output (was Re: [PATCH 1/2] init/console: Use ttynull as a fallback when there is no console)

[Vineet Gupta]

+CC Buildroot folks

Hi Petr,

On 11/11/20 5:54 AM, Petr Mladek wrote:
> stdin, stdout, and stderr standard I/O stream are created for the init
> process. They are not available when there is no console registered
> for /dev/console. It might lead to a crash when the init process
> tries to use them, see the commit 48021f98130880dd742 ("printk: handle
> blank console arguments passed in.").
> 
> Normally, ttySX and ttyX consoles are used as a fallback when no consoles
> are defined via the command line, device tree, or SPCR. But there
> will be no console registered when an invalid console name is configured
> or when the configured consoles do not exist on the system.
> 
> Users even try to avoid the console intentionally, for example,
> by using console="" or console=null. It is used on production
> systems where the serial port or terminal are not visible to
> users. Pushing messages to these consoles would just unnecessary
> slowdown the system.
> 
> Make sure that stdin, stdout, stderr, and /dev/console are always
> available by a fallback to the existing ttynull driver. It has
> been implemented for exactly this purpose but it was used only
> when explicitly configured.
> 
> Signed-off-by: Petr Mladek <pmladek@suse.com>

> --- a/init/main.c
> +++ b/init/main.c
> @@ -1470,8 +1470,14 @@ void __init console_on_rootfs(void)
>   	struct file *file = filp_open("/dev/console", O_RDWR, 0);
>   
>   	if (IS_ERR(file)) {
> -		pr_err("Warning: unable to open an initial console.\n");
> -		return;
> +		pr_err("Warning: unable to open an initial console. Fallback to ttynull.\n");
> +		register_ttynull_console();
> +
> +		file = filp_open("/dev/console", O_RDWR, 0);
> +		if (IS_ERR(file)) {
> +			pr_err("Warning: Failed to add ttynull console. No stdin, stdout, and stderr for the init process!\n");
> +			return;
> +		}


This breaks ARC booting (no output on console).

Our Buildroot based setup has dynamic /dev where /dev/console doesn't 
exist statically and there's a primoridla /init shell script which does 
following

/bin/mount -t devtmpfs devtmpfs /dev
exec 0</dev/console
exec 1>/dev/console
exec 2>/dev/console
exec /sbin/init "$@"

Buildroot has had this way of handling missing /dev/console since 2011 
[1] and [2].

Please advise what needs to be done to unbork boot. Otherwise this seems 
like a kernel change which breaks user-space and needs to be backed-out 
(or perhaps conditionalize on CONFIG_NULL_TTY. I'm surprised it hasn't 
been reported by any other  embedded folks

Thx,
-Vineet

[1] http://lists.busybox.net/pipermail/buildroot/2011-July/044505.html
[2] http://lists.busybox.net/pipermail/buildroot/2011-August/044832.html

[Buildroot] ARC no console output (was Re: [PATCH 1/2] init/console: Use ttynull as a fallback when there is no console)

[Yann E. MORIN]

Vineet, All,

On 2021-01-06 15:36 -0800, Vineet Gupta spake thusly:
> On 11/11/20 5:54 AM, Petr Mladek wrote:
[--SNIP--]
> >Make sure that stdin, stdout, stderr, and /dev/console are always
> >available by a fallback to the existing ttynull driver. It has
> >been implemented for exactly this purpose but it was used only
> >when explicitly configured.
> >
> >Signed-off-by: Petr Mladek <pmladek@suse.com>
> 
> >--- a/init/main.c
> >+++ b/init/main.c
> >@@ -1470,8 +1470,14 @@ void __init console_on_rootfs(void)
> >  	struct file *file = filp_open("/dev/console", O_RDWR, 0);
> >  	if (IS_ERR(file)) {
> >-		pr_err("Warning: unable to open an initial console.\n");
> >-		return;
> >+		pr_err("Warning: unable to open an initial console. Fallback to ttynull.\n");
> >+		register_ttynull_console();
> >+
> >+		file = filp_open("/dev/console", O_RDWR, 0);
> >+		if (IS_ERR(file)) {
> >+			pr_err("Warning: Failed to add ttynull console. No stdin, stdout, and stderr for the init process!\n");
> >+			return;
> >+		}
> 
> This breaks ARC booting (no output on console).
> 
> Our Buildroot based setup has dynamic /dev where /dev/console doesn't exist
> statically and there's a primoridla /init shell script which does following
> 
> /bin/mount -t devtmpfs devtmpfs /dev
> exec 0</dev/console
> exec 1>/dev/console
> exec 2>/dev/console
> exec /sbin/init "$@"

I guess you are speaking about the initramfs (cpio) case, right?

We've changed that code last August:

    https://git.buildroot.org/buildroot/commit/fs/cpio/init?id=b9026e83f

I.e. if we can't do the redirection, then we don't redirect anything.
The change was done for people who explicitly pass an empty console= on
their kernel command line.

Now, I haven't looked at nulltty yet, and I have (so far) no idea on how
it works. Thanks for the hint, I'll have a look.

> Buildroot has had this way of handling missing /dev/console since 2011 [1]
> and [2].

See also more archaelogy on that topic, referenced in that commit:
    https://git.buildroot.org/buildroot/commit/fs/cpio/?id=98a6f1fc02e41

> Please advise what needs to be done to unbork boot.

This has been present since the 2020.08 release, and has been backported
to the maintenance branches:
    2020.02.x (LTS) -> f1a83afe2df2a
    2020.05.x       -> 797f9e40224c9

> Otherwise this seems
> like a kernel change which breaks user-space and needs to be backed-out (or
> perhaps conditionalize on CONFIG_NULL_TTY. I'm surprised it hasn't been
> reported by any other  embedded folks

I won't speak about whether this is a kernel regression or not, not my
call.

Regards,
Yann E. MORIN.

-- 
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| +33 662 376 056 | Software  Designer | \ / CAMPAIGN     |  ___               |
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[Buildroot] ARC no console output (was Re: [PATCH 1/2] init/console: Use ttynull as a fallback when there is no console)

[Vineet Gupta]

Hi John,

On 1/7/21 1:02 AM, John Ogness wrote:
> Hi Vineet,
> 
> On 2021-01-06, Vineet Gupta <vgupta@synopsys.com> wrote:
>> This breaks ARC booting (no output on console).
> 
> Could you provide the kernel boot arguments that you use? This series is
> partly about addressing users that have used boot arguments that are
> technically incorrect (even if had worked). Seeing the boot arguments of
> users that are not experiencing problems may help to reveal some of the
> unusual console usages until now.


Kernel command line: earlycon=uart8250,mmio32,0xf0005000,115200n8 
console=ttyS0,115200n8 debug print-fatal-signals=1

And we do have serial driver built-in.

Thx,
-Vineet

[Buildroot] ARC no console output (was Re: [PATCH 1/2] init/console: Use ttynull as a fallback when there is no console)

[Vineet Gupta]

On 1/7/21 9:04 AM, Petr Mladek wrote:
> On Thu 2021-01-07 08:43:16, Vineet Gupta wrote:
>> Hi John,
>>
>> On 1/7/21 1:02 AM, John Ogness wrote:
>>> Hi Vineet,
>>>
>>> On 2021-01-06, Vineet Gupta <vgupta@synopsys.com> wrote:
>>>> This breaks ARC booting (no output on console).
>>>
>>> Could you provide the kernel boot arguments that you use? This series is
>>> partly about addressing users that have used boot arguments that are
>>> technically incorrect (even if had worked). Seeing the boot arguments of
>>> users that are not experiencing problems may help to reveal some of the
>>> unusual console usages until now.
>>
>>
>> Kernel command line: earlycon=uart8250,mmio32,0xf0005000,115200n8
>> console=ttyS0,115200n8 debug print-fatal-signals=1
> 
> This is strange, the problematic patch should use ttynull
> only as a fallback. It should not be used when a particular console
> is defined on the command line.

What happens in my case is console_on_rootfs() doesn't find /dev/console 
and switching to ttynull. /dev is not present because devtmpfs doesn't 
automount for initramfs.

> The only explanation would be that ttyS0 gets registered too late
> and ttynull is added as a fallback in the meantime.

I don't know if ttyS0 console should have registered already but even if 
it did - the /dev node missing would not have helped ?

> 
> Anyway, I propose the revert the problematic patch for 5.11-rc3,
> see
> https://lore.kernel.org/lkml/20210107164400.17904-2-pmladek at suse.com/
> This mystery is a good reason to avoid bigger changes at this stage.
> 
> Best Regards,
> Petr
> 

[Buildroot] ARC no console output (was Re: [PATCH 1/2] init/console: Use ttynull as a fallback when there is no console)

[Vineet Gupta]

On 1/7/21 7:48 PM, Sergey Senozhatsky wrote:
> On (21/01/07 09:58), Vineet Gupta wrote:
>> On 1/7/21 9:04 AM, Petr Mladek wrote:
>>> On Thu 2021-01-07 08:43:16, Vineet Gupta wrote:
>>>> Hi John,
>>>>
>>>> On 1/7/21 1:02 AM, John Ogness wrote:
>>>>> Hi Vineet,
>>>>>
>>>>> On 2021-01-06, Vineet Gupta <vgupta@synopsys.com> wrote:
>>>>>> This breaks ARC booting (no output on console).
>>>>>
>>>>> Could you provide the kernel boot arguments that you use? This series is
>>>>> partly about addressing users that have used boot arguments that are
>>>>> technically incorrect (even if had worked). Seeing the boot arguments of
>>>>> users that are not experiencing problems may help to reveal some of the
>>>>> unusual console usages until now.
>>>>
>>>>
>>>> Kernel command line: earlycon=uart8250,mmio32,0xf0005000,115200n8
>>>> console=ttyS0,115200n8 debug print-fatal-signals=1
>>>
>>> This is strange, the problematic patch should use ttynull
>>> only as a fallback. It should not be used when a particular console
>>> is defined on the command line.
>>
>> What happens in my case is console_on_rootfs() doesn't find /dev/console and
>> switching to ttynull. /dev is not present because devtmpfs doesn't automount
>> for initramfs.
> 
> I wonder if we'll move the nulltty fallback logic into printk code [1]
> will it fix the problem?
> 
> [1] https://lore.kernel.org/lkml/X6x%2FAxD1qanC6evJ at jagdpanzerIV.localdomain/

Your reasoning in the post above makes total sense.

I tired the patch: adding register_ttynull_console() call in 
console_device(), removing from console_on_rootfs() band that works too.


| Warning: unable to open an initial console. Fallback to ttynull.
| Warning: Failed to add ttynull console. No stdin, stdout, and stderr 
for the init process!
| Freeing unused kernel memory: 3096K
| This architecture does not have kernel memory protection.
| Run /init as init process
| with arguments:
|    /init
|  with environment:
|    HOME=/
|    TERM=linux
| Starting System logger (syslogd)
| Bringing up loopback device
| Starting inetd
| Mounting Posix Mqueue filesys
| 
CONFIG_INITRAMFS_SOURCE="~/arc/RAMFS/archs/ramfs_2011-GNU-2020-03-glibc-2.32-tiny"
| **********************************************************************
|			Welcome to ARCLinux
| **********************************************************************
| [ARCLinux]#

[Buildroot] ARC no console output (was Re: [PATCH 1/2] init/console: Use ttynull as a fallback when there is no console)

[Peter Korsgaard]

>>>>> "Vineet" == Vineet Gupta <vgupta@synopsys.com> writes:

 > On 1/7/21 9:04 AM, Petr Mladek wrote:
 >> On Thu 2021-01-07 08:43:16, Vineet Gupta wrote:
 >>> Hi John,
 >>> 
 >>> On 1/7/21 1:02 AM, John Ogness wrote:
 >>>> Hi Vineet,
 >>>> 
 >>>> On 2021-01-06, Vineet Gupta <vgupta@synopsys.com> wrote:
 >>>>> This breaks ARC booting (no output on console).
 >>>> 
 >>>> Could you provide the kernel boot arguments that you use? This series is
 >>>> partly about addressing users that have used boot arguments that are
 >>>> technically incorrect (even if had worked). Seeing the boot arguments of
 >>>> users that are not experiencing problems may help to reveal some of the
 >>>> unusual console usages until now.
 >>> 
 >>> 
 >>> Kernel command line: earlycon=uart8250,mmio32,0xf0005000,115200n8
 >>> console=ttyS0,115200n8 debug print-fatal-signals=1
 >> 
 >> This is strange, the problematic patch should use ttynull
 >> only as a fallback. It should not be used when a particular console
 >> is defined on the command line.

 > What happens in my case is console_on_rootfs() doesn't find
 > /dev/console and switching to ttynull. /dev is not present because
 > devtmpfs doesn't automount for initramfs.

But our initramfs/cpio logic ensures that the initramfs has a static
/dev/console device node, so how can that be?

https://git.buildroot.net/buildroot/tree/fs/cpio/cpio.mk#n25

-- 
Bye, Peter Korsgaard