linux-kernel - Re: [RFC] printk/sysrq: Don't play with console

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Message-ID: <e564ee00-6a93-defd-4eab-e306bbfe8b01@i-love.sakura.ne.jp>
Date:   Tue, 28 May 2019 23:21:17 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Dmitry Safonov <dima@...sta.com>, linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC] printk/sysrq: Don't play with console_loglevel

On 2019/05/28 22:42, Petr Mladek wrote:
>> Ahh.. OK, now I sort of remember why I gave up on this idea (see [1]
>> at the bottom, when it comes to uv_nmi_dump_state()) - printk_NMI and
>> printk-safe redirections.
>>
>> 	NMI
>> 		loglevel = NEW
>> 		printk -> printk_safe_nmi
>> 		loglevel = OLD
>>
>> 	iret
>>
>> 	IRQ
>> 		flush printk_safe_nmi -> printk
>> 		// At this point we don't remember about
>> 		// loglevel manipulation anymore
>> 	iret
> 
> printk_safe buffer preserves KERN_* headers. It should be
> possible to insert KERN_UNSUPPRESSED there.

But is context dependent buffer large enough to hold SysRq-t output?
I think that only main logbuf can become large enough to hold SysRq-t output.

We can add KERN_UNSUPPRESSED to SysRq's header line. But I don't think
that we can automatically add KERN_UNSUPPRESSED to SysRq's body lines
based on some context information. If we want to avoid manipulating
console_loglevel, we need to think about how to make sure that
KERN_UNSUPPRESSED is added to all lines from such context without
overflowing capacity of that buffer.