linux-kernel - Re: [PATCH] sysrq: Restore original console

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Message-ID: <20190111130729.GA31790@tigerII.localdomain>
Date:   Fri, 11 Jan 2019 22:07:29 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sysrq: Restore original console_loglevel when sysrq
 disabled

On (01/11/19 13:45), Petr Mladek wrote:
> The sysrq header line is printed with an increased loglevel
> to provide users some positive feedback.
> 
> The original loglevel is not restored when the sysrq operation
> is disabled. This bug was introduced in 2.6.12 (pre-git-history)
> by the commit ("Allow admin to enable only some of the Magic-Sysrq
> functions").

Good find, and the patch looks OK to me. A small comment below.
FWIW,
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>

---

A side note (nitpick, etc.); it's Friday night in here, I'm enjoying
my beer; so maybe I'm wrong about the whole thing.

> @@ -553,6 +553,7 @@ void __handle_sysrq(int key, bool check_mask)
>  			op_p->handler(key);
>  		} else {
>  			pr_cont("This sysrq operation is disabled.\n");
> +			console_loglevel = orig_log_level;
>  		}

This looks a bit racy.

When we do

	printk("FOO\n");
	console_loglevel = XYZ;

We don't have any real guarantees that printk("FOO\n") will print
anything straight ahead. It is possible that console_sem is already
locked and the owner is preempted, so by the time the console_sem
owner picks up that FOO\n messages, console_loglevel is back to
orig_log_level and suppress_message_printing() will just tell us
to skip the message.

Do we need pr_cont() there? Maybe we can just have a normal pr_err()
which would always tell that "key" sysrq is disabled? (we also
would need to change the error message a bit).

	-ss