linux-kernel - Re: [syzbot? printk?] no WARN_ON() messages printed before "Kernel panic

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Date:   Thu, 9 May 2019 19:26:18 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: [syzbot? printk?] no WARN_ON() messages printed before "Kernel
 panic - not syncing: panic_on_warn set ..."

On 2019/05/09 18:58, Sergey Senozhatsky wrote:
>> +#ifdef CONFIG_DEBUG_AID_FOR_SYZBOT
>> +static int initial_loglevel;
>> +static void check_loglevel(struct timer_list *timer)
>> +{
>> +	if (console_loglevel < initial_loglevel)
>> +		panic("Console loglevel changed (%d->%d)!", initial_loglevel,
>> +		      console_loglevel);
>> +	mod_timer(timer, jiffies + HZ);
>> +}
>> +static int __init loglevelcheck_init(void)
>> +{
>> +	static DEFINE_TIMER(timer, check_loglevel);
>> +
>> +	initial_loglevel = console_loglevel;
>> +	mod_timer(&timer, jiffies + HZ);
>> +	return 0;
>> +}
>> +late_initcall(loglevelcheck_init);
>> +#endif
> 
> I suppose this patch is for internal testing at Google only. I don't
> think we can consider upstreaming it.

Right. There is CONFIG_DEBUG_AID_FOR_SYZBOT option for testing at linux-next.git only.



> 
>> By the way, recently we are hitting false positives caused by "WARNING:"
>> string from not WARN() messages but plain printk() messages (e.g.
>>
>>   https://syzkaller.appspot.com/bug?id=31bdef63e48688854fde93e6edf390922b70f8a4
>>   https://syzkaller.appspot.com/bug?id=faae4720a75cadb8cd0dbda5c4d3542228d37340
>>
>> ) and we need to avoid emitting "WARNING:" string from plain printk() messages
>> during fuzzing testing. I guess we want to add something like
>> CONFIG_DEBUG_AID_FOR_SYZBOT to all kernels in order to mask such string...
> 
> I thought that we have MSG_FORMAT_SYSLOG exactly for things like these,
> so you can look at actual message level <%d> and then decide if it's a
> warning or a false alarm.

Since syzbot needs to use console output, message level is not available.

> 
> These are pr_info() level messages, but the text contains "WARNING: "
> 
> [..]
> pvr2_trace(PVR2_TRACE_ERROR_LEGS,
> 	   "WARNING: Detected a wedged cx25840 chip; the device will not work.");
> [..]
> 
> I would suggest to fix pvrusb2-i2c-core.c. I don't think we really
> want to $text =~ s/WARNING//g in printk.

Of course, I don't want to try $text =~ s/WARNING//g in printk().
What I meant is guard the callers like
https://git.kernel.org/pub/scm/linux/kernel/git/jmorris/linux-security.git/commit/?h=next-tomoyo&id=5c6b31e31adc31bd12636b196d3311f845dcc9d8
using a kernel config option which will be acceptable for upstreaming, for
we need to apply to all kernels which syzbot is testing.