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From: Fengguang Wu <fengguang.wu@intel.com>
To: Dmitry Vyukov <dvyukov@google.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
	Petr Mladek <pmladek@suse.com>,
	Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	syzkaller <syzkaller@googlegroups.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] printk: inject caller information into the body of message
Date: Wed, 20 Jun 2018 19:37:31 +0800
Message-ID: <20180620113731.y7apmqoh6ke6ar2v@wfg-t540p.sh.intel.com> (raw)
In-Reply-To: <CACT4Y+a6nUqdpSyYGGcXKAgVbcw+uQOz2kYx8tUS+8T5-bzqkg@mail.gmail.com>

On Wed, Jun 20, 2018 at 11:30:05AM +0200, Dmitry Vyukov wrote:
>On Wed, Jun 20, 2018 at 11:06 AM, Sergey Senozhatsky
><sergey.senozhatsky.work@gmail.com> wrote:
>> Hi Dmitry,
>>
>> On (06/20/18 10:45), Dmitry Vyukov wrote:
>>> Hi Sergey,
>>>
>>> What are the visible differences between this patch and Tetsuo's
>>> patch?
>>
>> I guess none, and looking at your requirements below I tend to agree
>> that Tetsuo's approach is probably what you need at the end of the day.
>>
>>> The only thing that will matter for syzkaller parsing in the
>>> end is the resulting text format as it appears on console. But you say
>>> "I'm not pushing for this particular message format", so what exactly
>>> do you want me to provide feedback on?
>>> I guess we need to handle pr_cont properly whatever approach we take.
>>
>> Mostly, was wondering about if:
>> a) you need pr_cont() handling
>> b) you need printk_safe() handling
>>
>> The reasons I left those things behind:
>>
>> a) pr_cont() is officially hated. It was never supposed to be used
>>    on SMP systems. So I wasn't sure if we need all that effort and
>>    add tricky code to handle pr_cont(). Given that syzkaller is
>>    probably the only user of that functionality.
>
>Well, if I put my syzkaller hat on, then I don't care what exactly
>happens in the kernel, the only thing I care is well-formed output on
>console that can be parsed unambiguously in all cases.

+1 for 0day kernel testing.

I admit that goal may never be 100% achievable -- at least some serial
console logs can sometimes become messy. So we'll have to write dmesg
parsing code in defensive ways.

But some unnecessary pr_cont() broken-up messages can obviously be
avoided. For example,

arch/x86/mm/fault.c:

	printk(KERN_ALERT "BUG: unable to handle kernel ");
	if (address < PAGE_SIZE)
		printk(KERN_CONT "NULL pointer dereference");
	else
		printk(KERN_CONT "paging request");

I've actually proposed to remove the above KERN_CONT, unfortunately the
patch was silently ignored.

>From this point of view I guess pr_cont is actually syzkaller's worst
>enemy. If pr_const is officially hated, and it causes corrupted crash
>reports, then we can resolve it by just getting rid of more pr_cont's.
>So potentially we do not need any support for pr_cont in this patch.
>However, we also need to be practical and if there are tons of
>pr_cont's then we need some intermediate support of them, just because
>we won't be able to get rid of all of them overnight.
>
>But even if we attach context to pr_cont, it still causes problems for
>crash parsing, because today we see:
>
>BUG: unable to handle
>... 10 lines ...
>kernel
>... 10 lines ...
>paging request
>... 10 lines ...
>at ADDR
>
>Which is not too friendly for parsing regardless of contexts.

We met exactly the same issue and ended up with special handling in
https://github.com/intel/lkp-tests/blob/master/lib/dmesg.rb:

       /(BUG: unable to handle kernel)/,
       /(BUG: unable to handle kernel) NULL pointer dereference/,
       /(BUG: unable to handle kernel) paging request/,

>So I am leaning towards to getting rid of pr_cont's as the solution to
>the problem.

+1 for reducing unnecessary pr_cont() uses.

Thanks,
Fengguang

  parent reply index

Thread overview: 94+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <201804232233.CIC65675.OJSOMFQOFFHVtL@I-love.SAKURA.ne.jp>
     [not found] ` <CACT4Y+boyw_Qy=y-iTnsKZrtTgF0Hk3nHN_xtqUdX4etgiYDQw@mail.gmail.com>
2018-04-24  1:33   ` printk feature for syzbot? Sergey Senozhatsky
2018-04-24 14:40     ` Steven Rostedt
2018-04-26 10:06     ` Petr Mladek
2018-05-10  4:22       ` Sergey Senozhatsky
2018-05-10 11:30         ` Petr Mladek
2018-05-10 12:11           ` Sergey Senozhatsky
2018-05-10 14:22             ` Steven Rostedt
2018-05-10 14:50         ` Tetsuo Handa
2018-05-11  1:45           ` Sergey Senozhatsky
     [not found]             ` <201805110238.w4B2cIGH079602@www262.sakura.ne.jp>
2018-05-11  6:21               ` Sergey Senozhatsky
2018-05-11  9:17                 ` Dmitry Vyukov
2018-05-11  9:50                   ` Sergey Senozhatsky
2018-05-11 11:58                     ` [PATCH] printk: inject caller information into the body of message Tetsuo Handa
2018-05-17 11:21                       ` Sergey Senozhatsky
2018-05-17 11:52                         ` Sergey Senozhatsky
2018-05-18 12:15                         ` Petr Mladek
2018-05-18 12:25                           ` Dmitry Vyukov
2018-05-18 12:54                             ` Petr Mladek
2018-05-18 13:08                               ` Dmitry Vyukov
2018-05-24  2:21                                 ` Sergey Senozhatsky
2018-05-23 10:19                           ` Tetsuo Handa
2018-05-24  2:14                           ` Sergey Senozhatsky
2018-05-26  6:36                             ` Dmitry Vyukov
2018-06-20  5:44                               ` Dmitry Vyukov
2018-06-20  8:31                                 ` Sergey Senozhatsky
2018-06-20  8:45                                   ` Dmitry Vyukov
2018-06-20  9:06                                     ` Sergey Senozhatsky
2018-06-20  9:18                                       ` Sergey Senozhatsky
2018-06-20  9:31                                         ` Dmitry Vyukov
2018-06-20 11:07                                           ` Sergey Senozhatsky
2018-06-20 11:32                                             ` Dmitry Vyukov
2018-06-20 13:06                                               ` Sergey Senozhatsky
2018-06-22 13:06                                                 ` Tetsuo Handa
2018-06-25  1:41                                                   ` Sergey Senozhatsky
2018-06-25  9:36                                                     ` Dmitry Vyukov
2018-06-27 10:29                                                       ` Tetsuo Handa
2018-09-10 11:20                                                 ` Alexander Potapenko
2018-09-12  6:53                                                   ` Sergey Senozhatsky
2018-09-12 16:05                                                     ` Steven Rostedt
2018-09-13  7:12                                                       ` Sergey Senozhatsky
2018-09-13 12:26                                                         ` Petr Mladek
2018-09-13 14:28                                                           ` Sergey Senozhatsky
2018-09-14  1:22                                                             ` Steven Rostedt
2018-09-14  2:15                                                               ` Sergey Senozhatsky
2018-09-14  6:57                                                             ` Sergey Senozhatsky
2018-09-14 10:37                                                               ` Tetsuo Handa
2018-09-14 11:50                                                                 ` Sergey Senozhatsky
2018-09-14 12:03                                                                   ` Tetsuo Handa
2018-09-14 12:22                                                                     ` Sergey Senozhatsky
2018-09-19 11:02                                                                       ` Tetsuo Handa
2018-09-24  8:11                                                                         ` Tetsuo Handa
2018-09-27 16:10                                                                           ` Tetsuo Handa
2018-09-28  9:02                                                                             ` Sergey Senozhatsky
2018-09-28  9:09                                                                           ` Sergey Senozhatsky
2018-09-28 11:01                                                                             ` Tetsuo Handa
2018-09-29 10:51                                                                               ` Sergey Senozhatsky
2018-09-29 11:15                                                                                 ` Tetsuo Handa
2018-10-01  2:37                                                                                   ` Sergey Senozhatsky
2018-10-01  2:58                                                                                     ` Sergey Senozhatsky
2018-10-01 11:21                                                                                     ` Tetsuo Handa
2018-10-02  6:38                                                                                       ` Sergey Senozhatsky
2018-10-08 10:31                                                                                         ` Tetsuo Handa
2018-10-08 16:03                                                                                           ` Petr Mladek
2018-10-08 20:48                                                                                             ` Tetsuo Handa
2018-10-09 14:52                                                                                               ` Petr Mladek
2018-10-09 21:19                                                                                                 ` Tetsuo Handa
2018-10-10 10:14                                                                                                   ` Tetsuo Handa
2018-10-11 10:20                                                                                                     ` Tetsuo Handa
2018-10-11 13:47                                                                                                       ` Steven Rostedt
2018-10-08 15:43                                                                                         ` Petr Mladek
2018-09-28  8:56                                                                         ` Sergey Senozhatsky
2018-09-28 11:21                                                                           ` Tetsuo Handa
2018-09-29 11:13                                                                             ` Sergey Senozhatsky
2018-09-29 11:39                                                                               ` Tetsuo Handa
2018-10-01  5:52                                                                               ` Sergey Senozhatsky
2018-10-01  8:37                                                                                 ` Sergey Senozhatsky
2018-10-01 18:06                                                                               ` Steven Rostedt
2018-09-14  1:12                                                         ` Steven Rostedt
2018-09-14  1:55                                                           ` Sergey Senozhatsky
2018-06-21  8:29                                               ` Sergey Senozhatsky
2018-06-20  9:30                                       ` Dmitry Vyukov
2018-06-20 11:19                                         ` Sergey Senozhatsky
2018-06-20 11:25                                           ` Dmitry Vyukov
2018-06-20 11:37                                         ` Fengguang Wu [this message]
2018-06-20 12:31                                           ` Dmitry Vyukov
2018-06-20 12:41                                             ` Fengguang Wu
2018-06-20 12:45                                               ` Dmitry Vyukov
2018-06-20 12:48                                                 ` Fengguang Wu
2018-05-11 13:37                     ` printk feature for syzbot? Steven Rostedt
2018-05-15  5:20                       ` Sergey Senozhatsky
2018-05-15 14:39                         ` Steven Rostedt
2018-05-11 11:02                 ` [PATCH] printk: fix possible reuse of va_list variable Tetsuo Handa
2018-05-11 11:27                   ` Sergey Senozhatsky
2018-05-17 11:57                   ` Petr Mladek

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