linux-kernel - Re: [PATCH] x86: Avoid pr_cont() in show

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Message-ID: <ac094aa6-3b8d-6a96-b635-0634052bf3cf@i-love.sakura.ne.jp>
Date:   Tue, 10 Jul 2018 20:55:41 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>, mingo@...hat.com,
        linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...capital.net>,
        Borislav Petkov <bp@...e.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] x86: Avoid pr_cont() in show_opcodes()

On 2018/07/10 4:11, Josh Poimboeuf wrote:
> On Mon, Jul 09, 2018 at 10:49:53AM +0200, Peter Zijlstra wrote:
>> On Sat, Jul 07, 2018 at 10:54:28PM +0900, Tetsuo Handa wrote:
>>>>> Since syzbot is confused by concurrent printk() messages [1],
>>>>> this patch changes show_opcodes() to use snprintf().
>>
>> But how big of a problem is that really? We can't very well remove all
>> pr_cont stuff from the kernel.
> 
> I'd say we should try to make oopses as legible as possible.
> 
> Also KERN_CONT is inherently broken, and we should avoid using it in
> general, IMO.
> 

We can't afford removing all pr_cont()/KERN_CONT.
But show_opcodes() is an example of function which is expected to be SMP-safe.

  /*
   * Annotation for a "continued" line of log printout (only done after a
   * line that had no enclosing \n). Only to be used by core/arch code
   * during early bootup (a continued line is not SMP-safe otherwise).
   */
  #define KERN_CONT       KERN_SOH "c"