Re: [PATCH] x86: Avoid pr_cont() in show_opcodes()

[Tetsuo Handa]

On 2018/07/07 20:12, Ingo Molnar wrote:
> 
> * Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote:
> 
>> From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
>>
>> Since syzbot is confused by concurrent printk() messages [1],
>> this patch changes show_opcodes() to use snprintf().
>>
>> When we start adding prefix to each line of printk() output,
>> we will be able to handle concurrent printk() messages.
>>
>> [1] https://syzkaller.appspot.com/text?tag=CrashReport&x=139d342c400000
> 
> Does this change the output?
> 
> - If yes, could you show the before/after output in the changelog,
> 
> - If not (i.e. if only the number of printk calls is changed, the output is the 
>   same), could you say so in the changelog?

This patch will not change the output unless multiple threads concurrently
call printk(). The purpose of this patch is to help parsing kernel messages
when multiple threads are concurrently calling printk() for multiple times
(e.g. backtrace) by reducing pr_cont()/KERN_CONT usage.

> 
> Also, 3*OPCODE_BUFSIZE+2+1 is 195 bytes - isn't that a bit too much on-stack 
> footprint?

Then, we can reduce it by OPCODE_BUFSIZE bytes by unionizing opcodes[] and buf[].