linux-kernel - Re: [syzbot] WARNING in c

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Date:   Sun, 16 Oct 2022 13:10:18 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Borislav Petkov <bp@...en8.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        syzbot <syzbot+d0fd2bf0dd6da72496dd@...kaller.appspotmail.com>,
        syzkaller-bugs@...glegroups.com,
        Andrew Jones <ajones@...tanamicro.com>, netdev@...r.kernel.org,
        "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Menglong Dong <imagedong@...cent.com>,
        Kuniyuki Iwashima <kuniyu@...zon.com>,
        Petr Machata <petrm@...dia.com>,
        Guo Ren <guoren@...ux.alibaba.com>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        andriy.shevchenko@...ux.intel.com, linux@...musvillemoes.dk,
        caraitto@...gle.com, willemb@...gle.com, jonolson@...gle.com,
        amritha.nambiar@...el.com, linux-kernel@...r.kernel.org
Subject: Re: [syzbot] WARNING in c_start

On 2022/10/16 10:12, Yury Norov wrote:
> On Sun, Oct 16, 2022 at 09:24:57AM +0900, Tetsuo Handa wrote:
>> I'm asking you not to use BUG_ON()/WARN_ON() etc. which breaks syzkaller.
> 
> It's not me who added WARN_ON() in the cpumask_check(). You're asking
> a wrong person. 

Because you broke the kernel testing. It was obvious that passing "cpu + 1"
instead of "cpu" will trivially hit cpu == nr_cpumask_bits condition.
If your patch were reviewed and tested, we would not have done this discussion.

> 
> What for do we have WARN_ON(), if we can't use it?

WARN_ON() could be used which should not happen.
But cpu == nr_cpumask_bits condition shall happen in your patch.

RCs are not for fixing bugs that causes boot failures. Such bugs
should have been tested and fixed before patches are sent to linux.git .

> 
>> Just printing messages (without "BUG:"/"WARNING:" string which also breaks
>> syzkaller) like below diff is sufficient for people to realize that they're
>> doing something wrong.
>>
>> Again, please do revert "cpumask: fix checking valid cpu range" immediately.
> 
> The revert is already in Jakub's batch for -rc2, AFAIK.

OK.