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

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Date:   Sun, 16 Oct 2022 09:34:09 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     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,
        Yury Norov <yury.norov@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [syzbot] WARNING in c_start

On 2022/10/16 9:28, Randy Dunlap wrote:
>> +/*
>> + * We want to avoid passing -1 as a valid cpu argument.
>> + * But we should not crash the kernel until all in-tree callers are fixed.
>> + */
> 
> Why not say that any negative cpu argument is invalid?

Currently only -1 is accepted as exception.

>>  	if (n != -1)
>>  		cpumask_check(n);
>> +	else
>> +		report_negative_cpuid();

> Or is it OK to pass -2 as the cpu arg?

Passing -2 will hit WARN_ON_ONCE(cpu >= nr_cpumask_bits) path.