linux-kernel - Re: [PATCH] hpet: Fix division by zero in hpet_time

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Message-ID: <54205e31-30e3-ec2f-015d-7ba65d0d4927@huawei.com>
Date:   Thu, 11 Jul 2019 21:21:19 +0800
From:   Kefeng Wang <wangkefeng.wang@...wei.com>
To:     Arnd Bergmann <arnd@...db.de>
CC:     Clemens Ladisch <clemens@...isch.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] hpet: Fix division by zero in hpet_time_div()



On 2019/7/11 20:32, Arnd Bergmann wrote:
> On Thu, Jul 11, 2019 at 1:20 PM Kefeng Wang <wangkefeng.wang@...wei.com> wrote:
>> The base value in do_div() called by hpet_time_div() is truncated from
>> unsigned long to uint32_t, resulting in a divide-by-zero exception.
> 
> Good catch!
> 
>> --- a/drivers/char/hpet.c
>> +++ b/drivers/char/hpet.c
>> @@ -567,7 +567,7 @@ static inline unsigned long hpet_time_div(struct hpets *hpets,
>>         unsigned long long m;
>>
>>         m = hpets->hp_tick_freq + (dis >> 1);
>> -       do_div(m, dis);
>> +       div64_ul(m, dis);
>>         return (unsigned long)m;
>>  }
> 
> This still looks wrong to me: div64_ul() unlike do_div() does not
> modify its argument, so you have to assign the output like
> 
>        return div64_ul(m, dis);

right, should check div64_ul more carefully， will resend v2, thanks

> 
>        Arnd
> 
>