linux-kernel - Re: do_div() silently truncates "base" to 32bit

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Date:	Fri, 30 Aug 2013 15:14:47 -0700
From:	Anatol Pomozov <anatol.pomozov@...il.com>
To:	Randy Dunlap <rdunlap@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
	bernie@...eler.com
Subject: Re: do_div() silently truncates "base" to 32bit

Hi

On Fri, Aug 30, 2013 at 2:23 PM, Randy Dunlap <rdunlap@...radead.org> wrote:
> On 08/30/13 10:21, Anatol Pomozov wrote:
>> Hi,
>>
>>
>> I was debugging weird "zero divide" problem in CFQ code below
>>
>>
>> static u64 cfqg_prfill_avg_queue_size(struct seq_file *sf,
>>       struct blkg_policy_data *pd, int off)
>> {
>>     struct cfq_group *cfqg = pd_to_cfqg(pd);
>>     u64 samples = blkg_stat_read(&cfqg->stats.avg_queue_size_samples);
>>     u64 v = 0;
>>
>>     if (samples) {
>>         v = blkg_stat_read(&cfqg->stats.avg_queue_size_sum);
>>         do_div(v, samples);
>>     }
>>     __blkg_prfill_u64(sf, pd, v);
>>     return 0;
>> }
>>
>>
>> do_div() crashes says "zero divide". It is weird because just a few
>> lines above we check divider for zero.
>>
>>
>> The problem comes from include/asm-generic/div64.h file that
>> implements do_div() as macros:
>>
>> # define do_div(n,base) ({ \
>>     uint32_t __base = (base); \
>>     uint32_t __rem; \
>>     __rem = ((uint64_t)(n)) % __base; \
>>     (n) = ((uint64_t)(n)) / __base; \
>>     __rem; \
>>  })
>>
>>
>> Do you see the problem?
>>
>> The problem here is that "base" argument is truncated to 32bit, but in
>> the function above "sample" is 64bit variable. If sample's 32 low bits
>> are zero - we have a crash. in fact we have incorrect behavior any
>> time when high 32bits are non-zero.
>>
>>
>> My question is why the base is 32bit? Why not to use 64bit arguments?
>
> Maybe performance related?
>
> If you want 64-bit values, don't use do_div() from asm-generic/div64.h.
>
> Instead look at linux/math64.h and use div_u64_rem() et al
> or the recently posted div64_u64_rem().
> [posted by Mike Snitzer on Aug. 21 2013]
>
> I.e., use exactly the function(s) that you need to use.
>
> Does that fix the problem?

It definitely fixes the crash. I've already sent a patch to CFQ
maillist http://news.gmane.org/gmane.linux.kernel.cgroups

But another question still remains: why compiler does not warn that
size truncation happens? How to prevent bugs like CFQ one in the
future? Should we add a compile-time assert to do_div() to prevent
passing 64 numbers in "base" macro parameter?
--
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