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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <52210D34.5030807@infradead.org>
Date:	Fri, 30 Aug 2013 14:23:00 -0700
From:	Randy Dunlap <rdunlap@...radead.org>
To:	Anatol Pomozov <anatol.pomozov@...il.com>
CC:	LKML <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
	bernie@...eler.com
Subject: Re: do_div() silently truncates "base" to 32bit

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?


> Ideally if this macros is converted to a function so compiler will
> warn us about unexpected truncation like this. But in this case it
> will be hard to do as "n" parameter both input and output.
> --


-- 
~Randy
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/