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

[Randy Dunlap <rdunlap@infradead.org>]

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