linux-kernel - Re: [PATCH] linux/kernel.h: fix overflow for DIV_ROUND_UP

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Date:   Tue, 25 Jun 2019 15:32:31 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Vinod Koul <vkoul@...nel.org>, linux-arm-msm@...r.kernel.org,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] linux/kernel.h: fix overflow for DIV_ROUND_UP_ULL

On Tue, 25 Jun 2019 15:29:38 -0700 Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Tue, 25 Jun 2019 15:35:18 +0530 Vinod Koul <vkoul@...nel.org> wrote:
> 
> > DIV_ROUND_UP_ULL adds the two arguments and then invokes
> > DIV_ROUND_DOWN_ULL. But on a 32bit system the addition of two 32 bit
> > values can overflow. DIV_ROUND_DOWN_ULL does it correctly and stashes
> > the addition into a unsigned long long so cast the result to unsigned
> > long long here to avoid the overflow condition.
> >
> > ...
> >
> > --- a/include/linux/kernel.h
> > +++ b/include/linux/kernel.h
> > @@ -93,7 +93,8 @@
> >  #define DIV_ROUND_DOWN_ULL(ll, d) \
> >  	({ unsigned long long _tmp = (ll); do_div(_tmp, d); _tmp; })
> >  
> > -#define DIV_ROUND_UP_ULL(ll, d)		DIV_ROUND_DOWN_ULL((ll) + (d) - 1, (d))
> > +#define DIV_ROUND_UP_ULL(ll, d) \
> > +	({ DIV_ROUND_DOWN_ULL((unsigned long long)(ll) + (d) - 1, (d)) })
> >  
> 
> This clearly wasn't tested :(
> 
> fs/fs-writeback.c: In function wb_split_bdi_pages:
> ./include/linux/kernel.h:97:65: error: expected ; before } token
>   ({ DIV_ROUND_DOWN_ULL((unsigned long long)(ll) + (d) - 1, (d)) })
>                                                                  ^
> fs/fs-writeback.c:811:10: note: in expansion of macro DIV_ROUND_UP_ULL
>    return DIV_ROUND_UP_ULL((u64)nr_pages * this_bw, tot_bw);
> 
> 
> From: Andrew Morton <akpm@...ux-foundation.org>
> Subject: linux-kernelh-fix-overflow-for-div_round_up_ull-fix
> 
> DIV_ROUND_UP_ULL must be an rval
> 
> Cc: Bjorn Andersson <bjorn.andersson@...aro.org>
> Cc: Randy Dunlap <rdunlap@...radead.org>
> Cc: Vinod Koul <vkoul@...nel.org>
> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
> ---
> 
>  include/linux/kernel.h |    6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> --- a/include/linux/kernel.h~linux-kernelh-fix-overflow-for-div_round_up_ull-fix
> +++ a/include/linux/kernel.h
> @@ -93,8 +93,10 @@
>  #define DIV_ROUND_DOWN_ULL(ll, d) \
>  	({ unsigned long long _tmp = (ll); do_div(_tmp, d); _tmp; })
>  
> -#define DIV_ROUND_UP_ULL(ll, d) \
> -	({ DIV_ROUND_DOWN_ULL((unsigned long long)(ll) + (d) - 1, (d)) })
> +#define DIV_ROUND_UP_ULL(ll, d) ({ \
> +	unsigned long long _tmp; \
> +	_tmp = DIV_ROUND_DOWN_ULL((unsigned long long)(ll) + (d) - 1, (d)); \
> +	_tmp; })

Simpler:

--- a/include/linux/kernel.h~linux-kernelh-fix-overflow-for-div_round_up_ull-fix
+++ a/include/linux/kernel.h
@@ -94,7 +94,7 @@
 	({ unsigned long long _tmp = (ll); do_div(_tmp, d); _tmp; })
 
 #define DIV_ROUND_UP_ULL(ll, d) \
-	({ DIV_ROUND_DOWN_ULL((unsigned long long)(ll) + (d) - 1, (d)) })
+	DIV_ROUND_DOWN_ULL((unsigned long long)(ll) + (d) - 1, (d))
 
 #if BITS_PER_LONG == 32
 # define DIV_ROUND_UP_SECTOR_T(ll,d) DIV_ROUND_UP_ULL(ll, d)
_