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

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Date:   Tue, 25 Jun 2019 15:29:38 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Vinod Koul <vkoul@...nel.org>
Cc:     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 15:35:18 +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; })
 
 #if BITS_PER_LONG == 32
 # define DIV_ROUND_UP_SECTOR_T(ll,d) DIV_ROUND_UP_ULL(ll, d)
_