linux-kernel - Re: [PATCH][next] bcache: Use 64-bit arithmetic instead of 32-bit

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Date:   Fri, 12 Feb 2021 22:22:44 +0800
From:   Coly Li <colyli@...e.de>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Kent Overstreet <kent.overstreet@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Christina Jacob <cjacob@...vell.com>,
        Hariprasad Kelam <hkelam@...vell.com>,
        Sunil Goutham <sgoutham@...vell.com>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>
Cc:     linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] bcache: Use 64-bit arithmetic instead of 32-bit

On 2/12/21 8:50 PM, Gustavo A. R. Silva wrote:
> Cast multiple variables to (int64_t) in order to give the compiler
> complete information about the proper arithmetic to use. Notice that
> these variables are being used in contexts that expect expressions of
> type int64_t  (64 bit, signed). And currently, such expressions are
> being evaluated using 32-bit arithmetic.
> 
> Fixes: d0cf9503e908 ("octeontx2-pf: ethtool fec mode support")
> Addresses-Coverity-ID: 1501724 ("Unintentional integer overflow")
> Addresses-Coverity-ID: 1501725 ("Unintentional integer overflow")
> Addresses-Coverity-ID: 1501726 ("Unintentional integer overflow")
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> ---
>  drivers/md/bcache/writeback.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
> index 82d4e0880a99..4fb635c0baa0 100644
> --- a/drivers/md/bcache/writeback.c
> +++ b/drivers/md/bcache/writeback.c
> @@ -110,13 +110,13 @@ static void __update_writeback_rate(struct cached_dev *dc)
>  		int64_t fps;
>  
>  		if (c->gc_stats.in_use <= BCH_WRITEBACK_FRAGMENT_THRESHOLD_MID) {
> -			fp_term = dc->writeback_rate_fp_term_low *
> +			fp_term = (int64_t)dc->writeback_rate_fp_term_low *
>  			(c->gc_stats.in_use - BCH_WRITEBACK_FRAGMENT_THRESHOLD_LOW);
>  		} else if (c->gc_stats.in_use <= BCH_WRITEBACK_FRAGMENT_THRESHOLD_HIGH) {
> -			fp_term = dc->writeback_rate_fp_term_mid *
> +			fp_term = (int64_t)dc->writeback_rate_fp_term_mid *
>  			(c->gc_stats.in_use - BCH_WRITEBACK_FRAGMENT_THRESHOLD_MID);
>  		} else {
> -			fp_term = dc->writeback_rate_fp_term_high *
> +			fp_term = (int64_t)dc->writeback_rate_fp_term_high *
>  			(c->gc_stats.in_use - BCH_WRITEBACK_FRAGMENT_THRESHOLD_HIGH);
>  		}
>  		fps = div_s64(dirty, dirty_buckets) * fp_term;
> 

Hmm, should such thing be handled by compiler ?  Otherwise this kind of
potential overflow issue will be endless time to time.

I am not a compiler expert, should we have to do such explicit type cast
all the time ?

Coly Li