linux-kernel - Re: [PATCH 1/2][next] mm: memcontrol: Use flex_array

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Message-ID: <20200804132526.GC7306@dhcp22.suse.cz>
Date:   Tue, 4 Aug 2020 15:25:26 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        cgroups@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 1/2][next] mm: memcontrol: Use flex_array_size() helper
 in memcpy()

On Fri 31-07-20 12:12:53, Gustavo A. R. Silva wrote:
> Make use of the flex_array_size() helper to calculate the size of a
> flexible array member within an enclosing structure.
> 
> This helper offers defense-in-depth against potential integer
> overflows, while at the same time makes it explicitly clear that
> we are dealing with a flexible array member.
> 
> Also, remove unnecessary braces.

I was not aware of this helper. The code looks slightly better.
 
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>

Acked-by: Michal Hocko <mhocko@...e.com>
> ---
>  mm/memcontrol.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index e84c2b5596f2..bd7f972ceea4 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4246,10 +4246,9 @@ static int __mem_cgroup_usage_register_event(struct mem_cgroup *memcg,
>  	new->size = size;
>  
>  	/* Copy thresholds (if any) to new array */
> -	if (thresholds->primary) {
> -		memcpy(new->entries, thresholds->primary->entries, (size - 1) *
> -				sizeof(struct mem_cgroup_threshold));
> -	}
> +	if (thresholds->primary)
> +		memcpy(new->entries, thresholds->primary->entries,
> +		       flex_array_size(new, entries, size - 1));
>  
>  	/* Add new threshold */
>  	new->entries[size - 1].eventfd = eventfd;
> -- 
> 2.27.0

-- 
Michal Hocko
SUSE Labs