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Subject: Re: [PATCH bpf-next] bpf: Update max_entries for array maps
To:     Florian Lehner <dev@der-flo.net>, <bpf@vger.kernel.org>
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References: <20221025092843.81572-1-dev@der-flo.net>
From:   Hou Tao <houtao1@huawei.com>
Message-ID: <e250349c-cd3c-ac2e-f0fd-da083aa87ceb@huawei.com>
Date:   Wed, 26 Oct 2022 11:35:19 +0800
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Hi,

On 10/25/2022 5:28 PM, Florian Lehner wrote:
> To improve memory handling and alignment max_entries is rounded up
> before using its value to allocate memory.
> This can lead to a situation where more memory is allocated than usable
> if max_entries is no adjusted accordingly. So this change updates
> max_entries in order to make the allocated memory available.
>
> Signed-off-by: Florian Lehner <dev@der-flo.net>
> ---
>  kernel/bpf/arraymap.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
> index 832b2659e96e..9411fa255ccc 100644
> --- a/kernel/bpf/arraymap.c
> +++ b/kernel/bpf/arraymap.c
> @@ -145,6 +145,7 @@ static struct bpf_map *array_map_alloc(union bpf_attr *attr)
>  	/* copy mandatory map attributes */
>  	bpf_map_init_from_attr(&array->map, attr);
>  	array->elem_size = elem_size;
> +	array->map.max_entries = max_entries;
>  
>  	if (percpu && bpf_array_alloc_percpu(array)) {
>  		bpf_map_area_free(array);
The override of max_entries is unnecessary and is also wrong.
bpf_array_alloc_percpu() will use array->map.max_entries to allocate per-cpu
value, and if using the rounded-up max_entries, there will be memory waste
because the extra allocated per-cpu values should not be accessible to bpf
program or user-space program.