linux-kernel - Re: [PATCH bpf-next] bpf: Don't use idx variable when registering kfunc dtors

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Date:   Wed, 23 Nov 2022 07:14:04 -0800
From:   Yonghong Song <yhs@...a.com>
To:     David Vernet <void@...ifault.com>, bpf@...r.kernel.org
Cc:     ast@...nel.org, andrii@...nel.org, daniel@...earbox.net,
        martin.lau@...ux.dev, yhs@...com, song@...nel.org, sdf@...gle.com,
        john.fastabend@...il.com, kpsingh@...nel.org, jolsa@...nel.org,
        haoluo@...gle.com, tj@...nel.org, kernel-team@...com,
        linux-kernel@...r.kernel.org, kernel test robot <lkp@...el.com>
Subject: Re: [PATCH bpf-next] bpf: Don't use idx variable when registering
 kfunc dtors



On 11/23/22 5:52 AM, David Vernet wrote:
> In commit fda01efc6160 ("bpf: Enable cgroups to be used as kptrs"), I
> added an 'int idx' variable to kfunc_init() which was meant to
> dynamically set the index of the btf id entries of the
> 'generic_dtor_ids' array. This was done to make the code slightly less
> brittle as the struct cgroup * kptr kfuncs such as bpf_cgroup_aquire()
> are compiled out if CONFIG_CGROUPS is not defined. This, however, causes
> an lkp build warning:
> 
>>> kernel/bpf/helpers.c:2005:40: warning: multiple unsequenced
>     modifications to 'idx' [-Wunsequenced]
> 	.btf_id       = generic_dtor_ids[idx++],
> 
> Fix the warning by just hard-coding the indices.
> 
> Fixes: fda01efc6160 ("bpf: Enable cgroups to be used as kptrs")
> Reported-by: kernel test robot <lkp@...el.com>
> Signed-off-by: David Vernet <void@...ifault.com>

I hit the same issue with clang build and used the same workaround below.

Acked-by: Yonghong Song <yhs@...com>

> ---
>   kernel/bpf/helpers.c | 10 +++++-----
>   1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index e4e9db301db5..da2681ebb7c3 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -2019,16 +2019,16 @@ static const struct btf_kfunc_id_set common_kfunc_set = {
>   
>   static int __init kfunc_init(void)
>   {
> -	int ret, idx = 0;
> +	int ret;
>   	const struct btf_id_dtor_kfunc generic_dtors[] = {
>   		{
> -			.btf_id       = generic_dtor_ids[idx++],
> -			.kfunc_btf_id = generic_dtor_ids[idx++]
> +			.btf_id       = generic_dtor_ids[0],
> +			.kfunc_btf_id = generic_dtor_ids[1]
>   		},
>   #ifdef CONFIG_CGROUPS
>   		{
> -			.btf_id       = generic_dtor_ids[idx++],
> -			.kfunc_btf_id = generic_dtor_ids[idx++]
> +			.btf_id       = generic_dtor_ids[2],
> +			.kfunc_btf_id = generic_dtor_ids[3]
>   		},
>   #endif
>   	};