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

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

[David Vernet]

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@intel.com>
Signed-off-by: David Vernet <void@manifault.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
 	};
-- 
2.38.1

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

[Yonghong Song]

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@intel.com>
> Signed-off-by: David Vernet <void@manifault.com>

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

Acked-by: Yonghong Song <yhs@fb.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
>   	};

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

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to bpf/bpf-next.git (master)
by Alexei Starovoitov <ast@kernel.org>:

On Wed, 23 Nov 2022 07:52:53 -0600 you 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:
> 
> [...]

Here is the summary with links:
  - [bpf-next] bpf: Don't use idx variable when registering kfunc dtors
    https://git.kernel.org/bpf/bpf-next/c/2fcc6081a7bf

You are awesome, thank you!
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