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Subject: Re: [PATCH v2] btf: warn but return no error for NULL btf from
 __register_btf_kfunc_id_set()
To:     SeongJae Park <sj@kernel.org>, martin.lau@linux.dev
Cc:     Alexander.Egorenkov@ibm.com, ast@kernel.org, memxor@gmail.com,
        olsajiri@gmail.com, bpf@vger.kernel.org, stable@vger.kernel.org,
        linux-kernel@vger.kernel.org, Jiri Olsa <jolsa@kernel.org>
References: <20230628164611.83038-1-sj@kernel.org>
From:   Daniel Borkmann <daniel@iogearbox.net>
Message-ID: <bae7e6d7-d7f4-a507-ee80-44e45aa4f2b4@iogearbox.net>
Date:   Fri, 30 Jun 2023 16:53:38 +0200
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On 6/28/23 6:46 PM, SeongJae Park wrote:
> __register_btf_kfunc_id_set() assumes .BTF to be part of the module's
> .ko file if CONFIG_DEBUG_INFO_BTF is enabled.  If that's not the case,
> the function prints an error message and return an error.  As a result,
> such modules cannot be loaded.
> 
> However, the section could be stripped out during a build process.  It
> would be better to let the modules loaded, because their basic
> functionalities have no problem[1], though the BTF functionalities will
> not be supported.  Make the function to lower the level of the message
> from error to warn, and return no error.
> 
> [1] https://lore.kernel.org/bpf/20220219082037.ow2kbq5brktf4f2u@apollo.legion/
> 
> Reported-by: Alexander Egorenkov <Alexander.Egorenkov@ibm.com>
> Link: https://lore.kernel.org/bpf/87y228q66f.fsf@oc8242746057.ibm.com/
> Suggested-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> Link: https://lore.kernel.org/bpf/20220219082037.ow2kbq5brktf4f2u@apollo.legion/
> Fixes: c446fdacb10d ("bpf: fix register_btf_kfunc_id_set for !CONFIG_DEBUG_INFO_BTF")
> Cc: <stable@vger.kernel.org> # 5.18.x
> Signed-off-by: SeongJae Park <sj@kernel.org>
> Acked-by: Jiri Olsa <jolsa@kernel.org>

I presume this one is targeted at bpf (rather than bpf-next) tree, right?

> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 6b682b8e4b50..d683f034996f 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -7848,14 +7848,10 @@ static int __register_btf_kfunc_id_set(enum btf_kfunc_hook hook,
>   
>   	btf = btf_get_module_btf(kset->owner);
>   	if (!btf) {
> -		if (!kset->owner && IS_ENABLED(CONFIG_DEBUG_INFO_BTF)) {
> -			pr_err("missing vmlinux BTF, cannot register kfuncs\n");
> -			return -ENOENT;
> -		}

Why the above one needs to be changed? Do you also run into this case? vmlinux BTF
should be built-in in this case. I understand it's rather the one below for BTF +
modules instead, no?

> -		if (kset->owner && IS_ENABLED(CONFIG_DEBUG_INFO_BTF_MODULES)) {
> -			pr_err("missing module BTF, cannot register kfuncs\n");
> -			return -ENOENT;
> -		}
> +		if (!kset->owner && IS_ENABLED(CONFIG_DEBUG_INFO_BTF))
> +			pr_warn("missing vmlinux BTF, cannot register kfuncs\n");
> +		if (kset->owner && IS_ENABLED(CONFIG_DEBUG_INFO_BTF_MODULES))
> +			pr_warn("missing module BTF, cannot register kfuncs\n");
>   		return 0;
>   	}
>   	if (IS_ERR(btf))
>