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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>
Cc:     martin.lau@linux.dev, 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: <20230630194859.100332-1-sj@kernel.org>
From:   Daniel Borkmann <daniel@iogearbox.net>
Message-ID: <75483b53-bd8e-692e-ed18-a4c87cf20a1b@iogearbox.net>
Date:   Fri, 30 Jun 2023 22:52:24 +0200
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On 6/30/23 9:48 PM, SeongJae Park wrote:
> On Fri, 30 Jun 2023 16:53:38 +0200 Daniel Borkmann <daniel@iogearbox.net> wrote:
>> 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?
> 
> You're correct.  It's not urgent for us, but I would prefer it to be merged
> into all affected kernels as early as possible.

Ok, sounds good, bpf tree it is then.

>>> 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?
> 
> Again, you're correct.  This change is not really needed.  I was interpreting
> Kumar's suggestion merely into code without thinking about his real meaning,
> sorry.  I will restore this in the next spin.

Perfect, I think after your v3 respin it should be good to land.

Thanks,
Daniel