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From:   SeongJae Park <sj@kernel.org>
To:     Daniel Borkmann <daniel@iogearbox.net>
Cc:     SeongJae Park <sj@kernel.org>, 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>
Subject: Re: [PATCH v2] btf: warn but return no error for NULL btf from __register_btf_kfunc_id_set()
Date:   Fri, 30 Jun 2023 21:02:51 +0000
Message-Id: <20230630210251.126928-1-sj@kernel.org>
In-Reply-To: <75483b53-bd8e-692e-ed18-a4c87cf20a1b@iogearbox.net>
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On Fri, 30 Jun 2023 22:52:24 +0200 Daniel Borkmann <daniel@iogearbox.net> wrote:

> 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.

Thank you!  I will send it by tomorrow, to give people enough time to comment.
If you don't want to wait, please let me know :)

Also, please note that this will not cleanly applicable on 6.1.y.  I will
provide the backport to stable@ as soon as this is merged into the mainline.


Thanks,
SJ

> 
> Thanks,
> Daniel
>