linux-kernel - Re: [PATCH v2] btf: warn but return no error for NULL btf from __register_btf_kfunc_id

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Message-Id: <20230630194859.100332-1-sj@kernel.org>
Date:   Fri, 30 Jun 2023 19:48:59 +0000
From:   SeongJae Park <sj@...nel.org>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     SeongJae Park <sj@...nel.org>, martin.lau@...ux.dev,
        Alexander.Egorenkov@....com, ast@...nel.org, memxor@...il.com,
        olsajiri@...il.com, bpf@...r.kernel.org, stable@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jiri Olsa <jolsa@...nel.org>
Subject: Re: [PATCH v2] btf: warn but return no error for NULL btf from __register_btf_kfunc_id_set()

Hi Daniel,

On Fri, 30 Jun 2023 16:53:38 +0200 Daniel Borkmann <daniel@...earbox.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@....com>
> > Link: https://lore.kernel.org/bpf/87y228q66f.fsf@oc8242746057.ibm.com/
> > Suggested-by: Kumar Kartikeya Dwivedi <memxor@...il.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@...r.kernel.org> # 5.18.x
> > Signed-off-by: SeongJae Park <sj@...nel.org>
> > Acked-by: Jiri Olsa <jolsa@...nel.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.

> 
> > 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.


Thanks,
SJ

> 
> > -		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))
> > 
> 
>