netdev - Re: [PATCH net v2] bpf: Fix build when CONFIG_BPF

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Date:   Thu, 11 Nov 2021 07:41:33 +0530
From:   Kumar Kartikeya Dwivedi <memxor@...il.com>
To:     Vinicius Costa Gomes <vinicius.gomes@...el.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>
Subject: Re: [PATCH net v2] bpf: Fix build when CONFIG_BPF_SYSCALL is disabled

On Thu, Nov 11, 2021 at 07:34:38AM IST, Vinicius Costa Gomes wrote:
> Hi Kartikeya,
>
> Kumar Kartikeya Dwivedi <memxor@...il.com> writes:
>
> > On Thu, Nov 11, 2021 at 05:21:53AM IST, Vinicius Costa Gomes wrote:
> >> Alexei Starovoitov <alexei.starovoitov@...il.com> writes:
> >>
> >> >> Thanks for the fix.
> >> >>
> >> >> But instead of moving this to core.c, you can probably make the btf.h
> >> >> declaration conditional on CONFIG_BPF_SYSCALL, since this is not useful in
> >> >> isolation (only used by verifier for module kfunc support). For the case of
> >> >> kfunc_btf_id_list variables, just define it as an empty struct and static
> >> >> variables, since the definition is still inside btf.c. So it becomes a noop for
> >> >> !CONFIG_BPF_SYSCALL.
> >> >>
> >> >> I am also not sure whether BTF is useful without BPF support, but maybe I'm
> >> >> missing some usecase.
> >> >
> >> > Unlikely. I would just disallow such config instead of sprinkling
> >> > the code with ifdefs.
> >>
> >> Is something like this what you have in mind?
> >>
> >> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> >> index 6fdbf9613aec..eae860c86e26 100644
> >> --- a/lib/Kconfig.debug
> >> +++ b/lib/Kconfig.debug
> >> @@ -316,6 +316,7 @@ config DEBUG_INFO_BTF
> >>  	bool "Generate BTF typeinfo"
> >>  	depends on !DEBUG_INFO_SPLIT && !DEBUG_INFO_REDUCED
> >>  	depends on !GCC_PLUGIN_RANDSTRUCT || COMPILE_TEST
> >> +	depends on BPF_SYSCALL
> >>  	help
> >>  	  Generate deduplicated BTF type information from DWARF debug info.
> >>  	  Turning this on expects presence of pahole tool, which will convert
> >>
> >>
> >
> > BTW, you will need a little more than that, I suspect the compiler optimizes out
> > the register/unregister call so we don't see a build failure, but adding a side
> > effect gives me errors, so something like this should resolve the problem (since
> > kfunc_btf_id_list variable definition is behind CONFIG_BPF_SYSCALL).
> >
> > diff --git a/include/linux/btf.h b/include/linux/btf.h
> > index 203eef993d76..e9881ef9e9aa 100644
> > --- a/include/linux/btf.h
> > +++ b/include/linux/btf.h
> > @@ -254,6 +254,8 @@ void unregister_kfunc_btf_id_set(struct kfunc_btf_id_list *l,
> >                                  struct kfunc_btf_id_set *s);
> >  bool bpf_check_mod_kfunc_call(struct kfunc_btf_id_list *klist, u32 kfunc_id,
> >                               struct module *owner);
> > +extern struct kfunc_btf_id_list bpf_tcp_ca_kfunc_list;
> > +extern struct kfunc_btf_id_list prog_test_kfunc_list;
> >  #else
> >  static inline void register_kfunc_btf_id_set(struct kfunc_btf_id_list *l,
> >                                              struct kfunc_btf_id_set *s)
> > @@ -268,13 +270,13 @@ static inline bool bpf_check_mod_kfunc_call(struct kfunc_btf_id_list *klist,
> >  {
> >         return false;
> >  }
> > +struct kfunc_btf_id_list {};
> > +static struct kfunc_btf_id_list bpf_tcp_ca_kfunc_list __maybe_unused;
> > +static struct kfunc_btf_id_list prog_test_kfunc_list __maybe_unused;
> > +
> >  #endif
> >
> >  #define DEFINE_KFUNC_BTF_ID_SET(set, name)                                     \
> >         struct kfunc_btf_id_set name = { LIST_HEAD_INIT(name.list), (set),     \
> >                                          THIS_MODULE }
> > -
> > -extern struct kfunc_btf_id_list bpf_tcp_ca_kfunc_list;
> > -extern struct kfunc_btf_id_list prog_test_kfunc_list;
> > -
> >  #endif
> >
>
> I could not reproduce the build failure here even when adding some side
> effects, but I didn't try very hard.
>
> As you are more familiar with the code, I would be glad if you could
> take it from here and propose a patch.
>

Sure, no worries!

>
> Cheers,
> --
> Vinicius

--
Kartikeya