netdev - Re: [PATCH v4 bpf-next 07/12] libbpf: add BPF static linker BTF and BTF.ext support

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Date:   Sun, 28 Mar 2021 11:29:27 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Andrii Nakryiko <andrii@...nel.org>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v4 bpf-next 07/12] libbpf: add BPF static linker BTF and
 BTF.ext support

On Sun, Mar 28, 2021 at 5:03 AM Jiri Olsa <jolsa@...hat.com> wrote:
>
> On Fri, Mar 19, 2021 at 07:58:13PM +0100, Jiri Olsa wrote:
> > On Fri, Mar 19, 2021 at 11:39:01AM -0700, Andrii Nakryiko wrote:
> > > On Fri, Mar 19, 2021 at 9:23 AM Jiri Olsa <jolsa@...hat.com> wrote:
> > > >
> > > > On Thu, Mar 18, 2021 at 12:40:31PM -0700, Andrii Nakryiko wrote:
> > > >
> > > > SNIP
> > > >
> > > > > +
> > > > > +     return NULL;
> > > > > +}
> > > > > +
> > > > > +static int linker_fixup_btf(struct src_obj *obj)
> > > > > +{
> > > > > +     const char *sec_name;
> > > > > +     struct src_sec *sec;
> > > > > +     int i, j, n, m;
> > > > > +
> > > > > +     n = btf__get_nr_types(obj->btf);
> > > >
> > > > hi,
> > > > I'm getting bpftool crash when building tests,
> > > >
> > > > looks like above obj->btf can be NULL:
> > >
> > > I lost if (!obj->btf) return 0; somewhere along the rebases. I'll send
> > > a fix shortly. But how did you end up with selftests BPF objects built
> > > without BTF?
> >
> > no idea.. I haven't even updated llvm for almost 3 days now ;-)
>
> sorry for late follow up on this, and it's actually forgotten empty
> object in progs directory that was causing this
>
> I wonder we should add empty object like below to catch these cases,

well, feel free to chime in on [0] then

  [0] https://patchwork.kernel.org/project/netdevbpf/patch/20210319205909.1748642-4-andrii@kernel.org/

> because there's another place that bpftool is crashing on with it
>
> I can send full patch for that if you think it's worth having

sure, but see my comment below

>
> jirka
>
>
> ---
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 7aad78dbb4b4..aecb6ca52bce 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -3165,6 +3165,9 @@ static int add_dummy_ksym_var(struct btf *btf)
>         const struct btf_var_secinfo *vs;
>         const struct btf_type *sec;
>
> +       if (!btf)
> +               return 0;
> +

add_dummy_ksym_var() shouldn't be called, if there is no btf, so the
fix should be outside of this fix.

>         sec_btf_id = btf__find_by_name_kind(btf, KSYMS_SEC,
>                                             BTF_KIND_DATASEC);
>         if (sec_btf_id < 0)
> diff --git a/tools/testing/selftests/bpf/progs/empty.c b/tools/testing/selftests/bpf/progs/empty.c
> new file mode 100644
> index 000000000000..e69de29bb2d1
>