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 14:03:38 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.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 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,
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

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;
+
 	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