libbpf: Add NULL check to add_dummy_ksym_var

Avoids a segv if btf isn't present. Seen on the call path
__bpf_object__open calling bpf_object__collect_externs.

Fixes: 5bd022ec01f0 (libbpf: Support extern kernel function)
Suggested-by: Stanislav Fomichev <sdf@google.com>
Suggested-by: Petar Penkov <ppenkov@google.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210504234910.976501-1-irogers@google.com

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index e2a3cf4..c41d9b2 100644 (file)
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -3216,6 +3216,9 @@ static int add_dummy_ksym_var(struct btf *btf)
        const struct btf_var_secinfo *vs;
        const struct btf_type *sec;
 
        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)
        sec_btf_id = btf__find_by_name_kind(btf, KSYMS_SEC,
                                            BTF_KIND_DATASEC);
        if (sec_btf_id < 0)