libbpf: Don't call libc APIs with NULL pointers

Sanitizer complains about qsort(), bsearch(), and memcpy() being called
with NULL pointer. This can only happen when the associated number of
elements is zero, so no harm should be done. But still prevent this from
happening to keep sanitizer runs clean from extra noise.
Signed-off-by: NAndrii Nakryiko <andrii@kernel.org>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211124002325.1737739-5-andrii@kernel.org

tools/lib/bpf/libbpf.c

@@ -3369,6 +3369,7 @@ static int bpf_object__elf_collect(struct bpf_object *obj)
 
 	/* sort BPF programs by section name and in-section instruction offset
 	 * for faster search */
+	if (obj->nr_programs)
 		qsort(obj->programs, obj->nr_programs, sizeof(*obj->programs), cmp_progs);
 
 	return bpf_object__init_btf(obj, btf_data, btf_ext_data);
@@ -5823,6 +5824,8 @@ static int cmp_relo_by_insn_idx(const void *key, const void *elem)
 
 static struct reloc_desc *find_prog_insn_relo(const struct bpf_program *prog, size_t insn_idx)
 {
+	if (!prog->nr_reloc)
+		return NULL;
 	return bsearch(&insn_idx, prog->reloc_desc, prog->nr_reloc,
 		       sizeof(*prog->reloc_desc), cmp_relo_by_insn_idx);
 }
@@ -5838,6 +5841,7 @@ static int append_subprog_relos(struct bpf_program *main_prog, struct bpf_progra
 	relos = libbpf_reallocarray(main_prog->reloc_desc, new_cnt, sizeof(*relos));
 	if (!relos)
 		return -ENOMEM;
+	if (subprog->nr_reloc)
 		memcpy(relos + main_prog->nr_reloc, subprog->reloc_desc,
 		       sizeof(*relos) * subprog->nr_reloc);