libbpf: use negative fd to specify missing BTF

Hello Andrii Nakryiko,

The patch fba01a0689a9: "libbpf: use negative fd to specify missing BTF" from May 29, 2019, leads to the following static checker warning:

	./tools/lib/bpf/libbpf.c:1757 bpf_object__create_maps()
	warn: always true condition '(create_attr.btf_fd >= 0) => (0-u32max >= 0)'

./tools/lib/bpf/libbpf.c
1735 if (obj->caps.name)
1736 create_attr.name = map->name;

  1737                  create_attr.map_ifindex = map->map_ifindex;
  1738                  create_attr.map_type = def->type;
  1739                  create_attr.map_flags = def->map_flags;
  1740                  create_attr.key_size = def->key_size;
  1741                  create_attr.value_size = def->value_size;
  1742                  create_attr.max_entries = def->max_entries;
  1743                  create_attr.btf_fd = -1;
                        ^^^^^^^^^^^^^^^^^^^^^^^

.btf_fd is a __u32

  1744                  create_attr.btf_key_type_id = 0;
  1745                  create_attr.btf_value_type_id = 0;
  1746                  if (bpf_map_type__is_map_in_map(def->type) &&
  1747                      map->inner_map_fd >= 0)
  1748                          create_attr.inner_map_fd = map->inner_map_fd;
  1749  
  1750                  if (obj->btf && !bpf_map_find_btf_info(map, obj->btf)) {
  1751                          create_attr.btf_fd = btf__fd(obj->btf);
  1752                          create_attr.btf_key_type_id = map->btf_key_type_id;
  1753                          create_attr.btf_value_type_id = map->btf_value_type_id;
  1754                  }
  1755  
  1756                  *pfd = bpf_create_map_xattr(&create_attr);
  1757                  if (*pfd < 0 && create_attr.btf_fd >= 0) {
                                        ^^^^^^^^^^^^^^^^^^^^^^^

Always true condition.

  1758                          cp = libbpf_strerror_r(errno, errmsg, sizeof(errmsg));
  1759                          pr_warning("Error in bpf_create_map_xattr(%s):%s(%d). Retrying without BTF.\n",
  1760                                     map->name, cp, errno);
  1761                          create_attr.btf_fd = -1;
  1762                          create_attr.btf_key_type_id = 0;
  1763                          create_attr.btf_value_type_id = 0;
  1764                          map->btf_key_type_id = 0;
  1765                          map->btf_value_type_id = 0;
  1766                          *pfd = bpf_create_map_xattr(&create_attr);
  1767                  }

regards,
dan carpenter

Thanks for reporting this! I'll post a fix shortly.