libbpf: Fix segfault in light skeleton for objects without BTF

When fed an empty BPF object, bpftool gen skeleton -L crashes at
btf__set_fd() since it assumes presence of obj->btf, however for
the sequence below clang adds no .BTF section (hence no BTF).

Reproducer:

  $ touch a.bpf.c
  $ clang -O2 -g -target bpf -c a.bpf.c
  $ bpftool gen skeleton -L a.bpf.o
  /* SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) */
  /* THIS FILE IS AUTOGENERATED! */

  struct a_bpf {
	struct bpf_loader_ctx ctx;
  Segmentation fault (core dumped)

The same occurs for files compiled without BTF info, i.e. without
clang's -g flag.

Fixes: 67234743736a (libbpf: Generate loader program out of BPF ELF file.)
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210930061634.1840768-1-memxor@gmail.com

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 88d8825..e4f83c3 100644 (file)
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -6894,7 +6894,8 @@ int bpf_object__load_xattr(struct bpf_object_load_attr *attr)
 
        if (obj->gen_loader) {
                /* reset FDs */
-               btf__set_fd(obj->btf, -1);
+               if (obj->btf)
+                       btf__set_fd(obj->btf, -1);
                for (i = 0; i < obj->nr_maps; i++)
                        obj->maps[i].fd = -1;
                if (!err)