From: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Date: Thu, 30 Sep 2021 11:46:34 +0530
Subject: libbpf: Fix segfault in light skeleton for objects without BTF
Patch-mainline: v5.15 or v5.15-rc5 (next release)
Git-commit: 4729445b47efebf089da4ccbcd1b116ffa2ad4af
References: git-fixes stable-5.14.12

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
Acked-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
---
 tools/lib/bpf/libbpf.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -8060,7 +8060,8 @@ int bpf_object__load_xattr(struct bpf_ob
 
 	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)