linux-kernel - [PATCH 5.14 066/151] libbpf: Fix segfault in light skeleton for objects without BTF

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Message-Id: <20211011134519.983304176@linuxfoundation.org>
Date:   Mon, 11 Oct 2021 15:45:38 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Kumar Kartikeya Dwivedi <memxor@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.14 066/151] libbpf: Fix segfault in light skeleton for objects without BTF

From: Kumar Kartikeya Dwivedi <memxor@...il.com>

[ Upstream commit 4729445b47efebf089da4ccbcd1b116ffa2ad4af ]

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@...il.com>
Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
Link: https://lore.kernel.org/bpf/20210930061634.1840768-1-memxor@gmail.com
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 tools/lib/bpf/libbpf.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index d27e017ebfbe..f1bc09e606cd 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -8115,7 +8115,8 @@ 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)
-- 
2.33.0