[PATCH] libbpf: fix possible NULL pointer dereference when destroy skelton

[Yafang Shao <laoar.shao@gmail.com>]

When I checked the code in skelton header file generated with my own bpf
prog, I found there may be possible NULL pointer derefence when destroy
skelton. Then I checked the in-tree bpf progs, finding that is a common
issue. Let's take the generated samples/bpf/xdp_redirect_cpu.skel.h for
example. Below is the generated code in
xdp_redirect_cpu__create_skeleton(),
	xdp_redirect_cpu__create_skeleton
		struct bpf_object_skeleton *s;
		s = (struct bpf_object_skeleton *)calloc(1, sizeof(*s));
		if (!s)
			goto error;
		...
	error:
		bpf_object__destroy_skeleton(s);
		return  -ENOMEM;

After goto error, the NULL 's' will be deferenced in
bpf_object__destroy_skeleton().

We can simply fix this issue by just adding a NULL check in
bpf_object__destroy_skeleton().

Fixes: d66562fba ("libbpf: Add BPF object skeleton support")
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Cc: Andrii Nakryiko <andrii@kernel.org>
---
 tools/lib/bpf/libbpf.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 7c74342bb668..a07fbd59e4b8 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -11464,6 +11464,9 @@ void bpf_object__detach_skeleton(struct bpf_object_skeleton *s)
 
 void bpf_object__destroy_skeleton(struct bpf_object_skeleton *s)
 {
+	if (!s)
+		return;
+
 	if (s->progs)
 		bpf_object__detach_skeleton(s);
 	if (s->obj)
-- 
2.17.1