[PATCH bpf-next v2 1/2] bpf: use proper target btf when exporting attach_btf_obj_id

[Stanislav Fomichev <sdf@google.com>]

When attaching to program, the program itself might not be attached
to anything (and, hence, might not have attach_btf), so we can't
unconditionally use 'prog->aux->dst_prog->aux->attach_btf'.
Instead, use bpf_prog_get_target_btf to pick proper target btf:

* when attached to dst_prog, use dst_prog->aux->btf
* when attached to kernel btf, use prog->aux->attach_btf

Fixes: b79c9fc9551b ("bpf: implement BPF_PROG_QUERY for BPF_LSM_CGROUP")
Acked-by: Hao Luo <haoluo@google.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
---
 kernel/bpf/syscall.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 83c7136c5788..7dc3f8003631 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -3886,6 +3886,7 @@ static int bpf_prog_get_info_by_fd(struct file *file,
 				   union bpf_attr __user *uattr)
 {
 	struct bpf_prog_info __user *uinfo = u64_to_user_ptr(attr->info.info);
+	struct btf *attach_btf = bpf_prog_get_target_btf(prog);
 	struct bpf_prog_info info;
 	u32 info_len = attr->info.info_len;
 	struct bpf_prog_kstats stats;
@@ -4088,10 +4089,8 @@ static int bpf_prog_get_info_by_fd(struct file *file,
 	if (prog->aux->btf)
 		info.btf_id = btf_obj_id(prog->aux->btf);
 	info.attach_btf_id = prog->aux->attach_btf_id;
-	if (prog->aux->attach_btf)
-		info.attach_btf_obj_id = btf_obj_id(prog->aux->attach_btf);
-	else if (prog->aux->dst_prog)
-		info.attach_btf_obj_id = btf_obj_id(prog->aux->dst_prog->aux->attach_btf);
+	if (attach_btf)
+		info.attach_btf_obj_id = btf_obj_id(attach_btf);
 
 	ulen = info.nr_func_info;
 	info.nr_func_info = prog->aux->func_info_cnt;
-- 
2.37.1.455.g008518b4e5-goog