bpf: Check link_create.flags parameter for multi_kprobe

[ Upstream commit 243911982aa9faf4361aa952f879331ad66933fe ]

The link_create.flags are currently not used for multi-kprobes, so return
-EINVAL if it is set, same as for other attach APIs.

We allow target_fd, on the other hand, to have an arbitrary value for
multi-kprobe, as there are existing users (libbpf) relying on this.

Fixes: 0dcac27254 ("bpf: Add multi kprobe link")
Signed-off-by: Tao Chen <chen.dylane@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250407035752.1108927-1-chen.dylane@linux.dev
Signed-off-by: Sasha Levin <sashal@kernel.org>

kernel/trace/bpf_trace.c

@@ -2932,6 +2932,9 @@ int bpf_kprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *pr
 	if (sizeof(u64) != sizeof(void *))
 		return -EOPNOTSUPP;
 
+	if (attr->link_create.flags)
+		return -EINVAL;
+
 	if (!is_kprobe_multi(prog))
 		return -EINVAL;