bpf: Allow bpf_d_path in bpf_iter program

task_file and task_vma iter programs have access to file->f_path. Enable
bpf_d_path to print paths of these file.
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210212183107.50963-3-songliubraving@fb.com

kernel/trace/bpf_trace.c

@@ -1191,6 +1191,10 @@ BTF_SET_END(btf_allowlist_d_path)
 
 static bool bpf_d_path_allowed(const struct bpf_prog *prog)
 {
+	if (prog->type == BPF_PROG_TYPE_TRACING &&
+	    prog->expected_attach_type == BPF_TRACE_ITER)
+		return true;
+
 	if (prog->type == BPF_PROG_TYPE_LSM)
 		return bpf_lsm_is_sleepable_hook(prog->aux->attach_btf_id);