selftests/bpf: Test task_file iterator without visiting pthreads

Modified existing bpf_iter_test_file.c program to check whether
all accessed files from the main thread or not.

Modified existing bpf_iter_test_file program to check
whether all accessed files from the main thread or not.
  $ ./test_progs -n 4
  ...
  #4/7 task_file:OK
  ...
  #4 bpf_iter:OK
  Summary: 1/24 PASSED, 0 SKIPPED, 0 FAILED
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200902023113.1672863-1-yhs@fb.com

tools/testing/selftests/bpf/prog_tests/bpf_iter.c

@@ -132,17 +132,38 @@ static void test_task_stack(void)
 	bpf_iter_task_stack__destroy(skel);
 }
 
+static void *do_nothing(void *arg)
+{
+	pthread_exit(arg);
+}
+
 static void test_task_file(void)
 {
 	struct bpf_iter_task_file *skel;
+	pthread_t thread_id;
+	void *ret;
 
 	skel = bpf_iter_task_file__open_and_load();
 	if (CHECK(!skel, "bpf_iter_task_file__open_and_load",
 		  "skeleton open_and_load failed\n"))
 		return;
 
+	skel->bss->tgid = getpid();
+
+	if (CHECK(pthread_create(&thread_id, NULL, &do_nothing, NULL),
+		  "pthread_create", "pthread_create failed\n"))
+		goto done;
+
 	do_dummy_read(skel->progs.dump_task_file);
 
+	if (CHECK(pthread_join(thread_id, &ret) || ret != NULL,
+		  "pthread_join", "pthread_join failed\n"))
+		goto done;
+
+	CHECK(skel->bss->count != 0, "check_count",
+	      "invalid non pthread file visit count %d\n", skel->bss->count);
+
+done:
 	bpf_iter_task_file__destroy(skel);
 }
 

tools/testing/selftests/bpf/progs/bpf_iter_task_file.c

@@ -6,6 +6,9 @@
 
 char _license[] SEC("license") = "GPL";
 
+int count = 0;
+int tgid = 0;
+
 SEC("iter/task_file")
 int dump_task_file(struct bpf_iter__task_file *ctx)
 {
@@ -17,8 +20,13 @@ int dump_task_file(struct bpf_iter__task_file *ctx)
 	if (task == (void *)0 || file == (void *)0)
 		return 0;
 
-	if (ctx->meta->seq_num == 0)
+	if (ctx->meta->seq_num == 0) {
+		count = 0;
 		BPF_SEQ_PRINTF(seq, "    tgid      gid       fd      file\n");
+	}
+
+	if (tgid == task->tgid && task->tgid != task->pid)
+		count++;
 
 	BPF_SEQ_PRINTF(seq, "%8d %8d %8d %lx\n", task->tgid, task->pid, fd,
 		       (long)file->f_op);