1// SPDX-License-Identifier: GPL-2.0
2/* Copyright (c) 2021, Oracle and/or its affiliates. */
3
4#include "vmlinux.h"
5
6#include <bpf/bpf_helpers.h>
7#include <bpf/bpf_tracing.h>
8#include <bpf/bpf_core_read.h>
9
10char _license[] SEC("license") = "GPL";
11
12unsigned int exception_triggered;
13int test_pid;
14
15/* TRACE_EVENT(task_newtask,
16 *         TP_PROTO(struct task_struct *p, u64 clone_flags)
17 */
18SEC("tp_btf/task_newtask")
19int BPF_PROG(trace_task_newtask, struct task_struct *task, u64 clone_flags)
20{
21	int pid = bpf_get_current_pid_tgid() >> 32;
22	struct callback_head *work;
23	void *func;
24
25	if (test_pid != pid)
26		return 0;
27
28	/* To verify we hit an exception we dereference task->task_works->func.
29	 * If task work has been added,
30	 * - task->task_works is non-NULL; and
31	 * - task->task_works->func is non-NULL also (the callback function
32	 *   must be specified for the task work.
33	 *
34	 * However, for a newly-created task, task->task_works is NULLed,
35	 * so we know the exception handler triggered if task_works is
36	 * NULL and func is NULL.
37	 */
38	work = task->task_works;
39	func = work->func;
40	/* Currently verifier will fail for `btf_ptr |= btf_ptr` * instruction.
41	 * To workaround the issue, use barrier_var() and rewrite as below to
42	 * prevent compiler from generating verifier-unfriendly code.
43	 */
44	barrier_var(work);
45	if (work)
46		return 0;
47	barrier_var(func);
48	if (func)
49		return 0;
50	exception_triggered++;
51	return 0;
52}
53