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 
10 char _license[] SEC("license") = "GPL";
11 
12 unsigned int exception_triggered;
13 int test_pid;
14 
15 /* TRACE_EVENT(task_newtask,
16  *         TP_PROTO(struct task_struct *p, u64 clone_flags)
17  */
18 SEC("tp_btf/task_newtask")
BPF_PROG(trace_task_newtask,struct task_struct * task,u64 clone_flags)19 int 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