selftests/bpf: Add exception handling selftests for tp_bpf program

Exception handling is triggered in BPF tracing programs when a NULL pointer
is dereferenced; the exception handler zeroes the target register and
execution of the BPF program progresses.

To test exception handling then, we need to trigger a NULL pointer dereference
for a field which should never be zero; if it is, the only explanation is the
exception handler ran. task->task_works is the NULL pointer chosen (for a new
task from fork() no work is associated), and the task_works->func field should
not be zero if task_works is non-NULL. The test verifies that task_works and
task_works->func are 0.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/1636131046-5982-3-git-send-email-alan.maguire@oracle.com

1 files changed, 43 insertions, 0 deletions

diff --git a/tools/testing/selftests/bpf/progs/exhandler_kern.c b/tools/testing/selftests/bpf/progs/exhandler_kern.c
new file mode 100644
index 000000000000..f5ca142abf8f
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/exhandler_kern.c
@@ -0,0 +1,43 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2021, Oracle and/or its affiliates. */
+
+#include "vmlinux.h"
+
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include <bpf/bpf_core_read.h>
+
+char _license[] SEC("license") = "GPL";
+
+unsigned int exception_triggered;
+int test_pid;
+
+/* TRACE_EVENT(task_newtask,
+ *         TP_PROTO(struct task_struct *p, u64 clone_flags)
+ */
+SEC("tp_btf/task_newtask")
+int BPF_PROG(trace_task_newtask, struct task_struct *task, u64 clone_flags)
+{
+	int pid = bpf_get_current_pid_tgid() >> 32;
+	struct callback_head *work;
+	void *func;
+
+	if (test_pid != pid)
+		return 0;
+
+	/* To verify we hit an exception we dereference task->task_works->func.
+	 * If task work has been added,
+	 * - task->task_works is non-NULL; and
+	 * - task->task_works->func is non-NULL also (the callback function
+	 *   must be specified for the task work.
+	 *
+	 * However, for a newly-created task, task->task_works is NULLed,
+	 * so we know the exception handler triggered if task_works is
+	 * NULL and func is NULL.
+	 */
+	work = task->task_works;
+	func = work->func;
+	if (!work && !func)
+		exception_triggered++;
+	return 0;
+}