[PATCH v2 bpf-next 2/2] selftests/bpf: add exception handling selftests for tp_bpf program

[Alan Maguire]

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.  Test verifies task_works and
task_works->func are 0.

Signed-off-by: Alan Maguire <alan.maguire@xxxxxxxxxx>
---
 tools/testing/selftests/bpf/prog_tests/exhandler.c | 43 ++++++++++++++++++++++
 tools/testing/selftests/bpf/progs/exhandler_kern.c | 43 ++++++++++++++++++++++
 2 files changed, 86 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/exhandler.c
 create mode 100644 tools/testing/selftests/bpf/progs/exhandler_kern.c

diff --git a/tools/testing/selftests/bpf/prog_tests/exhandler.c b/tools/testing/selftests/bpf/prog_tests/exhandler.c
new file mode 100644
index 0000000..118bb18
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/exhandler.c
@@ -0,0 +1,43 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2021, Oracle and/or its affiliates. */
+
+#include <test_progs.h>
+
+/* Test that verifies exception handling is working. fork()
+ * triggers task_newtask tracepoint; that new task will have a
+ * NULL pointer task_works, and the associated task->task_works->func
+ * should not be NULL if task_works itself is non-NULL.
+ *
+ * So to verify exception handling we want to see a NULL task_works
+ * and task_works->func; if we see this we can conclude that the
+ * exception handler ran when we attempted to dereference task->task_works
+ * and zeroed the destination register.
+ */
+#include "exhandler_kern.skel.h"
+
+void test_exhandler(void)
+{
+	int err = 0, duration = 0, status;
+	struct exhandler_kern *skel;
+	pid_t cpid;
+
+	skel = exhandler_kern__open_and_load();
+	if (CHECK(!skel, "skel_load", "skeleton failed: %d\n", err))
+		goto cleanup;
+
+	skel->bss->test_pid = getpid();
+
+	err = exhandler_kern__attach(skel);
+	if (!ASSERT_OK(err, "attach"))
+		goto cleanup;
+	cpid = fork();
+	if (!ASSERT_GT(cpid, -1, "fork failed"))
+		goto cleanup;
+	if (cpid == 0)
+		_exit(0);
+	waitpid(cpid, &status, 0);
+
+	ASSERT_NEQ(skel->bss->exception_triggered, 0, "verify exceptions occurred");
+cleanup:
+	exhandler_kern__destroy(skel);
+}
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 0000000..f5ca142
--- /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;
+}
-- 
1.8.3.1