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/prog_tests/exhandler.c b/tools/testing/selftests/bpf/prog_tests/exhandler.c
new file mode 100644
index 000000000000..118bb182ee20
--- /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);
+}