Re: [PATCH bpf-next 2/2] 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.  The skb->sk is
the NULL pointer chosen (for a ping received for 127.0.0.1 there
is no associated socket), and the sk_sndbuf size is chosen as the
"should never be 0" field.  Test verifies sk is NULL and sk_sndbuf
is zero.

Signed-off-by: Alan Maguire <redacted>
---
 tools/testing/selftests/bpf/prog_tests/exhandler.c | 45 ++++++++++++++++++++++
 tools/testing/selftests/bpf/progs/exhandler_kern.c | 35 +++++++++++++++++
 2 files changed, 80 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..5999498
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/exhandler.c

@@ -0,0 +1,45 @@
+// 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; ping to localhost
+ * will result in a receive with a NULL skb->sk; our BPF program
+ * then dereferences the an sk field which shouldn't be 0, and if we
+ * see 0 we can conclude the exception handler ran when we attempted to
+ * dereference the NULL sk and zeroed the destination register.
+ */
+#include "exhandler_kern.skel.h"
+
+#define SYSTEM(...)    \
+       (env.verbosity >= VERBOSE_VERY ?        \
+        system(__VA_ARGS__) : system(__VA_ARGS__ " >/dev/null 2>&1"))
+
+void test_exhandler(void)
+{
+       struct exhandler_kern *skel;
+       struct exhandler_kern__bss *bss;
+       int err = 0, duration = 0;
+
+       skel = exhandler_kern__open_and_load();
+       if (CHECK(!skel, "skel_load", "skeleton failed: %d\n", err))
+               goto cleanup;
+
+       bss = skel->bss;

+                 "verify exceptions were triggered",
+                 "no exceptions were triggered\n"))
+               goto cleanup;
+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..4049450
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/exhandler_kern.c

@@ -0,0 +1,35 @@
+// 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;
+
+/* TRACE_EVENT(netif_rx,
+ *         TP_PROTO(struct sk_buff *skb),
+ */
+SEC("tp_btf/netif_rx")
+int BPF_PROG(trace_netif_rx, struct sk_buff *skb)
+{
+       struct sock *sk;
+       int sndbuf;
+
+       /* To verify we hit an exception we dereference skb->sk->sk_sndbuf;
+        * sndbuf size should never be zero, so if it is we know the exception
+        * handler triggered and zeroed the destination register.
+        */
+       __builtin_preserve_access_index(({
+               sk = skb->sk;
+               sndbuf = sk->sk_sndbuf;
+       }));