Add a test BPF program with a ring buffer.

This adds an example BPF program with a ring buffer. The program using
the ring buffer is limited to kernel versions 5.8 and above (the same as
the ringbuffer internally). The program is marked critical to ensure
that the platform can support ringbuffers in the bpfloader.

This is only done for userdebug builds currently since tests that use it
can only be run in userdebug builds and statically allocates a 4KB ring
buffer.

Bug: 246985031
Test: build and flash on 4.19 and 5.10
Change-Id: I3cdb4a03051f832915bb784d43c01392c087f54c

bpfloader/Android.bp

@@ -59,4 +59,11 @@ cc_binary {
         "timeInState.o"
     ],
 
+    product_variables: {
+        debuggable: {
+            required: [
+                "bpfRingbufProg.o",
+            ],
+        },
+    }
 }

progs/Android.bp

@@ -26,3 +26,12 @@ cc_library_headers {
     name: "bpf_prog_headers",
     export_include_dirs: ["include"],
 }
+
+bpf {
+    name: "bpfRingbufProg.o",
+    srcs: ["bpfRingbufProg.c"],
+    cflags: [
+        "-Wall",
+        "-Werror",
+    ],
+}

progs/bpfRingbufProg.c

@@ -0,0 +1,40 @@
+/*
+ * Copyright (C) 2022 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "bpf_helpers.h"
+
+// This can't be easily changed since the program is loaded on boot and may be
+// run against tests at a slightly different version.
+#define TEST_RINGBUF_MAGIC_NUM 12345
+
+// This ring buffer is for testing purposes only.
+DEFINE_BPF_RINGBUF_EXT(test_ringbuf, __u64, 4096, AID_ROOT, AID_ROOT, 0660, "", "", false);
+
+// This program is for test purposes only - it should never be attached to a
+// socket, only executed manually with BPF_PROG_RUN.
+DEFINE_BPF_PROG_KVER("skfilter/ringbuf_test", AID_ROOT, AID_ROOT, test_ringbuf_prog, KVER(5, 8, 0))
+(void* unused_ctx) {
+    __u64* output = bpf_test_ringbuf_reserve();
+    if (output == NULL) return 1;
+
+    (*output) = TEST_RINGBUF_MAGIC_NUM;
+    bpf_test_ringbuf_submit(output);
+
+    return 0;
+}
+
+LICENSE("Apache 2.0");
+CRITICAL("BPF Ringbuf test");