NetBpfLoad: create/populate/pin kernel bugs bpf array

This is an array of two ints:
- index 0 should have value 0
  (a value of 1 implies a kernel uapi bug)
- index 1 should have value 123
  (a value of 0 implies bpf array writes at non-zero index fail)

Note that these bugs can only be present on Android S & Sv2,
as these kernel bugs are considered fatal on Android T+.

(btw. we need to do this write manually as we cannot write into
maps we pin and then reopen - due to sepolicy)

Test: TreeHugger
Flag: EXEMPT mainline
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I9b2ada7f3e299014d39461a8d0718396daf0c91a

bpf/loader/NetBpfLoad.cpp

@@ -1910,13 +1910,30 @@ static int doLoad(char** argv, char * const envp[]) {
         return 2;
     }
 
-    int key = 1;
-    int value = 123;
-    unique_fd map(
-            createMap(BPF_MAP_TYPE_ARRAY, sizeof(key), sizeof(value), 2, 0));
-    if (writeToMapEntry(map, &key, &value, BPF_ANY)) {
-        ALOGE("Critical kernel bug - failure to write into index 1 of 2 element bpf map array.");
-        if (isAtLeastT) return 1;
+    {
+        // Create a trivial bpf map: a two element array [int->int]
+        unique_fd map(createMap(BPF_MAP_TYPE_ARRAY, sizeof(int), sizeof(int), 2, 0));
+
+        int zero = 0;
+        int kernel_bugs = bpfCmdFixupIsNeeded;
+        if (writeToMapEntry(map, &zero, &kernel_bugs, BPF_ANY)) {
+            ALOGE("Failure to write into index 0 of kernel bugs array.");
+            return 1;
+        }
+
+        int one = 1;
+        int value = 123;
+        if (writeToMapEntry(map, &one, &value, BPF_ANY)) {
+            ALOGE("Critical kernel bug - failure to write into index 1 of 2 element bpf map array.");
+            if (isAtLeastT) return 1;
+        }
+
+        int ret = bpfFdPin(map, "/sys/fs/bpf/tethering/map_kernel_bugs");
+        if (ret) {
+            const int err = errno;
+            ALOGE("pin -> %d [%d:%s]", ret, err, strerror(err));
+            return -err;
+        }
     }
 
     // leave a flag that we're done

bpf/tests/mts/bpf_existence_test.cpp

@@ -60,6 +60,7 @@ static const set<string> PLATFORM_ONLY_IN_R = {
 
 // Provided by *current* mainline module for S+ devices
 static const set<string> MAINLINE_FOR_S_PLUS = {
+    TETHERING "map_kernel_bugs",
     TETHERING "map_offload_tether_dev_map",
     TETHERING "map_offload_tether_downstream4_map",
     TETHERING "map_offload_tether_downstream64_map",