Partially revert "bpfloader - remove dead code"

* Restore support for k4.9 devices

This partially reverts commit 90a866e

Change-Id: I3650744f8e9ea1c86a90be2dccd5d10d868eee44
Signed-off-by: Nanhumly <admin@warbler.cc>

libbpf_android/Loader.cpp

@@ -689,6 +689,15 @@ static std::optional<unique_fd> getMapBtfInfo(const char* elfPath,
 
 static bool mapMatchesExpectations(const unique_fd& fd, const string& mapName,
                                    const struct bpf_map_def& mapDef, const enum bpf_map_type type) {
+    // bpfGetFd... family of functions require at minimum a 4.14 kernel,
+    // so on 4.9 kernels just pretend the map matches our expectations.
+    // This isn't really a problem as we only really support 4.14+ anyway...
+    // Additionally we'll get almost equivalent test coverage on newer devices/kernels.
+    // This is because the primary failure mode we're trying to detect here
+    // is either a source code misconfiguration (which is likely kernel independent)
+    // or a newly introduced kernel feature/bug (which is unlikely to get backported to 4.9).
+    if (!isAtLeastKernelVersion(4, 14, 0)) return true;
+
     // Assuming fd is a valid Bpf Map file descriptor then
     // all the following should always succeed on a 4.14+ kernel.
     // If they somehow do fail, they'll return -1 (and set errno),
@@ -828,6 +837,16 @@ static int createMaps(const char* elfPath, ifstream& elfFile, vector<unique_fd>&
         }
 
         enum bpf_map_type type = md[i].type;
+        if (type == BPF_MAP_TYPE_DEVMAP && !isAtLeastKernelVersion(4, 14, 0)) {
+            // On Linux Kernels older than 4.14 this map type doesn't exist, but it can kind
+            // of be approximated: ARRAY has the same userspace api, though it is not usable
+            // by the same ebpf programs.  However, that's okay because the bpf_redirect_map()
+            // helper doesn't exist on 4.9 anyway (so the bpf program would fail to load,
+            // and thus needs to be tagged as 4.14+ either way), so there's nothing useful you
+            // could do with a DEVMAP anyway (that isn't already provided by an ARRAY)...
+            // Hence using an ARRAY instead of a DEVMAP simply makes life easier for userspace.
+            type = BPF_MAP_TYPE_ARRAY;
+        }
         if (type == BPF_MAP_TYPE_DEVMAP_HASH && !isAtLeastKernelVersion(5, 4, 0)) {
             // On Linux Kernels older than 5.4 this map type doesn't exist, but it can kind
             // of be approximated: HASH has the same userspace visible api.