NetBpfLoad: eliminate 'bpfCmdFixupIsNeeded'

It turns out this doesn't work anyway:

Both our Java and C++ higher level map opening logic on 4.14+
kernels will check key/value sizes match, which will fail due to
the kernel uapi mismatch.

(Note: that the fixup was only ever applied/enabled inside of
the NetBpfLoad process itself, not in any later .so/Java/JNI)

This removes the fixup and simply chooses to not even attempt to
load bpf.  This will result in the mainline bpf provided
maps and programs simply not existing.  This would/will cause
a failure of 'bpf_existence_test' - but who cares, so long
as things keep on booting.

We need to transition to libbpf, and clearly making this work
isn't worth the considerable effort it would be, since S ebpf
functionality (ie. tethering offload) hasn't worked on the
affected devices in years.

Test: TreeHugger
Flag: EXEMPT mainline
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: Ic93a5a9d5b3ba8078500e20dfdf00ef68256874c

bpf/loader/NetBpfLoad.cpp

@@ -824,7 +824,7 @@ static int createMaps(ElfObject& elfObj, vector<struct bpf_map_def>& md, vector<
     vector<char> btfData;
     struct btf *btf = NULL;
     auto btfGuard = base::make_scope_guard([&btf] { if (btf) btf__free(btf); });
-    if (isAtLeastKernelVersion(4, 19, 0) && !bpfCmdFixupIsNeeded) {
+    if (isAtLeastKernelVersion(4, 19, 0)) {
         // On Linux Kernels older than 4.18 BPF_BTF_LOAD command doesn't exist.
         ret = elfObj.readSectionByName(".BTF", btfData);
         if (ret) {
@@ -1398,7 +1398,7 @@ static bool loadObject(const unsigned int bpfloader_ver,
 #define BPFROOT APEXROOT "/etc/bpf/mainline/"
 
 static bool loadAllObjects(const unsigned int bpfloader_ver) {
-    bool libbpf = !bpfCmdFixupIsNeeded && (isAtLeast25Q3 || USE_LIBBPF);
+    bool libbpf = isAtLeast25Q3 || USE_LIBBPF;
     if (!loadObject(bpfloader_ver, BPFROOT "offload.o")) return false;
     if (!loadObject(bpfloader_ver, BPFROOT "test.o", libbpf)) return false;
     if (isAtLeastT) {
@@ -1885,8 +1885,10 @@ static int doLoad(char** argv, char * const envp[]) {
             if (next || errno != ENOENT) return 35;
             if (isAtLeastT || isAtLeastKernelVersion(4, 20, 0)) return 36;
             // implies Android S with 4.14 or 4.19 kernel
-            ALOGW("Enabling bpfCmdFixupIsNeeded.");
-            bpfCmdFixupIsNeeded = true;
+            ALOGW("Detected kernel with invalid BPF UAPI - disabling mainline use of eBPF.");
+            // leave a flag that we're 'done'
+            if (!createDir("/sys/fs/bpf/netd_shared/mainline_done")) return 43;  // same as below
+            return 0;
         }
     } else {  // implies S/T with 4.9 kernel
         // nothing we can do.
@@ -1908,7 +1910,7 @@ static int doLoad(char** argv, char * const envp[]) {
         unique_fd map(createMap(BPF_MAP_TYPE_ARRAY, sizeof(int), sizeof(int), 2, 0));
 
         int zero = 0;
-        int kernel_bugs = bpfCmdFixupIsNeeded;
+        int kernel_bugs = 0;
         if (writeToMapEntry(map, &zero, &kernel_bugs, BPF_ANY)) {
             ALOGE("Failure to write into index 0 of kernel bugs array.");
             return 38;

bpf/syscall_wrappers/include/BpfSyscallWrappers.h

@@ -34,28 +34,18 @@ inline uint64_t ptr_to_u64(const void * const x) {
     return (uint64_t)(uintptr_t)x;
 }
 
-#ifdef BPF_SUPPORT_CMD_FIXUP
-static bool bpfCmdFixupIsNeeded = false;
-#else
-static constexpr bool bpfCmdFixupIsNeeded = false;
-#endif
-
-inline long bpfCmdFixup(enum bpf_cmd cmd) {
-    return cmd + (cmd >= BPF_PROG_TEST_RUN && bpfCmdFixupIsNeeded);
-}
-
 /* Note: bpf_attr is a union which might have a much larger size then the anonymous struct portion
  * of it that we are using.  The kernel's bpf() system call will perform a strict check to ensure
  * all unused portions are zero.  It will fail with E2BIG if we don't fully zero bpf_attr.
  */
 
 inline int bpf(enum bpf_cmd cmd, const bpf_attr& attr) {
-    return syscall(__NR_bpf, bpfCmdFixup(cmd), &attr, sizeof(attr));
+    return syscall(__NR_bpf, cmd, &attr, sizeof(attr));
 }
 
 // this version is meant for use with cmd's which mutate the argument
 inline int bpf(enum bpf_cmd cmd, bpf_attr *attr) {
-    return syscall(__NR_bpf, bpfCmdFixup(cmd), attr, sizeof(*attr));
+    return syscall(__NR_bpf, cmd, attr, sizeof(*attr));
 }
 
 inline int createMap(bpf_map_type map_type, uint32_t key_size, uint32_t value_size,