NetBpfLoad: add self-check for bpfGetNext{Prog,Map}Id()

This detects some kernel bugs, particularly on Xiaomi devices.

Bug: 414570528
Test: TreeHugger
Flag: EXEMPT mainline
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: Ic099c53dc1ad5f01bfebfddd29b1024f5b936fe5

bpf/loader/NetBpfLoad.cpp

@@ -1833,6 +1833,50 @@ static int doLoad(char** argv, char * const envp[]) {
         if (writeProcSysFile("/proc/sys/net/core/bpf_jit_kallsyms", "1\n")) return 1;
     }
 
+    if (runningAsRoot) {  // implies U QPR3+ and kernel 4.14+
+        // There should not be any programs or maps yet
+        errno = 0;
+        uint32_t progId = bpfGetNextProgId(0);  // expect 0 with errno == ENOENT
+        if (progId || errno != ENOENT) {
+            ALOGE("bpfGetNextProgId(zero) returned %u (errno %d)", progId, errno);
+            return 1;
+        }
+        errno = 0;
+        uint32_t mapId = bpfGetNextMapId(0);  // expect 0 with errno == ENOENT
+        if (mapId || errno != ENOENT) {
+            ALOGE("bpfGetNextMapId(zero) returned %u (errno %d)", mapId, errno);
+            return 1;
+        }
+    } else if (isAtLeastKernelVersion(4, 14, 0)) {  // implies S through U QPR2
+        // bpfGetNext{Prog,Map}Id require 4.14+
+        // furthermore since we're not running as root, we're not the initial
+        // platform bpfloader, so there may already be some maps & programs.
+        uint32_t mapId = 0;
+        while (true) {
+            errno = 0;
+            uint32_t next = bpfGetNextMapId(mapId);
+            if (!next && errno == ENOENT) break;
+            if (next <= mapId) {
+                ALOGE("bpfGetNextMapId(%u) returned %u errno %d", mapId, next, errno);
+                return 1;
+            }
+            mapId = next;
+        }
+        // mapId is now the last map id, creating a new map should change that
+        unique_fd map(createMap(BPF_MAP_TYPE_ARRAY, sizeof(int), sizeof(int), 1, 0));
+        errno = 0;
+        uint32_t next = bpfGetNextMapId(mapId);
+        if (next <= mapId) {
+            // We should fail here on Xiaomi S 4.14.180 due to kernel uapi bug,
+            // which causes bpfGetNextMapId to behave as bpfGetNextProgId,
+            // and thus it should return 0 with errno == ENOENT.
+            ALOGE("bpfGetNextMapId(final %d) returned %d errno %d", mapId, next, errno);
+            return 1;
+        }
+    } else {  // implies S/T with 4.9 kernel
+        // nothing we can do.
+    }
+
     // Create all the pin subdirectories
     // (this must be done first to allow selinux_context and pin_subdir functionality,
     //  which could otherwise fail with ENOENT during object pinning or renaming,

bpf/syscall_wrappers/include/BpfSyscallWrappers.h

@@ -251,6 +251,18 @@ inline int runProgram(const borrowed_fd &prog_fd, const void *data,
                });
 }
 
+// 4.14+: returns next id > prog_id, or 0 (and sets errno)
+inline uint32_t bpfGetNextProgId(const uint32_t prog_id) {
+    bpf_attr arg = { .start_id = prog_id };
+    return bpf(BPF_PROG_GET_NEXT_ID, &arg) ? 0 : arg.next_id;
+}
+
+// 4.14+: returns next id > map_id, or 0 (and sets errno)
+inline uint32_t bpfGetNextMapId(const uint32_t map_id) {
+    bpf_attr arg = { .start_id = map_id };
+    return bpf(BPF_MAP_GET_NEXT_ID, &arg) ? 0 : arg.next_id;
+}
+
 // BPF_OBJ_GET_INFO_BY_FD requires 4.14+ kernel
 //
 // Note: some fields are only defined in newer kernels (ie. the map_info struct grows