introduce mapRetrieveLocklessRW()

the clat verifyPerms() checker only wants to check the
existence and accessibility of the maps.

Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I3079ab316bcd88370d774fec5e88c1b5ff438266
diff --git a/service/jni/com_android_server_connectivity_ClatCoordinator.cpp b/service/jni/com_android_server_connectivity_ClatCoordinator.cpp
index 18a274a..c0082bb 100644
--- a/service/jni/com_android_server_connectivity_ClatCoordinator.cpp
+++ b/service/jni/com_android_server_connectivity_ClatCoordinator.cpp
@@ -80,7 +80,8 @@
       case VERIFY_BIN: return;
       case VERIFY_PROG:   fd = bpf::retrieveProgram(path); break;
       case VERIFY_MAP_RO: fd = bpf::mapRetrieveRO(path); break;
-      case VERIFY_MAP_RW: fd = bpf::mapRetrieveRW(path); break;
+      // lockless: we're just checking access rights and will immediately close the fd
+      case VERIFY_MAP_RW: fd = bpf::mapRetrieveLocklessRW(path); break;
     }
 
     if (fd < 0) ALOGF("bpf_obj_get '%s' failed, errno=%d", path, errno);
diff --git a/staticlibs/native/bpf_syscall_wrappers/include/BpfSyscallWrappers.h b/staticlibs/native/bpf_syscall_wrappers/include/BpfSyscallWrappers.h
index 5bb3100..73cef89 100644
--- a/staticlibs/native/bpf_syscall_wrappers/include/BpfSyscallWrappers.h
+++ b/staticlibs/native/bpf_syscall_wrappers/include/BpfSyscallWrappers.h
@@ -166,11 +166,16 @@
     return ret;  // most likely -1 with errno == EAGAIN, due to already held lock
 }
 
-inline int mapRetrieveExclusiveRW(const char* pathname) {
-    return bpfLock(bpfFdGet(pathname, 0), F_WRLCK);
+inline int mapRetrieveLocklessRW(const char* pathname) {
+    return bpfFdGet(pathname, 0);
 }
+
+inline int mapRetrieveExclusiveRW(const char* pathname) {
+    return bpfLock(mapRetrieveLocklessRW(pathname), F_WRLCK);
+}
+
 inline int mapRetrieveRW(const char* pathname) {
-    return bpfLock(bpfFdGet(pathname, 0), F_RDLCK);
+    return bpfLock(mapRetrieveLocklessRW(pathname), F_RDLCK);
 }
 
 inline int mapRetrieveRO(const char* pathname) {