Set idmap2 binary uid and gid after forking

The file permissions of the idmap2 binary are currently not set
correctly when the system forks and execs the idmap binary during
zygote. This chnages sets the uid and gid after forking to the same uid
and gid of the parent process.

Bug: 134897503
Test: device boots and generates idmap
Change-Id: Ic7fac49e5982f3c47713603b905c3a6be117a05b

libs/androidfw/PosixUtils.cpp

@@ -64,6 +64,9 @@ std::unique_ptr<ProcResult> ExecuteBinary(const std::vector<std::string>& argv)
     return nullptr;
   }
 
+  auto gid = getgid();
+  auto uid = getuid();
+
   char const** argv0 = (char const**)malloc(sizeof(char*) * (argv.size() + 1));
   for (size_t i = 0; i < argv.size(); i++) {
     argv0[i] = argv[i].c_str();
@@ -75,6 +78,16 @@ std::unique_ptr<ProcResult> ExecuteBinary(const std::vector<std::string>& argv)
       PLOG(ERROR) << "fork";
       return nullptr;
     case 0: // child
+      if (setgid(gid) != 0) {
+        PLOG(ERROR) << "setgid";
+        exit(1);
+      }
+
+      if (setuid(uid) != 0) {
+        PLOG(ERROR) << "setuid";
+        exit(1);
+      }
+
       close(stdout[0]);
       if (dup2(stdout[1], STDOUT_FILENO) == -1) {
         abort();