add rrcall to get current time

src/preload/rrcalls.h

@@ -78,3 +78,7 @@
  * process tree, such that it may run without seccomp.
  */
 #define SYS_rrcall_detach_teleport (RR_CALL_BASE + 9)
+/**
+ * Requests the current rr tick.
+ */
+#define SYS_rrcall_current_time (RR_CALL_BASE + 10)

src/record_syscall.cc

@@ -4771,6 +4771,19 @@ static Switchable rec_prepare_syscall_arch(RecordTask* t,
 
       return ALLOW_SWITCH;
     }
+    case SYS_rrcall_current_time: {
+      // Since this is "user" facing, we follow best practices for regular
+      // syscalls and make sure that unused arguments (in this case all of them)
+      // are zero.
+      bool arguments_are_zero = true;
+      Registers r = t->regs();
+      for (int i = 1; i <= 6; ++i) {
+        arguments_are_zero &= r.arg(i) == 0;
+      }
+      syscall_state.emulate_result(arguments_are_zero ? t->trace_time() : (uintptr_t)-EINVAL);
+      syscall_state.expect_errno = ENOSYS;
+      return PREVENT_SWITCH;
+    }
 
     case Arch::brk:
     case Arch::munmap:

src/test/util_internal.h

@@ -15,4 +15,8 @@ void rr_detach_teleport(void) {
   test_assert(err == 0);
 }
 
+int rr_current_time(void) {
+  return syscall(SYS_rrcall_current_time, 0, 0, 0, 0, 0, 0);
+}
+
 #endif /* RRUTIL_INTERNAL_H */