Fix issue where fcntl was not using last arg correctly (#24)

* Fix issue where fcntl was not using last arg correctly

* Comment update

Author: cjappl

compiler-rt/lib/radsan/radsan_interceptors.cpp

@@ -89,10 +89,20 @@ INTERCEPTOR(int, fcntl, int filedes, int cmd, ...) {
 
   va_list args;
   va_start(args, cmd);
-  void *arg = va_arg(args, void *);
+
+  // Following precedent here. The linux source (fcntl.c, do_fcntl) accepts the
+  // final argument in a variable that will hold the largest of the possible
+  // argument types (pointers and ints are typical in fcntl) It is then assumed
+  // that the implementation of fcntl will cast it properly depending on cmd.
+  //
+  // This is also similar to what is done in
+  // sanitizer_common/sanitizer_common_syscalls.inc
+  const unsigned long arg = va_arg(args, unsigned long);
+  int result = REAL(fcntl)(filedes, cmd, arg);
+
   va_end(args);
 
-  return fcntl(filedes, cmd, arg);
+  return result;
 }
 
 INTERCEPTOR(int, close, int filedes) {

compiler-rt/lib/radsan/tests/radsan_test_interceptors.cpp

@@ -261,6 +261,28 @@ TEST(TestRadsanInterceptors, CloseDiesWhenRealtime) {
   expectNonrealtimeSurvival(Func);
 }
 
+TEST(TestRadsanInterceptors, fcntlSetFdDiesWhenRealtime) {
+  int fd = creat(TemporaryFilePath(), S_IRUSR | S_IWUSR);
+  ASSERT_THAT(fd, Ne(-1));
+
+  auto func = [fd]() {
+    int old_flags = fcntl(fd, F_GETFD);
+    ASSERT_THAT(fcntl(fd, F_SETFD, FD_CLOEXEC), Eq(0));
+
+    int flags = fcntl(fd, F_GETFD);
+    ASSERT_THAT(flags, Ne(-1));
+    ASSERT_THAT(flags & FD_CLOEXEC, Eq(FD_CLOEXEC));
+
+    ASSERT_THAT(fcntl(fd, F_SETFD, old_flags), Eq(0));
+    ASSERT_THAT(fcntl(fd, F_GETFD), Eq(old_flags));
+  };
+
+  ExpectRealtimeDeath(func, "fcntl");
+  ExpectNonRealtimeSurvival(func);
+
+  close(fd);
+}
+
 TEST(TestRadsanInterceptors, FopenDiesWhenRealtime) {
   auto Func = []() {
     FILE *fd = fopen(TemporaryFilePath(), "w");