diff --git a/libc/bionic/__FD_chk.cpp b/libc/bionic/__FD_chk.cpp
index c4b55de..23d3084 100644
--- a/libc/bionic/__FD_chk.cpp
+++ b/libc/bionic/__FD_chk.cpp
@@ -30,32 +30,41 @@
 #include <sys/select.h>
 #include "libc_logging.h"
 
-extern "C" int __FD_ISSET_chk(int fd, fd_set* set) {
+extern "C" int __FD_ISSET_chk(int fd, fd_set* set, size_t set_size) {
   if (__predict_false(fd < 0)) {
     __fortify_chk_fail("file descriptor is negative for FD_ISSET", 0);
   }
   if (__predict_false(fd >= FD_SETSIZE)) {
     __fortify_chk_fail("file descriptor is too big for FD_ISSET", 0);
   }
+  if (__predict_false(set_size < sizeof(fd_set))) {
+    __fortify_chk_fail("set is too small", 0);
+  }
   return FD_ISSET(fd, set);
 }
 
-extern "C" void __FD_CLR_chk(int fd, fd_set* set) {
+extern "C" void __FD_CLR_chk(int fd, fd_set* set, size_t set_size) {
   if (__predict_false(fd < 0)) {
     __fortify_chk_fail("file descriptor is negative for FD_CLR", 0);
   }
   if (__predict_false(fd >= FD_SETSIZE)) {
     __fortify_chk_fail("file descriptor is too big for FD_CLR", 0);
   }
+  if (__predict_false(set_size < sizeof(fd_set))) {
+    __fortify_chk_fail("set is too small", 0);
+  }
   FD_CLR(fd, set);
 }
 
-extern "C" void __FD_SET_chk(int fd, fd_set* set) {
+extern "C" void __FD_SET_chk(int fd, fd_set* set, size_t set_size) {
   if (__predict_false(fd < 0)) {
     __fortify_chk_fail("file descriptor is negative for FD_SET", 0);
   }
   if (__predict_false(fd >= FD_SETSIZE)) {
     __fortify_chk_fail("file descriptor is too big for FD_SET", 0);
   }
+  if (__predict_false(set_size < sizeof(fd_set))) {
+    __fortify_chk_fail("set is too small", 0);
+  }
   FD_SET(fd, set);
 }
diff --git a/libc/include/sys/select.h b/libc/include/sys/select.h
index c8cd8e1..50ac228 100644
--- a/libc/include/sys/select.h
+++ b/libc/include/sys/select.h
@@ -32,6 +32,7 @@
 #include <sys/time.h>
 #include <sys/types.h>
 #include <signal.h>
+#include <string.h>
 
 __BEGIN_DECLS
 
@@ -46,21 +47,19 @@
 #define __FDELT(fd) ((fd) / __NFDBITS)
 #define __FDMASK(fd) (1UL << ((fd) % __NFDBITS))
 #define __FDS_BITS(set) (((fd_set*)(set))->fds_bits)
+#define __FD_ZERO(set) (memset(set, 0, sizeof(*(fd_set*)(set))))
+
+#if defined(__BIONIC_FORTIFY)
+extern void __FD_CLR_chk(int, fd_set*, size_t);
+extern void __FD_SET_chk(int, fd_set*, size_t);
+extern int  __FD_ISSET_chk(int, fd_set*, size_t);
+#define __FD_CLR(fd, set) __FD_CLR_chk(fd, set, __bos(set))
+#define __FD_SET(fd, set) __FD_SET_chk(fd, set, __bos(set))
+#define __FD_ISSET(fd, set) __FD_ISSET_chk(fd, set, __bos(set))
+#else
 #define __FD_CLR(fd, set) (__FDS_BITS(set)[__FDELT(fd)] &= ~__FDMASK(fd))
 #define __FD_SET(fd, set) (__FDS_BITS(set)[__FDELT(fd)] |= __FDMASK(fd))
 #define __FD_ISSET(fd, set) ((__FDS_BITS(set)[__FDELT(fd)] & __FDMASK(fd)) != 0)
-#define __FD_ZERO(set) (__builtin_memset(set, 0, sizeof(*(fd_set*)(set))))
-
-#if defined(__BIONIC_FORTIFY)
-extern void __FD_CLR_chk(int, fd_set*);
-extern void __FD_SET_chk(int, fd_set*);
-extern int  __FD_ISSET_chk(int, fd_set*);
-#undef __FD_CLR
-#undef __FD_SET
-#undef __FD_ISSET
-#define __FD_CLR(fd, set) __FD_CLR_chk(fd, set)
-#define __FD_SET(fd, set) __FD_SET_chk(fd, set)
-#define __FD_ISSET(fd, set) __FD_ISSET_chk(fd, set)
 #endif /* defined(__BIONIC_FORTIFY) */
 
 extern int select(int, fd_set*, fd_set*, fd_set*, struct timeval*);
diff --git a/tests/fortify_test.cpp b/tests/fortify_test.cpp
index abe2cae..d514a3d 100644
--- a/tests/fortify_test.cpp
+++ b/tests/fortify_test.cpp
@@ -554,6 +554,20 @@
   ASSERT_EXIT(FD_ISSET(-1, &set), testing::KilledBySignal(SIGABRT), "");
 }
 
+TEST(DEATHTEST, FD_ISSET_2_fortified) {
+  ::testing::FLAGS_gtest_death_test_style = "threadsafe";
+  char buf[1];
+  fd_set* set = (fd_set*) buf;
+  ASSERT_EXIT(FD_ISSET(0, set), testing::KilledBySignal(SIGABRT), "");
+}
+
+TEST(DEATHTEST, FD_ZERO_fortified) {
+  ::testing::FLAGS_gtest_death_test_style = "threadsafe";
+  char buf[1];
+  fd_set* set = (fd_set*) buf;
+  ASSERT_EXIT(FD_ZERO(set), testing::KilledBySignal(SIGABRT), "");
+}
+
 extern "C" char* __strncat_chk(char*, const char*, size_t, size_t);
 extern "C" char* __strcat_chk(char*, const char*, size_t);