diff --git a/NEWS b/NEWS
index 0820984547..df46560878 100644
--- a/NEWS
+++ b/NEWS
@@ -28,6 +28,12 @@ Major new features:
   The 32-bit RISC-V port requires at least Linux 5.4, GCC 7.1 and binutils
   2.28.
 
+* A new fortification level _FORTIFY_SOURCE=3 is available.  At this level,
+  glibc may use additional checks that may have an additional performance
+  overhead.  At present these checks are available only on LLVM 9 and later.
+  The latest GCC available at this time (10.2) does not support this level of
+  fortification.
+
 Deprecated and removed features, and other changes affecting compatibility:
 
 * The mallinfo function is marked deprecated.  Callers should call
diff --git a/include/features.h b/include/features.h
index f3e62d3362..b1225d5b3f 100644
--- a/include/features.h
+++ b/include/features.h
@@ -397,10 +397,17 @@
 #  warning _FORTIFY_SOURCE requires compiling with optimization (-O)
 # elif !__GNUC_PREREQ (4, 1)
 #  warning _FORTIFY_SOURCE requires GCC 4.1 or later
-# elif _FORTIFY_SOURCE > 1
+# elif _FORTIFY_SOURCE == 3 && !__glibc_clang_prereq(9, 0)
+#  warning _FORTIFY_SOURCE > 2 requires LLVM 9.0 or later, falling back to 2
 #  define __USE_FORTIFY_LEVEL 2
+# elif _FORTIFY_SOURCE > 2
+#  if __glibc_clang_prereq(9, 0)
+#   define __USE_FORTIFY_LEVEL 3
+#  else
+#   define __USE_FORTIFY_LEVEL 2
+#  endif
 # else
-#  define __USE_FORTIFY_LEVEL 1
+#  define __USE_FORTIFY_LEVEL _FORTIFY_SOURCE
 # endif
 #endif
 #ifndef __USE_FORTIFY_LEVEL
diff --git a/include/string.h b/include/string.h
index 7d344d77d4..841ee05a1d 100644
--- a/include/string.h
+++ b/include/string.h
@@ -123,10 +123,11 @@ libc_hidden_proto (__strerror_l)
 void __explicit_bzero_chk_internal (void *, size_t, size_t)
   __THROW __nonnull ((1)) attribute_hidden;
 # define explicit_bzero(buf, len) \
-  __explicit_bzero_chk_internal (buf, len, __bos0 (buf))
+  __explicit_bzero_chk_internal (buf, len, __objsize0 (buf))
 #elif !IS_IN (nonlib)
 void __explicit_bzero_chk (void *, size_t, size_t) __THROW __nonnull ((1));
-# define explicit_bzero(buf, len) __explicit_bzero_chk (buf, len, __bos0 (buf))
+# define explicit_bzero(buf, len) __explicit_bzero_chk (buf, len,	      \
+							__objsize0 (buf))
 #endif
 
 libc_hidden_builtin_proto (memchr)
diff --git a/manual/creature.texi b/manual/creature.texi
index be5050468b..31208ccb2b 100644
--- a/manual/creature.texi
+++ b/manual/creature.texi
@@ -254,7 +254,8 @@ included.
 @standards{GNU, (none)}
 If this macro is defined to @math{1}, security hardening is added to
 various library functions.  If defined to @math{2}, even stricter
-checks are applied.
+checks are applied. If defined to @math{3}, @theglibc{} may also use
+checks that may have an additional performance overhead.
 @end defvr
 
 @defvr Macro _REENTRANT
diff --git a/misc/sys/cdefs.h b/misc/sys/cdefs.h
index a06f1cfd91..ca51a5c3ad 100644
--- a/misc/sys/cdefs.h
+++ b/misc/sys/cdefs.h
@@ -127,6 +127,15 @@
 #define __bos(ptr) __builtin_object_size (ptr, __USE_FORTIFY_LEVEL > 1)
 #define __bos0(ptr) __builtin_object_size (ptr, 0)
 
+/* Use __builtin_dynamic_object_size at _FORTIFY_SOURCE=3 when available.  */
+#if __USE_FORTIFY_LEVEL == 3 && __glibc_clang_prereq (9, 0)
+# define __objsize0(__o) __builtin_dynamic_object_size (__o, 0)
+# define __objsize(__o) __builtin_dynamic_object_size (__o, 1)
+#else
+# define __objsize0(__o) __bos0 (__o)
+# define __objsize(__o) __bos (__o)
+#endif
+
 #if __GNUC_PREREQ (4,3)
 # define __warnattr(msg) __attribute__((__warning__ (msg)))
 # define __errordecl(name, msg) \
diff --git a/string/bits/string_fortified.h b/string/bits/string_fortified.h
index 4c1aeb45f1..c9f9197aef 100644
--- a/string/bits/string_fortified.h
+++ b/string/bits/string_fortified.h
@@ -26,13 +26,13 @@ __fortify_function void *
 __NTH (memcpy (void *__restrict __dest, const void *__restrict __src,
 	       size_t __len))
 {
-  return __builtin___memcpy_chk (__dest, __src, __len, __bos0 (__dest));
+  return __builtin___memcpy_chk (__dest, __src, __len, __objsize0 (__dest));
 }
 
 __fortify_function void *
 __NTH (memmove (void *__dest, const void *__src, size_t __len))
 {
-  return __builtin___memmove_chk (__dest, __src, __len, __bos0 (__dest));
+  return __builtin___memmove_chk (__dest, __src, __len, __objsize0 (__dest));
 }
 
 #ifdef __USE_GNU
@@ -40,7 +40,7 @@ __fortify_function void *
 __NTH (mempcpy (void *__restrict __dest, const void *__restrict __src,
 		size_t __len))
 {
-  return __builtin___mempcpy_chk (__dest, __src, __len, __bos0 (__dest));
+  return __builtin___mempcpy_chk (__dest, __src, __len, __objsize0 (__dest));
 }
 #endif
 
@@ -53,7 +53,7 @@ __NTH (mempcpy (void *__restrict __dest, const void *__restrict __src,
 __fortify_function void *
 __NTH (memset (void *__dest, int __ch, size_t __len))
 {
-  return __builtin___memset_chk (__dest, __ch, __len, __bos0 (__dest));
+  return __builtin___memset_chk (__dest, __ch, __len, __objsize0 (__dest));
 }
 
 #ifdef __USE_MISC
@@ -65,21 +65,21 @@ void __explicit_bzero_chk (void *__dest, size_t __len, size_t __destlen)
 __fortify_function void
 __NTH (explicit_bzero (void *__dest, size_t __len))
 {
-  __explicit_bzero_chk (__dest, __len, __bos0 (__dest));
+  __explicit_bzero_chk (__dest, __len, __objsize0 (__dest));
 }
 #endif
 
 __fortify_function char *
 __NTH (strcpy (char *__restrict __dest, const char *__restrict __src))
 {
-  return __builtin___strcpy_chk (__dest, __src, __bos (__dest));
+  return __builtin___strcpy_chk (__dest, __src, __objsize (__dest));
 }
 
 #ifdef __USE_GNU
 __fortify_function char *
 __NTH (stpcpy (char *__restrict __dest, const char *__restrict __src))
 {
-  return __builtin___stpcpy_chk (__dest, __src, __bos (__dest));
+  return __builtin___stpcpy_chk (__dest, __src, __objsize (__dest));
 }
 #endif
 
@@ -88,14 +88,14 @@ __fortify_function char *
 __NTH (strncpy (char *__restrict __dest, const char *__restrict __src,
 		size_t __len))
 {
-  return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
+  return __builtin___strncpy_chk (__dest, __src, __len, __objsize (__dest));
 }
 
 #if __GNUC_PREREQ (4, 7) || __glibc_clang_prereq (2, 6)
 __fortify_function char *
 __NTH (stpncpy (char *__dest, const char *__src, size_t __n))
 {
-  return __builtin___stpncpy_chk (__dest, __src, __n, __bos (__dest));
+  return __builtin___stpncpy_chk (__dest, __src, __n, __objsize (__dest));
 }
 #else
 extern char *__stpncpy_chk (char *__dest, const char *__src, size_t __n,
@@ -118,7 +118,7 @@ __NTH (stpncpy (char *__dest, const char *__src, size_t __n))
 __fortify_function char *
 __NTH (strcat (char *__restrict __dest, const char *__restrict __src))
 {
-  return __builtin___strcat_chk (__dest, __src, __bos (__dest));
+  return __builtin___strcat_chk (__dest, __src, __objsize (__dest));
 }
 
 
@@ -126,7 +126,7 @@ __fortify_function char *
 __NTH (strncat (char *__restrict __dest, const char *__restrict __src,
 		size_t __len))
 {
-  return __builtin___strncat_chk (__dest, __src, __len, __bos (__dest));
+  return __builtin___strncat_chk (__dest, __src, __len, __objsize (__dest));
 }
 
 #endif /* bits/string_fortified.h */
diff --git a/string/bits/strings_fortified.h b/string/bits/strings_fortified.h
index d4091f4f69..122199e036 100644
--- a/string/bits/strings_fortified.h
+++ b/string/bits/strings_fortified.h
@@ -22,13 +22,13 @@
 __fortify_function void
 __NTH (bcopy (const void *__src, void *__dest, size_t __len))
 {
-  (void) __builtin___memmove_chk (__dest, __src, __len, __bos0 (__dest));
+  (void) __builtin___memmove_chk (__dest, __src, __len, __objsize0 (__dest));
 }
 
 __fortify_function void
 __NTH (bzero (void *__dest, size_t __len))
 {
-  (void) __builtin___memset_chk (__dest, '\0', __len, __bos0 (__dest));
+  (void) __builtin___memset_chk (__dest, '\0', __len, __objsize0 (__dest));
 }
 
 #endif
