Index: lib/asan/asan_interceptors.h
===================================================================
--- lib/asan/asan_interceptors.h
+++ lib/asan/asan_interceptors.h
@@ -40,12 +40,6 @@
 # define ASAN_USE_ALIAS_ATTRIBUTE_FOR_INDEX 0
 #endif
 
-#if !SANITIZER_MAC
-# define ASAN_INTERCEPT_STRNLEN 1
-#else
-# define ASAN_INTERCEPT_STRNLEN 0
-#endif
-
 #if SANITIZER_LINUX && !SANITIZER_ANDROID
 # define ASAN_INTERCEPT_SWAPCONTEXT 1
 #else
Index: lib/asan/asan_interceptors.cc
===================================================================
--- lib/asan/asan_interceptors.cc
+++ lib/asan/asan_interceptors.cc
@@ -111,7 +111,7 @@
 } while (0)
 
 static inline uptr MaybeRealStrnlen(const char *s, uptr maxlen) {
-#if ASAN_INTERCEPT_STRNLEN
+#if SANITIZER_INTERCEPT_STRNLEN
   if (REAL(strnlen)) {
     return REAL(strnlen)(s, maxlen);
   }
@@ -583,19 +583,6 @@
   return REAL(strncpy)(to, from, size);
 }
 
-#if ASAN_INTERCEPT_STRNLEN
-INTERCEPTOR(uptr, strnlen, const char *s, uptr maxlen) {
-  void *ctx;
-  ASAN_INTERCEPTOR_ENTER(ctx, strnlen);
-  ENSURE_ASAN_INITED();
-  uptr length = REAL(strnlen)(s, maxlen);
-  if (flags()->replace_str) {
-    ASAN_READ_RANGE(ctx, s, Min(length + 1, maxlen));
-  }
-  return length;
-}
-#endif  // ASAN_INTERCEPT_STRNLEN
-
 INTERCEPTOR(long, strtol, const char *nptr,  // NOLINT
             char **endptr, int base) {
   void *ctx;
@@ -728,9 +715,6 @@
   ASAN_INTERCEPT_FUNC(strncat);
   ASAN_INTERCEPT_FUNC(strncpy);
   ASAN_INTERCEPT_FUNC(strdup);
-#if ASAN_INTERCEPT_STRNLEN
-  ASAN_INTERCEPT_FUNC(strnlen);
-#endif
 #if ASAN_INTERCEPT_INDEX && ASAN_USE_ALIAS_ATTRIBUTE_FOR_INDEX
   ASAN_INTERCEPT_FUNC(index);
 #endif
Index: lib/msan/msan_interceptors.cc
===================================================================
--- lib/msan/msan_interceptors.cc
+++ lib/msan/msan_interceptors.cc
@@ -44,6 +44,7 @@
 using __sanitizer::atomic_uintptr_t;
 
 DECLARE_REAL(SIZE_T, strlen, const char *s)
+DECLARE_REAL(SIZE_T, strnlen, const char *s, SIZE_T maxlen)
 
 #if SANITIZER_FREEBSD
 #define __errno_location __error
@@ -282,14 +283,6 @@
 #define MSAN_MAYBE_INTERCEPT_MALLOC_STATS
 #endif
 
-INTERCEPTOR(SIZE_T, strnlen, const char *s, SIZE_T n) {
-  ENSURE_MSAN_INITED();
-  SIZE_T res = REAL(strnlen)(s, n);
-  SIZE_T scan_size = (res == n) ? res : res + 1;
-  CHECK_UNPOISONED(s, scan_size);
-  return res;
-}
-
 INTERCEPTOR(char *, strcpy, char *dest, const char *src) {  // NOLINT
   ENSURE_MSAN_INITED();
   GET_STORE_STACK_TRACE;
@@ -1555,7 +1548,6 @@
   INTERCEPT_FUNCTION(strndup);
   MSAN_MAYBE_INTERCEPT___STRNDUP;
   INTERCEPT_FUNCTION(strncpy);  // NOLINT
-  INTERCEPT_FUNCTION(strnlen);
   INTERCEPT_FUNCTION(gcvt);
   INTERCEPT_FUNCTION(strcat);  // NOLINT
   INTERCEPT_FUNCTION(strncat);  // NOLINT
Index: lib/sanitizer_common/sanitizer_common_interceptors.inc
===================================================================
--- lib/sanitizer_common/sanitizer_common_interceptors.inc
+++ lib/sanitizer_common/sanitizer_common_interceptors.inc
@@ -218,6 +218,20 @@
 #define INIT_STRLEN
 #endif
 
+#if SANITIZER_INTERCEPT_STRNLEN
+INTERCEPTOR(SIZE_T, strnlen, const char *s, SIZE_T maxlen) {
+  void *ctx;
+  COMMON_INTERCEPTOR_ENTER(ctx, strnlen, s, maxlen);
+  SIZE_T length = REAL(strnlen)(s, maxlen);
+  if (common_flags()->intercept_strlen)
+    COMMON_INTERCEPTOR_READ_RANGE(ctx, s, Min(length + 1, maxlen));
+  return length;
+}
+#define INIT_STRNLEN COMMON_INTERCEPT_FUNCTION(strnlen)
+#else
+#define INIT_STRNLEN
+#endif
+
 #if SANITIZER_INTERCEPT_TEXTDOMAIN
 INTERCEPTOR(char*, textdomain, const char *domainname) {
   void *ctx;
@@ -5445,6 +5459,7 @@
 
   INIT_TEXTDOMAIN;
   INIT_STRLEN;
+  INIT_STRNLEN;
   INIT_STRCMP;
   INIT_STRNCMP;
   INIT_STRCASECMP;
Index: lib/sanitizer_common/sanitizer_flags.inc
===================================================================
--- lib/sanitizer_common/sanitizer_flags.inc
+++ lib/sanitizer_common/sanitizer_flags.inc
@@ -183,7 +183,7 @@
             "If set, uses custom wrappers for strpbrk function "
             "to find more errors.")
 COMMON_FLAG(bool, intercept_strlen, true,
-            "If set, uses custom wrappers for strlen function "
+            "If set, uses custom wrappers for strlen and strnlen functions "
             "to find more errors.")
 COMMON_FLAG(bool, intercept_strchr, true,
             "If set, uses custom wrappers for strchr, strchrnul, and strrchr "
Index: lib/sanitizer_common/sanitizer_platform_interceptors.h
===================================================================
--- lib/sanitizer_common/sanitizer_platform_interceptors.h
+++ lib/sanitizer_common/sanitizer_platform_interceptors.h
@@ -43,8 +43,10 @@
 
 #if SANITIZER_MAC
 # define SI_MAC 1
+# define SI_NOT_MAC 0
 #else
 # define SI_MAC 0
+# define SI_NOT_MAC 1
 #endif
 
 #if SANITIZER_IOS
@@ -60,6 +62,7 @@
 #endif
 
 #define SANITIZER_INTERCEPT_STRLEN 1
+#define SANITIZER_INTERCEPT_STRNLEN SI_NOT_MAC
 #define SANITIZER_INTERCEPT_STRCMP 1
 #define SANITIZER_INTERCEPT_STRSTR 1
 #define SANITIZER_INTERCEPT_STRCASESTR SI_NOT_WINDOWS
Index: test/sanitizer_common/TestCases/strnlen.c
===================================================================
--- /dev/null
+++ test/sanitizer_common/TestCases/strnlen.c
@@ -0,0 +1,12 @@
+// RUN: %clang %s -o %t && %run %t 2>&1
+
+#include <assert.h>
+#include <string.h>
+int main(int argc, char **argv) {
+  const char *s = "mytest";
+  assert(strnlen(s, 0) == 0UL);
+  assert(strnlen(s, 1) == 1UL);
+  assert(strnlen(s, 6) == strlen(s));
+  assert(strnlen(s, 7) == strlen(s));
+  return 0;
+}