diff --git a/compiler-rt/lib/dfsan/dfsan_custom.cpp b/compiler-rt/lib/dfsan/dfsan_custom.cpp
--- a/compiler-rt/lib/dfsan/dfsan_custom.cpp
+++ b/compiler-rt/lib/dfsan/dfsan_custom.cpp
@@ -652,6 +652,37 @@
   return ret;
 }
 
+SANITIZER_INTERFACE_ATTRIBUTE char *__dfsw_strncat(
+    char *dest, const char *src, size_t num, dfsan_label dest_label,
+    dfsan_label src_label, dfsan_label num_label, dfsan_label *ret_label) {
+  size_t src_len = strlen(src);
+  src_len = src_len < num ? src_len : num;
+  size_t dest_len = strlen(dest);
+
+  char *ret = strncat(dest, src, num);
+  dfsan_mem_shadow_transfer(dest + dest_len, src, src_len);
+  *ret_label = dest_label;
+  return ret;
+}
+
+SANITIZER_INTERFACE_ATTRIBUTE char *__dfso_strncat(
+    char *dest, const char *src, size_t num, dfsan_label dest_label,
+    dfsan_label src_label, dfsan_label num_label, dfsan_label *ret_label,
+    dfsan_origin dest_origin, dfsan_origin src_origin, dfsan_origin num_origin,
+    dfsan_origin *ret_origin) {
+  size_t src_len = strlen(src);
+  src_len = src_len < num ? src_len : num;
+  size_t dest_len = strlen(dest);
+
+  char *ret = strncat(dest, src, num);
+
+  dfsan_mem_origin_transfer(dest + dest_len, src, src_len);
+  dfsan_mem_shadow_transfer(dest + dest_len, src, src_len);
+  *ret_label = dest_label;
+  *ret_origin = dest_origin;
+  return ret;
+}
+
 SANITIZER_INTERFACE_ATTRIBUTE char *
 __dfsw_strdup(const char *s, dfsan_label s_label, dfsan_label *ret_label) {
   size_t len = strlen(s);
diff --git a/compiler-rt/lib/dfsan/done_abilist.txt b/compiler-rt/lib/dfsan/done_abilist.txt
--- a/compiler-rt/lib/dfsan/done_abilist.txt
+++ b/compiler-rt/lib/dfsan/done_abilist.txt
@@ -88,6 +88,7 @@
 fun:ispunct=functional
 fun:isspace=functional
 fun:tolower=functional
+fun:_tolower=functional
 fun:toupper=functional
 
 # Functions that return a value that is data-dependent on the input.
@@ -268,6 +269,7 @@
 fun:strtoul=custom
 fun:strtoull=custom
 fun:strcat=custom
+fun:strncat=custom
 
 # Functions that produce an output that is computed from the input, but is not
 # necessarily data dependent.
diff --git a/compiler-rt/test/dfsan/custom.cpp b/compiler-rt/test/dfsan/custom.cpp
--- a/compiler-rt/test/dfsan/custom.cpp
+++ b/compiler-rt/test/dfsan/custom.cpp
@@ -326,7 +326,7 @@
 #endif
 }
 
-void test_strcat() {
+void _test_strcat(int _strncat) {
   char src[] = "world";
   int volatile x = 0; // buffer to ensure src and dst do not share origins
   (void)x;
@@ -339,7 +339,12 @@
   dfsan_set_label(j_label, dst, sizeof(dst));
   dfsan_origin dst_o = dfsan_get_origin((long)dst[0]);
   (void)dst_o;
-  char *ret = strcat(p, src);
+  char *ret = 0;
+  if (!_strncat)
+    ret = strcat(p, src);
+  else
+    ret = strncat(p, src, strlen(src));
+
   ASSERT_LABEL(ret, k_label);
   ASSERT_EQ_ORIGIN(ret, p);
   assert(ret == dst);
@@ -367,6 +372,10 @@
   ASSERT_LABEL(dst[11], j_label);
 }
 
+void test_strcat() { _test_strcat(0); }
+
+void test_strncat() { _test_strcat(1); }
+
 void test_strlen() {
   char str1[] = "str1";
   dfsan_set_label(i_label, &str1[3], 1);
@@ -2082,6 +2091,7 @@
   test_strchr();
   test_strcmp();
   test_strcat();
+  test_strncat();
   test_strcpy();
   test_strdup();
   test_strlen();