diff --git a/compiler-rt/lib/dfsan/dfsan.h b/compiler-rt/lib/dfsan/dfsan.h
--- a/compiler-rt/lib/dfsan/dfsan.h
+++ b/compiler-rt/lib/dfsan/dfsan.h
@@ -46,6 +46,10 @@
 
 // Copy or move the origins of the len bytes from src to dst.
 void dfsan_mem_origin_transfer(const void *dst, const void *src, uptr len);
+
+// Copy shadow bytes from src to dst.
+// Note this preserves distinct taint labls at specific offsets.
+void dfsan_mem_shadow_transfer(void *dst, const void *src, uptr len);
 }  // extern "C"
 
 template <typename T>
diff --git a/compiler-rt/lib/dfsan/dfsan.cpp b/compiler-rt/lib/dfsan/dfsan.cpp
--- a/compiler-rt/lib/dfsan/dfsan.cpp
+++ b/compiler-rt/lib/dfsan/dfsan.cpp
@@ -401,12 +401,18 @@
   MoveOrigin(dst, src, len, &stack);
 }
 
-SANITIZER_INTERFACE_ATTRIBUTE void dfsan_mem_origin_transfer(const void *dst,
-                                                             const void *src,
-                                                             uptr len) {
+extern "C" SANITIZER_INTERFACE_ATTRIBUTE void dfsan_mem_origin_transfer(
+    const void *dst, const void *src, uptr len) {
   __dfsan_mem_origin_transfer(dst, src, len);
 }
 
+extern "C" SANITIZER_INTERFACE_ATTRIBUTE void dfsan_mem_shadow_transfer(
+    void *dst, const void *src, uptr len) {
+  internal_memcpy((void *)__dfsan::shadow_for(dst),
+                  (const void *)__dfsan::shadow_for(src),
+                  len * sizeof(dfsan_label));
+}
+
 namespace __dfsan {
 
 bool dfsan_inited = false;
@@ -414,8 +420,7 @@
 
 void dfsan_copy_memory(void *dst, const void *src, uptr size) {
   internal_memcpy(dst, src, size);
-  internal_memcpy((void *)shadow_for(dst), (const void *)shadow_for(src),
-                  size * sizeof(dfsan_label));
+  dfsan_mem_shadow_transfer(dst, src, size);
   if (dfsan_get_track_origins())
     dfsan_mem_origin_transfer(dst, src, size);
 }
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
@@ -497,9 +497,7 @@
 }
 
 static void *dfsan_memcpy(void *dest, const void *src, size_t n) {
-  dfsan_label *sdest = shadow_for(dest);
-  const dfsan_label *ssrc = shadow_for(src);
-  internal_memcpy((void *)sdest, (const void *)ssrc, n * sizeof(dfsan_label));
+  dfsan_mem_shadow_transfer(dest, src, n);
   return internal_memcpy(dest, src, n);
 }
 
@@ -584,10 +582,7 @@
                                                   dfsan_label *ret_label) {
   size_t dest_len = strlen(dest);
   char *ret = strcat(dest, src);
-  dfsan_label *sdest = shadow_for(dest + dest_len);
-  const dfsan_label *ssrc = shadow_for(src);
-  internal_memcpy((void *)sdest, (const void *)ssrc,
-                  strlen(src) * sizeof(dfsan_label));
+  dfsan_mem_shadow_transfer(dest + dest_len, src, strlen(src));
   *ret_label = dest_label;
   return ret;
 }
@@ -598,12 +593,9 @@
     dfsan_origin *ret_origin) {
   size_t dest_len = strlen(dest);
   char *ret = strcat(dest, src);
-  dfsan_label *sdest = shadow_for(dest + dest_len);
-  const dfsan_label *ssrc = shadow_for(src);
   size_t src_len = strlen(src);
   dfsan_mem_origin_transfer(dest + dest_len, src, src_len);
-  internal_memcpy((void *)sdest, (const void *)ssrc,
-                  src_len * sizeof(dfsan_label));
+  dfsan_mem_shadow_transfer(dest + dest_len, src, src_len);
   *ret_label = dest_label;
   *ret_origin = dest_origin;
   return ret;
@@ -1120,8 +1112,7 @@
                     dfsan_label src_label, dfsan_label *ret_label) {
   char *ret = strcpy(dest, src);
   if (ret) {
-    internal_memcpy(shadow_for(dest), shadow_for(src),
-                    sizeof(dfsan_label) * (strlen(src) + 1));
+    dfsan_mem_shadow_transfer(dest, src, strlen(src) + 1);
   }
   *ret_label = dst_label;
   return ret;
@@ -1136,8 +1127,7 @@
   if (ret) {
     size_t str_len = strlen(src) + 1;
     dfsan_mem_origin_transfer(dest, src, str_len);
-    internal_memcpy(shadow_for(dest), shadow_for(src),
-                    sizeof(dfsan_label) * str_len);
+    dfsan_mem_shadow_transfer(dest, src, str_len);
   }
   *ret_label = dst_label;
   *ret_origin = dst_origin;
@@ -2369,9 +2359,8 @@
                                       formatter.num_written_bytes(retval));
           }
           va_labels++;
-          internal_memcpy(shadow_for(formatter.str_cur()), shadow_for(arg),
-                          sizeof(dfsan_label) *
-                              formatter.num_written_bytes(retval));
+          dfsan_mem_shadow_transfer(formatter.str_cur(), arg,
+                                    formatter.num_written_bytes(retval));
           end_fmt = true;
           break;
         }