diff --git a/common/libs/fs/shared_fd.cpp b/common/libs/fs/shared_fd.cpp
index 690507c..d2b2ea0 100644
--- a/common/libs/fs/shared_fd.cpp
+++ b/common/libs/fs/shared_fd.cpp
@@ -22,6 +22,7 @@
 #include <fcntl.h>
 #include <netinet/in.h>
 #include <unistd.h>
+#include <algorithm>
 
 #include "common/libs/auto_resources/auto_resources.h"
 #include "common/libs/glog/logging.h"
@@ -76,6 +77,23 @@
   return true;
 }
 
+bool FileInstance::CopyFrom(FileInstance& in, size_t length) {
+  AutoFreeBuffer buffer;
+  buffer.Resize(8192);
+  while (length > 0) {
+    ssize_t num_read = in.Read(buffer.data(), std::min(buffer.size(), length));
+    length -= num_read;
+    if (num_read <= 0) {
+      return false;
+    }
+    if (Write(buffer.data(), num_read) != num_read) {
+      // The caller will have to log an appropriate message.
+      return false;
+    }
+  }
+  return true;
+}
+
 void FileInstance::Close() {
   AutoFreeBuffer message;
   if (fd_ == -1) {
diff --git a/common/libs/fs/shared_fd.h b/common/libs/fs/shared_fd.h
index 58d3db1..f5dd451 100644
--- a/common/libs/fs/shared_fd.h
+++ b/common/libs/fs/shared_fd.h
@@ -223,6 +223,7 @@
   // The non-const reference is needed to avoid binding this to a particular
   // reference type.
   bool CopyFrom(FileInstance& in);
+  bool CopyFrom(FileInstance& in, size_t length);
 
   int UNMANAGED_Dup() {
     errno = 0;