diff --git a/src/libutil/serialise.cc b/src/libutil/serialise.cc
index 640267a13..ba549c214 100644
--- a/src/libutil/serialise.cc
+++ b/src/libutil/serialise.cc
@@ -149,12 +149,17 @@ void writeLongLong(unsigned long long n, Sink & sink)
 }
 
 
+void writeString(const unsigned char * buf, size_t len, Sink & sink)
+{
+    writeInt(len, sink);
+    sink(buf, len);
+    writePadding(len, sink);
+}
+
+
 void writeString(const string & s, Sink & sink)
 {
-    size_t len = s.length();
-    writeInt(len, sink);
-    sink((const unsigned char *) s.c_str(), len);
-    writePadding(len, sink);
+    writeString((const unsigned char *) s.c_str(), s.size(), sink);
 }
 
 
@@ -208,6 +213,16 @@ unsigned long long readLongLong(Source & source)
 }
 
 
+size_t readString(unsigned char * buf, size_t max, Source & source)
+{
+    size_t len = readInt(source);
+    if (len > max) throw Error("string is too long");
+    source(buf, len);
+    readPadding(len, source);
+    return len;
+}
+
+ 
 string readString(Source & source)
 {
     size_t len = readInt(source);
diff --git a/src/libutil/serialise.hh b/src/libutil/serialise.hh
index 25398b09d..efd8e2a06 100644
--- a/src/libutil/serialise.hh
+++ b/src/libutil/serialise.hh
@@ -114,12 +114,14 @@ struct StringSource : Source
 void writePadding(size_t len, Sink & sink);
 void writeInt(unsigned int n, Sink & sink);
 void writeLongLong(unsigned long long n, Sink & sink);
+void writeString(const unsigned char * buf, size_t len, Sink & sink);
 void writeString(const string & s, Sink & sink);
 void writeStringSet(const StringSet & ss, Sink & sink);
 
 void readPadding(size_t len, Source & source);
 unsigned int readInt(Source & source);
 unsigned long long readLongLong(Source & source);
+size_t readString(unsigned char * buf, size_t max, Source & source);
 string readString(Source & source);
 StringSet readStringSet(Source & source);
 
diff --git a/src/nix-worker/nix-worker.cc b/src/nix-worker/nix-worker.cc
index 695e4c38d..85e2105b2 100644
--- a/src/nix-worker/nix-worker.cc
+++ b/src/nix-worker/nix-worker.cc
@@ -56,7 +56,7 @@ static void tunnelStderr(const unsigned char * buf, size_t count)
     if (canSendStderr && myPid == getpid()) {
         try {
             writeInt(STDERR_NEXT, to);
-            writeString(string((char *) buf, count), to);
+            writeString(buf, count, to);
             to.flush();
         } catch (...) {
             /* Write failed; that means that the other side is
@@ -205,7 +205,7 @@ struct TunnelSink : Sink
     virtual void operator () (const unsigned char * data, size_t len)
     {
         writeInt(STDERR_WRITE, to);
-        writeString(string((const char *) data, len), to);
+        writeString(data, len, to);
     }
 };
 
@@ -224,16 +224,11 @@ struct TunnelSource : BufferedSource
         writeInt(STDERR_READ, to);
         writeInt(len, to);
         to.flush();
-        string s = readString(from); // !!! inefficient
+        size_t n = readString(data, len, from);
 
         startWork();
-
-        if (s.empty()) throw EndOfFile("unexpected end-of-file");
-        if (s.size() > len) throw Error("client sent too much data");
-
-        memcpy(data, (const unsigned char *) s.c_str(), s.size());
-
-        return s.size();
+        if (n == 0) throw EndOfFile("unexpected end-of-file");
+        return n;
     }
 };