* When NIX_REMOTE is set to "slave", fork off nix-worker in slave

mode.  Presumably nix-worker would be setuid to the Nix store user.
  The worker performs all operations on the Nix store and database, so
  the caller can be completely unprivileged.

  This is already much more secure than the old setuid scheme, since
  the worker doesn't need to do Nix expression evaluation and so on.
  Most importantly, this means that it doesn't need to access any user
  files, with all resulting security risks; it only performs pure
  store operations.

  Once this works, it is easy to move to a daemon model that forks off
  a worker for connections established through a Unix domain socket.
  That would be even more secure.

src/libstore/remote-store.cc

@@ -1,12 +1,64 @@
+#include "serialise.hh"
+#include "util.hh"
 #include "remote-store.hh"
 
+#include <iostream>
+#include <unistd.h>
+
 
 namespace nix {
 
 
 RemoteStore::RemoteStore()
 {
-    throw Error("not implemented");
+    toChild.create();
+    fromChild.create();
+
+
+    /* Start the worker. */
+    string worker = "nix-worker";
+
+    child = fork();
+    
+    switch (child) {
+        
+    case -1:
+        throw SysError("unable to fork");
+
+    case 0:
+        try { /* child */
+
+            fromChild.readSide.close();
+            if (dup2(fromChild.writeSide, STDOUT_FILENO) == -1)
+                throw SysError("dupping write side");
+
+            toChild.writeSide.close();
+            if (dup2(toChild.readSide, STDIN_FILENO) == -1)
+                throw SysError("dupping read side");
+
+            execlp(worker.c_str(), worker.c_str(),
+                "-vvv", "--slave", NULL);
+            
+            throw SysError(format("executing `%1%'") % worker);
+            
+        } catch (std::exception & e) {
+            std::cerr << format("child error: %1%\n") % e.what();
+        }
+        quickExit(1);
+    }
+
+    fromChild.writeSide.close();
+    toChild.readSide.close();
+
+    from.fd = fromChild.readSide;
+    to.fd = toChild.writeSide;
+
+    
+    /* Send the magic greeting, check for the reply. */
+    writeInt(0x6e697864, to);
+    
+    unsigned int magic = readInt(from);
+    if (magic != 0x6478696e) throw Error("protocol mismatch");
 }
 
 

src/libstore/remote-store.hh

@@ -9,6 +9,12 @@
 namespace nix {
 
 
+class Pipe;
+class Pid;
+struct FdSink;
+struct FdSource;
+
+
 class RemoteStore : public StoreAPI
 {
 public:
@@ -44,6 +50,11 @@ public:
     void ensurePath(const Path & storePath);
 
 private:
+    Pipe toChild;
+    Pipe fromChild;
+    FdSink to;
+    FdSource from;
+    Pid child;
 };
 
 

src/libstore/store-api.cc

@@ -91,6 +91,7 @@ Path makeFixedOutputPath(bool recursive,
 
 
 #include "local-store.hh"
+#include "serialise.hh"
 #include "remote-store.hh"
 
 

src/libutil/serialise.hh

@@ -33,6 +33,11 @@ struct FdSink : Sink
 {
     int fd;
 
+    FdSink()
+    {
+        fd = 0;
+    }
+    
     FdSink(int fd) 
     {
         this->fd = fd;
@@ -47,6 +52,11 @@ struct FdSource : Source
 {
     int fd;
 
+    FdSource()
+    {
+        fd = 0;
+    }
+    
     FdSource(int fd) 
     {
         this->fd = fd;

src/nix-worker/main.cc

@@ -1,35 +1,21 @@
 #include "shared.hh"
 #include "local-store.hh"
 #include "util.hh"
+#include "serialise.hh"
 
 using namespace nix;
 
 
-/* !!! Mostly cut&pasted from util/archive.hh */
-/* Use buffered reads. */
-static unsigned int readInt(int fd)
+void processConnection(Source & from, Sink & to)
 {
-    unsigned char buf[8];
-    readFull(fd, buf, sizeof(buf));
-    if (buf[4] || buf[5] || buf[6] || buf[7])
-        throw Error("implementation cannot deal with > 32-bit integers");
-    return
-        buf[0] |
-        (buf[1] << 8) |
-        (buf[2] << 16) |
-        (buf[3] << 24);
-}
+    store = boost::shared_ptr<StoreAPI>(new LocalStore(true));
 
-
-void processConnection(int fdFrom, int fdTo)
-{
-    store = openStore();
-
-    unsigned int magic = readInt(fdFrom);
+    unsigned int magic = readInt(from);
     if (magic != 0x6e697864) throw Error("protocol mismatch");
 
+    writeInt(0x6478696e, to);
 
-    
+    debug("greeting exchanged");
 }
 
 
@@ -43,8 +29,11 @@ void run(Strings args)
         if (arg == "--slave") slave = true;
     }
 
-    if (slave)
-        processConnection(STDIN_FILENO, STDOUT_FILENO);
+    if (slave) {
+        FdSource source(STDIN_FILENO);
+        FdSink sink(STDOUT_FILENO);
+        processConnection(source, sink);
+    }
 
     else if (daemon)
         throw Error("daemon mode not implemented");