* Don't provide the whole Nix store in the chroot, but only the

closure of the inputs.  This really enforces that there can't be any
  undeclared dependencies on paths in the store.  This is done by
  creating a fake Nix store and creating bind-mounts or hard-links in
  the fake store for all paths in the closure.  After the build, the
  build output is moved from the fake store to the real store.  TODO:
  the chroot has to be on the same filesystem as the Nix store for
  this to work, but this isn't enforced yet.  (I.e. it only works
  currently if /tmp is on the same FS as /nix/store.)

src/libstore/build.cc

@@ -648,6 +648,8 @@ private:
 
     /* Whether we're currently doing a chroot build. */
     bool useChroot;
+    
+    Path chrootRootDir;
 
     /* RAII object to delete the chroot directory. */
     boost::shared_ptr<AutoDelete> autoDelChroot;
@@ -804,9 +806,7 @@ void DerivationGoal::haveDerivation()
     trace("loading derivation");
 
     if (nrFailed != 0) {
-        printMsg(lvlError,
+        printMsg(lvlError, format("cannot build missing derivation `%1%'") % drvPath);
-            format("cannot build missing derivation `%1%'")
-            % drvPath);
         amDone(ecFailed);
         return;
     }
@@ -1062,6 +1062,12 @@ void DerivationGoal::buildDone()
              i != drv.outputs.end(); ++i)
         {
             Path path = i->second.path;
+
+            if (useChroot && pathExists(chrootRootDir + path)) {
+                if (rename((chrootRootDir + path).c_str(), path.c_str()) == -1)
+                    throw SysError(format("moving build output `%1%' from the chroot to the Nix store") % path);
+            }
+            
             if (!pathExists(path)) continue;
 
             struct stat st;
@@ -1449,6 +1455,14 @@ DerivationGoal::PrepareBuildReply DerivationGoal::prepareBuild()
 }
 
 
+void chmod(const Path & path, mode_t mode)
+{
+    if (::chmod(path.c_str(), 01777) == -1)
+        throw SysError(format("setting permissions on `%1%'") % path);
+    
+}
+
+
 void DerivationGoal::startBuilder()
 {
     startNest(nest, lvlInfo,
@@ -1648,7 +1662,6 @@ void DerivationGoal::startBuilder()
        work properly.  Purity checking for fixed-output derivations
        is somewhat pointless anyway. */
     useChroot = queryBoolSetting("build-use-chroot", false);
-    Path chrootRootDir;
     Paths dirsInChroot;
 
     if (fixedOutput) useChroot = false;
@@ -1669,9 +1682,7 @@ void DerivationGoal::startBuilder()
            instead.) */
         Path chrootTmpDir = chrootRootDir + "/tmp";
         createDirs(chrootTmpDir);
-
+        chmod(chrootTmpDir, 01777);
-        if (chmod(chrootTmpDir.c_str(), 01777) == -1)
-            throw SysError("creating /tmp in the chroot");
 
         /* Create a /etc/passwd with entries for the build user and
            the nobody account.  The latter is kind of a hack to
@@ -1695,8 +1706,31 @@ void DerivationGoal::startBuilder()
         
         dirsInChroot = querySetting("build-chroot-dirs", defaultDirs);
 
-        dirsInChroot.push_front(nixStore);
         dirsInChroot.push_front(tmpDir);
+
+        /* Make the closure of the inputs available in the chroot,
+           rather than the whole Nix store.  This prevents any access
+           to undeclared dependencies.  Directories are bind-mounted,
+           while other inputs are hard-linked (since only directories
+           can be bind-mounted).  !!! As an extra security
+           precaution, make the fake Nix store only writable by the
+           build user. */
+        createDirs(chrootRootDir + nixStore);
+        chmod(chrootRootDir + nixStore, 01777);
+
+        foreach (PathSet::iterator, i, inputPaths) {
+            struct stat st;
+            if (lstat(i->c_str(), &st))
+                throw SysError(format("getting attributes of path `%1%'") % *i);
+            if (S_ISDIR(st.st_mode))
+                dirsInChroot.push_back(*i);
+            else {
+                Path p = chrootRootDir + *i;
+                if (link(i->c_str(), p.c_str()) == -1)
+                    throw SysError(format("linking `%1%' to `%2%'") % p % *i);
+            }
+        }
+        
 #else
         throw Error("chroot builds are not supported on this platform");
 #endif
@@ -1742,7 +1776,7 @@ void DerivationGoal::startBuilder()
                 foreach (Paths::iterator, i, dirsInChroot) {
                     Path source = *i;
                     Path target = chrootRootDir + source;
-                    printMsg(lvlError, format("bind mounting `%1%' to `%2%'") % source % target);
+                    debug(format("bind mounting `%1%' to `%2%'") % source % target);
                 
                     createDirs(target);
                 
@@ -1781,7 +1815,7 @@ void DerivationGoal::startBuilder()
                safe.  Also note that setuid() when run as root sets
                the real, effective and saved UIDs. */
             if (buildUser.enabled()) {
-                debug(format("switching to user `%1%'") % buildUser.getUser());
+                printMsg(lvlChatty, format("switching to user `%1%'") % buildUser.getUser());
 
                 if (amPrivileged()) {