* Put the chroots under /nix/var/nix/chroots to reduce the risk of

disasters involving `rm -rf' on bind mounts.  Will try the
  definitive fix (per-process mounts, apparently possible via the
  CLONE_NEWNS flag in clone()) some other time.

src/libmain/shared.cc

@@ -138,6 +138,7 @@ static void initAndRun(int argc, char * * argv)
     nixConfDir = canonPath(getEnv("NIX_CONF_DIR", NIX_CONF_DIR));
     nixLibexecDir = canonPath(getEnv("NIX_LIBEXEC_DIR", NIX_LIBEXEC_DIR));
     nixBinDir = canonPath(getEnv("NIX_BIN_DIR", NIX_BIN_DIR));
+    nixChrootsDir = canonPath(getEnv("NIX_CHROOTS_DIR", nixStateDir + "/chroots"));
 
     string subs = getEnv("NIX_SUBSTITUTERS", "default");
     if (subs == "default") {

src/libstore/build.cc

@@ -1710,16 +1710,13 @@ void DerivationGoal::startBuilder()
         /* Create a temporary directory in which we set up the chroot
            environment using bind-mounts.
 
-           !!! Big danger here: since we're doing this in /tmp, there
-           is a risk that the admin does something like "rm -rf
-           /tmp/chroot-nix-*" to clean up aborted builds, and if some
-           of the bind-mounts are still active, then "rm -rf" will
-           happily recurse into those mount points (thereby deleting,
-           say, /nix/store).  Ideally, chrootRootDir should be created in
-           some special location (maybe in /nix/var/nix) where Nix
-           takes care of unmounting / deleting old chroots
-           automatically. */
-        chrootRootDir = createTempDir("", "chroot-nix");
+           !!! Bind mounts are potentially dangerous: if the user
+           cleans up his system by doing "rm -rf
+           /nix/var/nix/chroots/*", this will recurse into /nix/store
+           via the bind mounts (and potentially other parts of the
+           filesystem, depending on the setting of the
+           `build-chroot-dirs' option). */
+        chrootRootDir = createTempDir(nixChrootsDir, "chroot-nix");
 
         /* Clean up the chroot directory automatically, but don't
            recurse; that would be very very bad if the unmount of a

src/libstore/globals.cc

@@ -16,6 +16,7 @@ string nixDBPath = "/UNINIT";
 string nixConfDir = "/UNINIT";
 string nixLibexecDir = "/UNINIT";
 string nixBinDir = "/UNINIT";
+string nixChrootsDir = "/UNINIT";
 
 bool keepFailed = false;
 bool keepGoing = false;

src/libstore/globals.hh

@@ -35,6 +35,12 @@ extern string nixLibexecDir;
 /* nixBinDir is the directory where the main programs are stored. */
 extern string nixBinDir;
 
+/* nixChrootsDir is the directory where we create chroot environments
+   (when chroot builds are enabled).  We don't put these under /tmp to
+   prevent "rm -rf /tmp" from recursing into /nix/store via the bind
+   mounts in the chroots. */
+extern string nixChrootsDir;
+
 
 /* Misc. global flags. */