Sandbox: Fix /dev/ptmx on recent kernels

This fixes "No such file or directory" when opening /dev/ptmx
(e.g. http://hydra.nixos.org/build/51094249).

The reason appears to be some changes to /dev/ptmx / /dev/pts handling
between Linux 4.4 and 4.9. See
https://patchwork.kernel.org/patch/7832531/.

The fix is to go back to mounting a proper /dev/pts instance inside
the sandbox. Happily, this now works inside user namespaces, even for
unprivileged users. So

  NIX_REMOTE=local?root=/tmp/nix nix-build \
    '<nixpkgs/nixos/tests/misc.nix>' -A test

works for non-root users.

The downside is that the fix breaks sandbox builds on older kernels
(probably pre-4.6), since mounting a devpts fails inside user
namespaces for some reason I've never been able to figure out. Builds
on those systems will fail with

  error: while setting up the build environment: mounting /dev/pts: Invalid argument

Ah well.

1 files changed, 1 insertions, 8 deletions

diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index fc840df81..43a8dadf8 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -2391,8 +2391,6 @@ void DerivationGoal::runChild()
                 ss.push_back("/dev/tty");
                 ss.push_back("/dev/urandom");
                 ss.push_back("/dev/zero");
-                ss.push_back("/dev/ptmx");
-                ss.push_back("/dev/pts");
                 createSymlink("/proc/self/fd", chrootRootDir + "/dev/fd");
                 createSymlink("/proc/self/fd/0", chrootRootDir + "/dev/stdin");
                 createSymlink("/proc/self/fd/1", chrootRootDir + "/dev/stdout");
@@ -2448,17 +2446,13 @@ void DerivationGoal::runChild()
                     fmt("size=%s", settings.get("sandbox-dev-shm-size", std::string("50%"))).c_str()) == -1)
                 throw SysError("mounting /dev/shm");
 
-#if 0
-            // FIXME: can't figure out how to do this in a user
-            // namespace.
-
             /* Mount a new devpts on /dev/pts.  Note that this
                requires the kernel to be compiled with
                CONFIG_DEVPTS_MULTIPLE_INSTANCES=y (which is the case
                if /dev/ptx/ptmx exists). */
             if (pathExists("/dev/pts/ptmx") &&
                 !pathExists(chrootRootDir + "/dev/ptmx")
-                && dirsInChroot.find("/dev/pts") == dirsInChroot.end())
+                && !dirsInChroot.count("/dev/pts"))
             {
                 if (mount("none", (chrootRootDir + "/dev/pts").c_str(), "devpts", 0, "newinstance,mode=0620") == -1)
                     throw SysError("mounting /dev/pts");
@@ -2468,7 +2462,6 @@ void DerivationGoal::runChild()
                    Linux versions, it is created with permissions 0.  */
                 chmod_(chrootRootDir + "/dev/pts/ptmx", 0666);
             }
-#endif
 
             /* Do the chroot(). */
             if (chdir(chrootRootDir.c_str()) == -1)