1 files changed, 264 insertions, 0 deletions
diff --git a/gnu/packages/aux-files/run-in-namespace.c b/gnu/packages/aux-files/run-in-namespace.c
new file mode 100644
index 0000000000..d0ab05c5db
--- /dev/null
+++ b/gnu/packages/aux-files/run-in-namespace.c
@@ -0,0 +1,264 @@
+/* GNU Guix --- Functional package management for GNU
+   Copyright (C) 2018 Ludovic Courtès <ludo@gnu.org>
+
+   This file is part of GNU Guix.
+
+   GNU Guix is free software; you can redistribute it and/or modify it
+   under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or (at
+   your option) any later version.
+
+   GNU Guix is distributed in the hope that it will be useful, but
+   WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* Make the given @WRAPPED_PROGRAM@ relocatable by executing it in a separate
+   mount namespace where the store is mounted in its right place.
+
+   We would happily do that in Scheme using 'call-with-container'.  However,
+   this very program needs to be relocatable, so it needs to be statically
+   linked, which complicates things (Guile's modules can hardly be "linked"
+   into a single executable.)  */
+
+#define _GNU_SOURCE
+#include <stdlib.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <sched.h>
+#include <sys/mount.h>
+#include <errno.h>
+#include <libgen.h>
+#include <limits.h>
+#include <string.h>
+#include <assert.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <fcntl.h>
+#include <dirent.h>
+
+/* Concatenate DIRECTORY, a slash, and FILE.  Return the result, which the
+   caller must eventually free.  */
+static char *
+concat (const char *directory, const char *file)
+{
+  char *result = malloc (strlen (directory) + 2 + strlen (file));
+  assert (result != NULL);
+
+  strcpy (result, directory);
+  strcat (result, "/");
+  strcat (result, file);
+  return result;
+}
+
+static void
+mkdir_p (const char *directory)
+{
+  if (strcmp (directory, "/") != 0)
+    {
+      char *parent = dirname (strdupa (directory));
+      mkdir_p (parent);
+      int err = mkdir (directory, 0700);
+      if (err < 0 && errno != EEXIST)
+	assert_perror (errno);
+    }
+}
+
+static void
+rm_rf (const char *directory)
+{
+  DIR *stream = opendir (directory);
+
+  for (struct dirent *entry = readdir (stream);
+       entry != NULL;
+       entry = readdir (stream))
+    {
+      if (strcmp (entry->d_name, ".") == 0
+	  || strcmp (entry->d_name, "..") == 0)
+	continue;
+
+      char *full = concat (directory, entry->d_name);
+
+      int err = unlink (full);
+      if (err < 0)
+	{
+	  if (errno == EISDIR)
+	    /* Recurse (we expect a shallow directory structure so there's
+	       little risk of stack overflow.)  */
+	    rm_rf (full);
+	  else
+	    assert_perror (errno);
+	}
+
+      free (full);
+    }
+
+  closedir (stream);
+
+  int err = rmdir (directory);
+  if (err < 0 && errno != ENOENT)
+    assert_perror (errno);
+}
+
+/* Bind mount all the top-level entries in SOURCE to TARGET.  */
+static void
+bind_mount (const char *source, const char *target)
+{
+  DIR *stream = opendir (source);
+
+  for (struct dirent *entry = readdir (stream);
+       entry != NULL;
+       entry = readdir (stream))
+    {
+      /* XXX: Some file systems may not report a useful 'd_type'.  Ignore them
+	 for now.  */
+      assert (entry->d_type != DT_UNKNOWN);
+
+      if (strcmp (entry->d_name, ".") == 0
+	  || strcmp (entry->d_name, "..") == 0)
+	continue;
+
+      char *abs_source = concat (source, entry->d_name);
+      char *new_entry = concat (target, entry->d_name);
+
+      if (entry->d_type == DT_LNK)
+	{
+	  char target[PATH_MAX];
+
+	  ssize_t result = readlink (abs_source, target, sizeof target - 1);
+	  if (result > 0)
+	    {
+	      target[result] = '\0';
+	      int err = symlink (target, new_entry);
+	      if (err < 0)
+		assert_perror (errno);
+	    }
+	}
+      else
+	{
+	  /* Create the mount point.  */
+	  if (entry->d_type == DT_DIR)
+	    {
+	      int err = mkdir (new_entry, 0700);
+	      if (err != 0)
+		assert_perror (errno);
+	    }
+	  else
+	    close (open (new_entry, O_WRONLY | O_CREAT));
+
+	  int err = mount (abs_source, new_entry, "none",
+			   MS_BIND | MS_REC | MS_RDONLY, NULL);
+
+	  /* It used to be that only directories could be bind-mounted.  Thus,
+	     keep going if we fail to bind-mount a non-directory entry.
+	     That's OK because regular files in the root file system are
+	     usually uninteresting.  */
+	  if (err != 0 && entry->d_type != DT_DIR)
+	    assert_perror (errno);
+
+	  free (new_entry);
+	  free (abs_source);
+	}
+    }
+
+  closedir (stream);
+}
+
+
+int
+main (int argc, char *argv[])
+{
+  ssize_t size;
+  char self[PATH_MAX];
+  size = readlink ("/proc/self/exe", self, sizeof self - 1);
+  assert (size > 0);
+
+  /* SELF is something like "/home/ludo/.local/gnu/store/…-foo/bin/ls" and we
+     want to extract "/home/ludo/.local/gnu/store".  */
+  size_t index = strlen (self)
+    - strlen ("@WRAPPED_PROGRAM@")
+    + strlen ("@STORE_DIRECTORY@");
+  char *store = strdup (self);
+  store[index] = '\0';
+
+  struct stat statbuf;
+
+  /* If STORE is already at the "right" place, we can execute
+     @WRAPPED_PROGRAM@ right away.  This is not just an optimization: it's
+     needed when running one of these wrappers from within an unshare'd
+     namespace, because 'unshare' fails with EPERM in that context.  */
+  if (strcmp (store, "@STORE_DIRECTORY@") != 0
+      && lstat ("@WRAPPED_PROGRAM@", &statbuf) != 0)
+    {
+      /* Spawn @WRAPPED_PROGRAM@ in a separate namespace where STORE is
+	 bind-mounted in the right place.  */
+      int err;
+      char *new_root = mkdtemp (strdup ("/tmp/guix-exec-XXXXXX"));
+      char *new_store = concat (new_root, "@STORE_DIRECTORY@");
+      char *cwd = get_current_dir_name ();
+
+      pid_t child = fork ();
+      switch (child)
+	{
+	case 0:
+	  /* Unshare namespaces in the child and set up bind-mounts from
+	     there.  That way, bind-mounts automatically disappear when the
+	     child exits, which simplifies cleanup for the parent.  */
+	  err = unshare (CLONE_NEWNS | CLONE_NEWUSER);
+	  if (err < 0)
+	    {
+	      fprintf (stderr, "%s: error: 'unshare' failed: %m\n", argv[0]);
+	      fprintf (stderr, "\
+This may be because \"user namespaces\" are not supported on this system.\n\
+Consequently, we cannot run '@WRAPPED_PROGRAM@',\n\
+unless you move it to the '@STORE_DIRECTORY@' directory.\n\
+\n\
+Please refer to the 'guix pack' documentation for more information.\n");
+	      return EXIT_FAILURE;
+	    }
+
+	  /* Note: Due to <https://bugzilla.kernel.org/show_bug.cgi?id=183461>
+	     we cannot make NEW_ROOT a tmpfs (which would have saved the need
+	     for 'rm_rf'.)  */
+	  bind_mount ("/", new_root);
+	  mkdir_p (new_store);
+	  err = mount (store, new_store, "none", MS_BIND | MS_REC | MS_RDONLY,
+		       NULL);
+	  if (err < 0)
+	    assert_perror (errno);
+
+	  chdir (new_root);
+	  err = chroot (new_root);
+	  if (err < 0)
+	    assert_perror (errno);
+
+	  /* Change back to where we were before chroot'ing.  */
+	  chdir (cwd);
+	  break;
+	case -1:
+	  assert_perror (errno);
+	  break;
+	default:
+	  {
+	    int status;
+	    waitpid (child, &status, 0);
+	    chdir ("/");			  /* avoid EBUSY */
+	    rm_rf (new_root);
+	    free (new_root);
+	    exit (status);
+	  }
+	}
+    }
+
+  /* The executable is available under @STORE_DIRECTORY@, so we can now
+     execute it.  */
+  int err = execv ("@WRAPPED_PROGRAM@", argv);
+  if (err < 0)
+    assert_perror (errno);
+
+  return EXIT_FAILURE;
+}