linux-user-chroot-newnet: New optional helper program

Some versions of the Linux kernel require large (order 4) contiguous
allocations per network namespace.  This optional helper program is a
workaround for that; one can create the empty network namespace just
once.

3 files changed, 142 insertions, 0 deletions

diff --git a/Makefile-user-chroot.am b/Makefile-user-chroot.am
index 5d29a2c..c3801f5 100644
--- a/Makefile-user-chroot.am
+++ b/Makefile-user-chroot.am
@@ -20,3 +20,11 @@ bin_PROGRAMS += linux-user-chroot
 linux_user_chroot_SOURCES = src/linux-user-chroot.c
 
 linux_user_chroot_CFLAGS = $(AM_CFLAGS)
+
+if BUILD_NEWNET_HELPER
+bin_PROGRAMS += linux-user-chroot-newnet
+endif
+
+linux_user_chroot_newnet_SOURCES = src/linux-user-chroot-newnet.c
+
+linux_user_chroot_newnet_CFLAGS = $(AM_CFLAGS)
diff --git a/configure.ac b/configure.ac
index 76b79c1..20c992b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -29,6 +29,12 @@ AC_CHECK_HEADER([linux/securebits.h],
 AC_PATH_PROG([XSLTPROC], [xsltproc])
 AM_CONDITIONAL(HAVE_XSLTPROC, test x"$XSLTPROC" != x)
 
+AC_ARG_ENABLE(newnet-helper,
+              AC_HELP_STRING([--enable-newnet-helper],
+                             [build newnet helper]),,
+              enable_newnet_helper=no)
+AM_CONDITIONAL(BUILD_NEWNET_HELPER, test x$enable_newnet_helper = xyes)
+
 AC_CONFIG_FILES([
 Makefile
 ])
diff --git a/src/linux-user-chroot-newnet.c b/src/linux-user-chroot-newnet.c
new file mode 100644
index 0000000..9cb0804
--- /dev/null
+++ b/src/linux-user-chroot-newnet.c
@@ -0,0 +1,128 @@
+/* -*- mode: c; tab-width: 2; indent-tabs-mode: nil -*-
+ *
+ * newnet-suid: Allow allocating a new empty network namespace as
+ * non-root.  This program is just a workaround for the kernel
+ * requiring large-order allocations (e.g. 4 pages) per network
+ * namespace.
+ *
+ * Copyright 2012 Colin Walters <walters@verbum.org>
+ *
+ * This program 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 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it would 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 this program; if not, write to the Free Software Foundation,
+ * Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ */
+
+#define _GNU_SOURCE
+#include <unistd.h>
+#include <stdio.h>
+#include <fcntl.h>
+#include <stdarg.h>
+#include <string.h>
+#include <assert.h>
+#include <stdlib.h>
+#include <sys/types.h>
+#include <sys/prctl.h>
+#include <sys/mount.h>
+#include <sys/syscall.h>
+#include <sys/wait.h>
+#include <sched.h>
+
+static void fatal (const char *message, ...) __attribute__ ((noreturn)) __attribute__ ((format (printf, 1, 2)));
+static void fatal_errno (const char *message) __attribute__ ((noreturn));
+
+static void
+fatal (const char *fmt,
+       ...)
+{
+  va_list args;
+  
+  va_start (args, fmt);
+
+  vfprintf (stderr, fmt, args);
+  putc ('\n', stderr);
+  
+  va_end (args);
+  exit (1);
+}
+
+static void
+fatal_errno (const char *message)
+{
+  perror (message);
+  exit (1);
+}
+
+
+int
+main (int      argc,
+      char   **argv)
+{
+  const char *program;
+  uid_t ruid, euid, suid;
+  gid_t rgid, egid, sgid;
+  char **program_argv;
+  int child_status = 0;
+  pid_t child;
+
+  if (argc <= 0)
+    return 1;
+
+  argc--;
+  argv++;
+
+  if (argc < 1)
+    fatal ("PROGRAM argument must be specified");
+
+  program = argv[0];
+  program_argv = argv + 1;
+
+  if (getresgid (&rgid, &egid, &sgid) < 0)
+    fatal_errno ("getresgid");
+  if (getresuid (&ruid, &euid, &suid) < 0)
+    fatal_errno ("getresuid");
+
+  if (rgid == 0)
+    rgid = ruid;
+
+  if ((child = syscall (__NR_clone, SIGCHLD | CLONE_NEWNET, NULL)) < 0)
+    perror ("clone");
+
+  if (child == 0)
+    {
+      /* Switch back to the uid of our invoking process.  These calls are
+       * irrevocable - see setuid(2) */
+      if (setgid (rgid) < 0)
+        fatal_errno ("setgid");
+      if (setuid (ruid) < 0)
+        fatal_errno ("setuid");
+
+      if (execvp (program, program_argv) < 0)
+        fatal_errno ("execv");
+    }
+
+  /* Let's also setuid back in the parent - there's no reason to stay uid 0, and
+   * it's just better to drop privileges. */
+  if (setgid (rgid) < 0)
+    fatal_errno ("setgid");
+  if (setuid (ruid) < 0)
+    fatal_errno ("setuid");
+
+  /* Kind of lame to sit around blocked in waitpid, but oh well. */
+  if (waitpid (child, &child_status, 0) < 0)
+    fatal_errno ("waitpid");
+  
+  if (WIFEXITED (child_status))
+    return WEXITSTATUS (child_status);
+  else
+    return 1;
+}