shm/dconf-shm.c

   1 /*
   2  * Copyright © 2010 Codethink Limited
   3  * Copyright © 2012 Canonical Limited
   4  *
   5  * This library is free software; you can redistribute it and/or
   6  * modify it under the terms of the GNU Lesser General Public
   7  * License as published by the Free Software Foundation; either
   8  * version 2 of the licence, or (at your option) any later version.
   9  *
  10  * This library is distributed in the hope that it will be useful,
  11  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  12  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13  * Lesser General Public License for more details.
  14  *
  15  * You should have received a copy of the GNU Lesser General Public
  16  * License along with this library; if not, write to the
  17  * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
  18  * Boston, MA 02111-1307, USA.
  19  *
  20  * Author: Ryan Lortie <desrt@desrt.ca>
  21  */
  22
  23 #include "config.h"
  24
  25 #include "dconf-shm.h"
  26
  27 #include <sys/mman.h>
  28 #include <unistd.h>
  29 #include <fcntl.h>
  30 #include <errno.h>
  31
  32 static gchar *
  33 dconf_shm_get_shmdir (void)
  34 {
  35   static gchar *shmdir;
  36
  37   if (g_once_init_enter (&shmdir))
  38     g_once_init_leave (&shmdir, g_build_filename (g_get_user_runtime_dir (), "dconf", NULL));
  39
  40   return shmdir;
  41 }
  42
  43 void
  44 dconf_shm_close (guint8 *shm)
  45 {
  46   if (shm)
  47     munmap (shm, 1);
  48 }
  49
  50 guint8 *
  51 dconf_shm_open (const gchar *name)
  52 {
  53   const gchar *shmdir;
  54   gchar *filename;
  55   void *memory;
  56   gint fd;
  57
  58   shmdir = dconf_shm_get_shmdir ();
  59   filename = g_build_filename (shmdir, name, NULL);
  60   memory = NULL;
  61   fd = -1;
  62
  63   if (g_mkdir_with_parents (shmdir, 0700) != 0)
  64     {
  65       g_critical ("unable to create directory '%s': %s.  dconf will not work properly.", shmdir, g_strerror (errno));
  66       goto out;
  67     }
  68
  69   fd = open (filename, O_RDWR | O_CREAT, 0600);
  70   if (fd == -1)
  71     {
  72       g_critical ("unable to create file '%s': %s.  dconf will not work properly.", filename, g_strerror (errno));
  73       goto out;
  74     }
  75
  76   /* ftruncate(fd, 1) is not sufficient because it does not actually
  77    * ensure that the space is available (which could give a SIGBUS
  78    * later).
  79    *
  80    * posix_fallocate() is also problematic because it is implemented in
  81    * a racy way in the libc if unavailable for a particular filesystem
  82    * (as is the case for tmpfs, which is where we probably are).
  83    *
  84    * By writing to the second byte in the file we ensure we don't
  85    * overwrite the first byte (which is the one we care about).
  86    */
  87   if (pwrite (fd, "", 1, 1) != 1)
  88     {
  89       g_critical ("failed to allocate file '%s': %s.  dconf will not work properly.", filename, g_strerror (errno));
  90       goto out;
  91     }
  92
  93   memory = mmap (NULL, 1, PROT_READ, MAP_SHARED, fd, 0);
  94   g_assert (memory != MAP_FAILED);
  95   g_assert (memory != NULL);
  96
  97  out:
  98   g_free (filename);
  99   close (fd);
 100
 101   return memory;
 102 }
 103
 104 void
 105 dconf_shm_flag (const gchar *name)
 106 {
 107   const gchar *shmdir;
 108   gchar *filename;
 109   gint fd;
 110
 111   shmdir = dconf_shm_get_shmdir ();
 112   filename = g_build_filename (shmdir, name, NULL);
 113
 114   /* We need O_RDWR for PROT_WRITE.
 115    *
 116    * This is probably due to the fact that some architectures can't make
 117    * write-only mappings (so they end up being readable as well).
 118    */
 119   fd = open (filename, O_RDWR);
 120   if (fd >= 0)
 121     {
 122       /* In theory we could have opened the file after a client created
 123        * it but before they called pwrite().  Do the pwrite() ourselves
 124        * to make sure (so we don't get SIGBUS in a moment).
 125        *
 126        * If this fails then it will probably fail for the client too.
 127        * If it doesn't then there's not really much we can do...
 128        */
 129       if (pwrite (fd, "", 1, 1) == 1)
 130         {
 131           guint8 *shm;
 132
 133           /* It would have been easier for us to do write(fd, "\1", 1);
 134            * but this causes problems on kernels (ie: OpenBSD) that
 135            * don't sync up their filesystem cache with mmap()ed regions.
 136            *
 137            * Using mmap() works everywhere.
 138            *
 139            * See https://bugzilla.gnome.org/show_bug.cgi?id=687334 about
 140            * why we need to have PROT_READ even though we only write.
 141            */
 142           shm = mmap (NULL, 1, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
 143           g_assert (shm != MAP_FAILED);
 144
 145           *shm = 1;
 146
 147           munmap (shm, 1);
 148         }
 149
 150       close (fd);
 151
 152       unlink (filename);
 153     }
 154
 155   g_free (filename);
 156 }