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, see <http://www.gnu.org/licenses/>.
  17  *
  18  * Author: Ryan Lortie <desrt@desrt.ca>
  19  */
  20
  21 #include "config.h"
  22
  23 #include "dconf-shm.h"
  24
  25 #include <sys/mman.h>
  26 #include <unistd.h>
  27 #include <fcntl.h>
  28 #include <errno.h>
  29
  30 static gchar *
  31 dconf_shm_get_shmdir (void)
  32 {
  33   static gchar *shmdir;
  34
  35   if (g_once_init_enter (&shmdir))
  36     g_once_init_leave (&shmdir, g_build_filename (g_get_user_runtime_dir (), "dconf", NULL));
  37
  38   return shmdir;
  39 }
  40
  41 void
  42 dconf_shm_close (guint8 *shm)
  43 {
  44   if (shm)
  45     munmap (shm, 1);
  46 }
  47
  48 guint8 *
  49 dconf_shm_open (const gchar *name)
  50 {
  51   const gchar *shmdir;
  52   gchar *filename;
  53   void *memory;
  54   gint fd;
  55
  56   shmdir = dconf_shm_get_shmdir ();
  57   filename = g_build_filename (shmdir, name, NULL);
  58   memory = NULL;
  59   fd = -1;
  60
  61   if (g_mkdir_with_parents (shmdir, 0700) != 0)
  62     {
  63       g_critical ("unable to create directory '%s': %s.  dconf will not work properly.", shmdir, g_strerror (errno));
  64       goto out;
  65     }
  66
  67   fd = open (filename, O_RDWR | O_CREAT, 0600);
  68   if (fd == -1)
  69     {
  70       g_critical ("unable to create file '%s': %s.  dconf will not work properly.", filename, g_strerror (errno));
  71       goto out;
  72     }
  73
  74   /* ftruncate(fd, 1) is not sufficient because it does not actually
  75    * ensure that the space is available (which could give a SIGBUS
  76    * later).
  77    *
  78    * posix_fallocate() is also problematic because it is implemented in
  79    * a racy way in the libc if unavailable for a particular filesystem
  80    * (as is the case for tmpfs, which is where we probably are).
  81    *
  82    * By writing to the second byte in the file we ensure we don't
  83    * overwrite the first byte (which is the one we care about).
  84    */
  85   if (pwrite (fd, "", 1, 1) != 1)
  86     {
  87       g_critical ("failed to allocate file '%s': %s.  dconf will not work properly.", filename, g_strerror (errno));
  88       goto out;
  89     }
  90
  91   memory = mmap (NULL, 1, PROT_READ, MAP_SHARED, fd, 0);
  92   g_assert (memory != MAP_FAILED);
  93   g_assert (memory != NULL);
  94
  95  out:
  96   g_free (filename);
  97   close (fd);
  98
  99   return memory;
 100 }
 101
 102 void
 103 dconf_shm_flag (const gchar *name)
 104 {
 105   const gchar *shmdir;
 106   gchar *filename;
 107   gint fd;
 108
 109   shmdir = dconf_shm_get_shmdir ();
 110   filename = g_build_filename (shmdir, name, NULL);
 111
 112   /* We need O_RDWR for PROT_WRITE.
 113    *
 114    * This is probably due to the fact that some architectures can't make
 115    * write-only mappings (so they end up being readable as well).
 116    */
 117   fd = open (filename, O_RDWR);
 118   if (fd >= 0)
 119     {
 120       /* In theory we could have opened the file after a client created
 121        * it but before they called pwrite().  Do the pwrite() ourselves
 122        * to make sure (so we don't get SIGBUS in a moment).
 123        *
 124        * If this fails then it will probably fail for the client too.
 125        * If it doesn't then there's not really much we can do...
 126        */
 127       if (pwrite (fd, "", 1, 1) == 1)
 128         {
 129           guint8 *shm;
 130
 131           /* It would have been easier for us to do write(fd, "\1", 1);
 132            * but this causes problems on kernels (ie: OpenBSD) that
 133            * don't sync up their filesystem cache with mmap()ed regions.
 134            *
 135            * Using mmap() works everywhere.
 136            *
 137            * See https://bugzilla.gnome.org/show_bug.cgi?id=687334 about
 138            * why we need to have PROT_READ even though we only write.
 139            */
 140           shm = mmap (NULL, 1, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
 141           g_assert (shm != MAP_FAILED);
 142
 143           *shm = 1;
 144
 145           munmap (shm, 1);
 146         }
 147
 148       close (fd);
 149
 150       unlink (filename);
 151     }
 152
 153   g_free (filename);
 154 }