sysdeps/unix/sysv/linux/shm_open.c

   1 /* Copyright (C) 2000-2014 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the License, or (at your option) any later version.
   8
   9    The GNU C Library is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, see
  16    <http://www.gnu.org/licenses/>.  */
  17
  18 #include <errno.h>
  19 #include <fcntl.h>
  20 #include <mntent.h>
  21 #include <paths.h>
  22 #include <stdio.h>
  23 #include <stdlib.h>
  24 #include <string.h>
  25 #include <unistd.h>
  26 #include <sys/mman.h>
  27 #include <sys/statfs.h>
  28 #include <bits/libc-lock.h>
  29 #include "linux_fsinfo.h"
  30
  31
  32 /* Mount point of the shared memory filesystem.  */
  33 static struct
  34 {
  35   char *dir;
  36   size_t dirlen;
  37 } mountpoint;
  38
  39 /* This is the default directory.  */
  40 static const char defaultdir[] = "/dev/shm/";
  41
  42 /* Protect the `mountpoint' variable above.  */
  43 __libc_once_define (static, once);
  44
  45
  46 /* Determine where the shmfs is mounted (if at all).  */
  47 static void
  48 where_is_shmfs (void)
  49 {
  50   char buf[512];
  51   struct statfs f;
  52   struct mntent resmem;
  53   struct mntent *mp;
  54   FILE *fp;
  55
  56   /* The canonical place is /dev/shm.  This is at least what the
  57      documentation tells everybody to do.  */
  58   if (__statfs (defaultdir, &f) == 0 && (f.f_type == SHMFS_SUPER_MAGIC
  59                                          || f.f_type == RAMFS_MAGIC))
  60     {
  61       /* It is in the normal place.  */
  62       mountpoint.dir = (char *) defaultdir;
  63       mountpoint.dirlen = sizeof (defaultdir) - 1;
  64
  65       return;
  66     }
  67
  68   /* OK, do it the hard way.  Look through the /proc/mounts file and if
  69      this does not exist through /etc/fstab to find the mount point.  */
  70   fp = __setmntent ("/proc/mounts", "r");
  71   if (__glibc_unlikely (fp == NULL))
  72     {
  73       fp = __setmntent (_PATH_MNTTAB, "r");
  74       if (__glibc_unlikely (fp == NULL))
  75         /* There is nothing we can do.  Blind guesses are not helpful.  */
  76         return;
  77     }
  78
  79   /* Now read the entries.  */
  80   while ((mp = __getmntent_r (fp, &resmem, buf, sizeof buf)) != NULL)
  81     /* The original name is "shm" but this got changed in early Linux
  82        2.4.x to "tmpfs".  */
  83     if (strcmp (mp->mnt_type, "tmpfs") == 0
  84         || strcmp (mp->mnt_type, "shm") == 0)
  85       {
  86         /* Found it.  There might be more than one place where the
  87            filesystem is mounted but one is enough for us.  */
  88         size_t namelen;
  89
  90         /* First make sure this really is the correct entry.  At least
  91            some versions of the kernel give wrong information because
  92            of the implicit mount of the shmfs for SysV IPC.  */
  93         if (__statfs (mp->mnt_dir, &f) != 0 || (f.f_type != SHMFS_SUPER_MAGIC
  94                                                 && f.f_type != RAMFS_MAGIC))
  95           continue;
  96
  97         namelen = strlen (mp->mnt_dir);
  98
  99         if (namelen == 0)
 100           /* Hum, maybe some crippled entry.  Keep on searching.  */
 101           continue;
 102
 103         mountpoint.dir = (char *) malloc (namelen + 2);
 104         if (mountpoint.dir != NULL)
 105           {
 106             char *cp = __mempcpy (mountpoint.dir, mp->mnt_dir, namelen);
 107             if (cp[-1] != '/')
 108               *cp++ = '/';
 109             *cp = '\0';
 110             mountpoint.dirlen = cp - mountpoint.dir;
 111           }
 112
 113         break;
 114       }
 115
 116   /* Close the stream.  */
 117   __endmntent (fp);
 118 }
 119
 120
 121 /* Open shared memory object.  This implementation assumes the shmfs
 122    implementation introduced in the late 2.3.x kernel series to be
 123    available.  Normally the filesystem will be mounted at /dev/shm but
 124    we fall back on searching for the actual mount point should opening
 125    such a file fail.  */
 126 int
 127 shm_open (const char *name, int oflag, mode_t mode)
 128 {
 129   size_t namelen;
 130   char *fname;
 131   int fd;
 132
 133   /* Determine where the shmfs is mounted.  */
 134   __libc_once (once, where_is_shmfs);
 135
 136   /* If we don't know the mount points there is nothing we can do.  Ever.  */
 137   if (mountpoint.dir == NULL)
 138     {
 139       __set_errno (ENOSYS);
 140       return -1;
 141     }
 142
 143   /* Construct the filename.  */
 144   while (name[0] == '/')
 145     ++name;
 146
 147   namelen = strlen (name);
 148
 149   /* Validate the filename.  */
 150   if (name[0] == '\0' || namelen > NAME_MAX || strchr (name, '/') != NULL)
 151     {
 152       __set_errno (EINVAL);
 153       return -1;
 154     }
 155
 156   fname = (char *) alloca (mountpoint.dirlen + namelen + 1);
 157   __mempcpy (__mempcpy (fname, mountpoint.dir, mountpoint.dirlen),
 158              name, namelen + 1);
 159
 160   /* And get the file descriptor.
 161      XXX Maybe we should test each descriptor whether it really is for a
 162      file on the shmfs.  If this is what should be done the whole function
 163      should be revamped since we can determine whether shmfs is available
 164      while trying to open the file, all in one turn.  */
 165   fd = open (fname, oflag | O_CLOEXEC | O_NOFOLLOW, mode);
 166   if (fd == -1 && __glibc_unlikely (errno == EISDIR))
 167     /* It might be better to fold this error with EINVAL since
 168        directory names are just another example for unsuitable shared
 169        object names and the standard does not mention EISDIR.  */
 170     __set_errno (EINVAL);
 171
 172   return fd;
 173 }
 174
 175
 176 /* Unlink a shared memory object.  */
 177 int
 178 shm_unlink (const char *name)
 179 {
 180   size_t namelen;
 181   char *fname;
 182
 183   /* Determine where the shmfs is mounted.  */
 184   __libc_once (once, where_is_shmfs);
 185
 186   if (mountpoint.dir == NULL)
 187     {
 188       /* We cannot find the shmfs.  If `name' is really a shared
 189          memory object it must have been created by another process
 190          and we have no idea where that process found the mountpoint.  */
 191       __set_errno (ENOENT);
 192       return -1;
 193     }
 194
 195   /* Construct the filename.  */
 196   while (name[0] == '/')
 197     ++name;
 198
 199   namelen = strlen (name);
 200
 201   /* Validate the filename.  */
 202   if (name[0] == '\0' || namelen > NAME_MAX || strchr (name, '/') != NULL)
 203     {
 204       __set_errno (ENOENT);
 205       return -1;
 206     }
 207
 208   fname = (char *) alloca (mountpoint.dirlen + namelen + 1);
 209   __mempcpy (__mempcpy (fname, mountpoint.dir, mountpoint.dirlen),
 210              name, namelen + 1);
 211
 212   /* And remove the file.  */
 213   int ret = unlink (fname);
 214   if (ret < 0 && errno == EPERM)
 215     __set_errno (EACCES);
 216   return ret;
 217 }
 218
 219
 220 /* Make sure the table is freed if we want to free everything before
 221    exiting.  */
 222 libc_freeres_fn (freeit)
 223 {
 224   if (mountpoint.dir != defaultdir)
 225     free (mountpoint.dir);
 226 }