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 #include <kernel-features.h>
  32
  33
  34 /* Mount point of the shared memory filesystem.  */
  35 static struct
  36 {
  37   char *dir;
  38   size_t dirlen;
  39 } mountpoint;
  40
  41 /* This is the default directory.  */
  42 static const char defaultdir[] = "/dev/shm/";
  43
  44 /* Protect the `mountpoint' variable above.  */
  45 __libc_once_define (static, once);
  46
  47
  48 #if defined O_CLOEXEC && !defined __ASSUME_O_CLOEXEC
  49 static bool have_o_cloexec;
  50 #endif
  51
  52
  53 /* Determine where the shmfs is mounted (if at all).  */
  54 static void
  55 where_is_shmfs (void)
  56 {
  57   char buf[512];
  58   struct statfs f;
  59   struct mntent resmem;
  60   struct mntent *mp;
  61   FILE *fp;
  62
  63   /* The canonical place is /dev/shm.  This is at least what the
  64      documentation tells everybody to do.  */
  65   if (__statfs (defaultdir, &f) == 0 && (f.f_type == SHMFS_SUPER_MAGIC
  66                                          || f.f_type == RAMFS_MAGIC))
  67     {
  68       /* It is in the normal place.  */
  69       mountpoint.dir = (char *) defaultdir;
  70       mountpoint.dirlen = sizeof (defaultdir) - 1;
  71
  72       return;
  73     }
  74
  75   /* OK, do it the hard way.  Look through the /proc/mounts file and if
  76      this does not exist through /etc/fstab to find the mount point.  */
  77   fp = __setmntent ("/proc/mounts", "r");
  78   if (__builtin_expect (fp == NULL, 0))
  79     {
  80       fp = __setmntent (_PATH_MNTTAB, "r");
  81       if (__builtin_expect (fp == NULL, 0))
  82         /* There is nothing we can do.  Blind guesses are not helpful.  */
  83         return;
  84     }
  85
  86   /* Now read the entries.  */
  87   while ((mp = __getmntent_r (fp, &resmem, buf, sizeof buf)) != NULL)
  88     /* The original name is "shm" but this got changed in early Linux
  89        2.4.x to "tmpfs".  */
  90     if (strcmp (mp->mnt_type, "tmpfs") == 0
  91         || strcmp (mp->mnt_type, "shm") == 0)
  92       {
  93         /* Found it.  There might be more than one place where the
  94            filesystem is mounted but one is enough for us.  */
  95         size_t namelen;
  96
  97         /* First make sure this really is the correct entry.  At least
  98            some versions of the kernel give wrong information because
  99            of the implicit mount of the shmfs for SysV IPC.  */
 100         if (__statfs (mp->mnt_dir, &f) != 0 || (f.f_type != SHMFS_SUPER_MAGIC
 101                                                 && f.f_type != RAMFS_MAGIC))
 102           continue;
 103
 104         namelen = strlen (mp->mnt_dir);
 105
 106         if (namelen == 0)
 107           /* Hum, maybe some crippled entry.  Keep on searching.  */
 108           continue;
 109
 110         mountpoint.dir = (char *) malloc (namelen + 2);
 111         if (mountpoint.dir != NULL)
 112           {
 113             char *cp = __mempcpy (mountpoint.dir, mp->mnt_dir, namelen);
 114             if (cp[-1] != '/')
 115               *cp++ = '/';
 116             *cp = '\0';
 117             mountpoint.dirlen = cp - mountpoint.dir;
 118           }
 119
 120         break;
 121       }
 122
 123   /* Close the stream.  */
 124   __endmntent (fp);
 125 }
 126
 127
 128 /* Open shared memory object.  This implementation assumes the shmfs
 129    implementation introduced in the late 2.3.x kernel series to be
 130    available.  Normally the filesystem will be mounted at /dev/shm but
 131    we fall back on searching for the actual mount point should opening
 132    such a file fail.  */
 133 int
 134 shm_open (const char *name, int oflag, mode_t mode)
 135 {
 136   size_t namelen;
 137   char *fname;
 138   int fd;
 139
 140   /* Determine where the shmfs is mounted.  */
 141   __libc_once (once, where_is_shmfs);
 142
 143   /* If we don't know the mount points there is nothing we can do.  Ever.  */
 144   if (mountpoint.dir == NULL)
 145     {
 146       __set_errno (ENOSYS);
 147       return -1;
 148     }
 149
 150   /* Construct the filename.  */
 151   while (name[0] == '/')
 152     ++name;
 153
 154   namelen = strlen (name);
 155
 156   /* Validate the filename.  */
 157   if (name[0] == '\0' || namelen > NAME_MAX || strchr (name, '/') != NULL)
 158     {
 159       __set_errno (EINVAL);
 160       return -1;
 161     }
 162
 163   fname = (char *) alloca (mountpoint.dirlen + namelen + 1);
 164   __mempcpy (__mempcpy (fname, mountpoint.dir, mountpoint.dirlen),
 165              name, namelen + 1);
 166
 167 #ifdef O_CLOEXEC
 168   oflag |= O_CLOEXEC;
 169 #endif
 170
 171   /* And get the file descriptor.
 172      XXX Maybe we should test each descriptor whether it really is for a
 173      file on the shmfs.  If this is what should be done the whole function
 174      should be revamped since we can determine whether shmfs is available
 175      while trying to open the file, all in one turn.  */
 176   fd = open (fname, oflag | O_NOFOLLOW, mode);
 177   if (fd != -1)
 178     {
 179 #if !defined O_CLOEXEC || !defined __ASSUME_O_CLOEXEC
 180 # ifdef O_CLOEXEC
 181       if (have_o_cloexec <= 0)
 182 # endif
 183         {
 184           /* We got a descriptor.  Now set the FD_CLOEXEC bit.  */
 185           int flags = fcntl (fd, F_GETFD, 0);
 186
 187           if (__builtin_expect (flags, 0) >= 0)
 188             {
 189 # ifdef O_CLOEXEC
 190               if (have_o_cloexec == 0)
 191                 have_o_cloexec = (flags & FD_CLOEXEC) == 0 ? -1 : 1;
 192               if (have_o_cloexec < 0)
 193 # endif
 194                 {
 195                   flags |= FD_CLOEXEC;
 196                   flags = fcntl (fd, F_SETFD, flags);
 197                 }
 198             }
 199
 200           if (flags == -1)
 201             {
 202               /* Something went wrong.  We cannot return the descriptor.  */
 203               int save_errno = errno;
 204               close (fd);
 205               fd = -1;
 206               __set_errno (save_errno);
 207             }
 208         }
 209 #endif
 210     }
 211   else if (__builtin_expect (errno == EISDIR, 0))
 212     /* It might be better to fold this error with EINVAL since
 213        directory names are just another example for unsuitable shared
 214        object names and the standard does not mention EISDIR.  */
 215     __set_errno (EINVAL);
 216
 217   return fd;
 218 }
 219
 220
 221 /* Unlink a shared memory object.  */
 222 int
 223 shm_unlink (const char *name)
 224 {
 225   size_t namelen;
 226   char *fname;
 227
 228   /* Determine where the shmfs is mounted.  */
 229   __libc_once (once, where_is_shmfs);
 230
 231   if (mountpoint.dir == NULL)
 232     {
 233       /* We cannot find the shmfs.  If `name' is really a shared
 234          memory object it must have been created by another process
 235          and we have no idea where that process found the mountpoint.  */
 236       __set_errno (ENOENT);
 237       return -1;
 238     }
 239
 240   /* Construct the filename.  */
 241   while (name[0] == '/')
 242     ++name;
 243
 244   namelen = strlen (name);
 245
 246   /* Validate the filename.  */
 247   if (name[0] == '\0' || namelen > NAME_MAX || strchr (name, '/') != NULL)
 248     {
 249       __set_errno (ENOENT);
 250       return -1;
 251     }
 252
 253   fname = (char *) alloca (mountpoint.dirlen + namelen + 1);
 254   __mempcpy (__mempcpy (fname, mountpoint.dir, mountpoint.dirlen),
 255              name, namelen + 1);
 256
 257   /* And remove the file.  */
 258   int ret = unlink (fname);
 259   if (ret < 0 && errno == EPERM)
 260     __set_errno (EACCES);
 261   return ret;
 262 }
 263
 264
 265 /* Make sure the table is freed if we want to free everything before
 266    exiting.  */
 267 libc_freeres_fn (freeit)
 268 {
 269   if (mountpoint.dir != defaultdir)
 270     free (mountpoint.dir);
 271 }