sysdeps/unix/sysv/linux/shm_open.c

   1 /* Copyright (C) 2000-2004,2006,2007 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     {
  67       /* It is in the normal place.  */
  68       mountpoint.dir = (char *) defaultdir;
  69       mountpoint.dirlen = sizeof (defaultdir) - 1;
  70
  71       return;
  72     }
  73
  74   /* OK, do it the hard way.  Look through the /proc/mounts file and if
  75      this does not exist through /etc/fstab to find the mount point.  */
  76   fp = __setmntent ("/proc/mounts", "r");
  77   if (__builtin_expect (fp == NULL, 0))
  78     {
  79       fp = __setmntent (_PATH_MNTTAB, "r");
  80       if (__builtin_expect (fp == NULL, 0))
  81         /* There is nothing we can do.  Blind guesses are not helpful.  */
  82         return;
  83     }
  84
  85   /* Now read the entries.  */
  86   while ((mp = __getmntent_r (fp, &resmem, buf, sizeof buf)) != NULL)
  87     /* The original name is "shm" but this got changed in early Linux
  88        2.4.x to "tmpfs".  */
  89     if (strcmp (mp->mnt_type, "tmpfs") == 0
  90 #ifndef __ASSUME_TMPFS_NAME
  91         || strcmp (mp->mnt_type, "shm") == 0
  92 #endif
  93         )
  94       {
  95         /* Found it.  There might be more than one place where the
  96            filesystem is mounted but one is enough for us.  */
  97         size_t namelen;
  98
  99         /* First make sure this really is the correct entry.  At least
 100            some versions of the kernel give wrong information because
 101            of the implicit mount of the shmfs for SysV IPC.  */
 102         if (__statfs (mp->mnt_dir, &f) != 0 || f.f_type != SHMFS_SUPER_MAGIC)
 103           continue;
 104
 105         namelen = strlen (mp->mnt_dir);
 106
 107         if (namelen == 0)
 108           /* Hum, maybe some crippled entry.  Keep on searching.  */
 109           continue;
 110
 111         mountpoint.dir = (char *) malloc (namelen + 2);
 112         if (mountpoint.dir != NULL)
 113           {
 114             char *cp = __mempcpy (mountpoint.dir, mp->mnt_dir, namelen);
 115             if (cp[-1] != '/')
 116               *cp++ = '/';
 117             *cp = '\0';
 118             mountpoint.dirlen = cp - mountpoint.dir;
 119           }
 120
 121         break;
 122       }
 123
 124   /* Close the stream.  */
 125   __endmntent (fp);
 126 }
 127
 128
 129 /* Open shared memory object.  This implementation assumes the shmfs
 130    implementation introduced in the late 2.3.x kernel series to be
 131    available.  Normally the filesystem will be mounted at /dev/shm but
 132    we fall back on searching for the actual mount point should opening
 133    such a file fail.  */
 134 int
 135 shm_open (const char *name, int oflag, mode_t mode)
 136 {
 137   size_t namelen;
 138   char *fname;
 139   int fd;
 140
 141   /* Determine where the shmfs is mounted.  */
 142   __libc_once (once, where_is_shmfs);
 143
 144   /* If we don't know the mount points there is nothing we can do.  Ever.  */
 145   if (mountpoint.dir == NULL)
 146     {
 147       __set_errno (ENOSYS);
 148       return -1;
 149     }
 150
 151   /* Construct the filename.  */
 152   while (name[0] == '/')
 153     ++name;
 154
 155   if (name[0] == '\0')
 156     {
 157       /* The name "/" is not supported.  */
 158       __set_errno (EINVAL);
 159       return -1;
 160     }
 161
 162   namelen = strlen (name);
 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   if (name[0] == '\0')
 245     {
 246       /* The name "/" is not supported.  */
 247       __set_errno (ENOENT);
 248       return -1;
 249     }
 250
 251   namelen = strlen (name);
 252   fname = (char *) alloca (mountpoint.dirlen + namelen + 1);
 253   __mempcpy (__mempcpy (fname, mountpoint.dir, mountpoint.dirlen),
 254              name, namelen + 1);
 255
 256   /* And remove the file.  */
 257   int ret = unlink (fname);
 258   if (ret < 0 && errno == EPERM)
 259     __set_errno (EACCES);
 260   return ret;
 261 }
 262
 263
 264 /* Make sure the table is freed if we want to free everything before
 265    exiting.  */
 266 libc_freeres_fn (freeit)
 267 {
 268   if (mountpoint.dir != defaultdir)
 269     free (mountpoint.dir);
 270 }