sysdeps/unix/sysv/linux/shm_open.c

   1 /* Copyright (C) 2000-2013 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       {
  91         /* Found it.  There might be more than one place where the
  92            filesystem is mounted but one is enough for us.  */
  93         size_t namelen;
  94
  95         /* First make sure this really is the correct entry.  At least
  96            some versions of the kernel give wrong information because
  97            of the implicit mount of the shmfs for SysV IPC.  */
  98         if (__statfs (mp->mnt_dir, &f) != 0 || f.f_type != SHMFS_SUPER_MAGIC)
  99           continue;
 100
 101         namelen = strlen (mp->mnt_dir);
 102
 103         if (namelen == 0)
 104           /* Hum, maybe some crippled entry.  Keep on searching.  */
 105           continue;
 106
 107         mountpoint.dir = (char *) malloc (namelen + 2);
 108         if (mountpoint.dir != NULL)
 109           {
 110             char *cp = __mempcpy (mountpoint.dir, mp->mnt_dir, namelen);
 111             if (cp[-1] != '/')
 112               *cp++ = '/';
 113             *cp = '\0';
 114             mountpoint.dirlen = cp - mountpoint.dir;
 115           }
 116
 117         break;
 118       }
 119
 120   /* Close the stream.  */
 121   __endmntent (fp);
 122 }
 123
 124
 125 /* Open shared memory object.  This implementation assumes the shmfs
 126    implementation introduced in the late 2.3.x kernel series to be
 127    available.  Normally the filesystem will be mounted at /dev/shm but
 128    we fall back on searching for the actual mount point should opening
 129    such a file fail.  */
 130 int
 131 shm_open (const char *name, int oflag, mode_t mode)
 132 {
 133   size_t namelen;
 134   char *fname;
 135   int fd;
 136
 137   /* Determine where the shmfs is mounted.  */
 138   __libc_once (once, where_is_shmfs);
 139
 140   /* If we don't know the mount points there is nothing we can do.  Ever.  */
 141   if (mountpoint.dir == NULL)
 142     {
 143       __set_errno (ENOSYS);
 144       return -1;
 145     }
 146
 147   /* Construct the filename.  */
 148   while (name[0] == '/')
 149     ++name;
 150
 151   if (name[0] == '\0')
 152     {
 153       /* The name "/" is not supported.  */
 154       __set_errno (EINVAL);
 155       return -1;
 156     }
 157
 158   namelen = strlen (name);
 159   fname = (char *) alloca (mountpoint.dirlen + namelen + 1);
 160   __mempcpy (__mempcpy (fname, mountpoint.dir, mountpoint.dirlen),
 161              name, namelen + 1);
 162
 163 #ifdef O_CLOEXEC
 164   oflag |= O_CLOEXEC;
 165 #endif
 166
 167   /* And get the file descriptor.
 168      XXX Maybe we should test each descriptor whether it really is for a
 169      file on the shmfs.  If this is what should be done the whole function
 170      should be revamped since we can determine whether shmfs is available
 171      while trying to open the file, all in one turn.  */
 172   fd = open (fname, oflag | O_NOFOLLOW, mode);
 173   if (fd != -1)
 174     {
 175 #if !defined O_CLOEXEC || !defined __ASSUME_O_CLOEXEC
 176 # ifdef O_CLOEXEC
 177       if (have_o_cloexec <= 0)
 178 # endif
 179         {
 180           /* We got a descriptor.  Now set the FD_CLOEXEC bit.  */
 181           int flags = fcntl (fd, F_GETFD, 0);
 182
 183           if (__builtin_expect (flags, 0) >= 0)
 184             {
 185 # ifdef O_CLOEXEC
 186               if (have_o_cloexec == 0)
 187                 have_o_cloexec = (flags & FD_CLOEXEC) == 0 ? -1 : 1;
 188               if (have_o_cloexec < 0)
 189 # endif
 190                 {
 191                   flags |= FD_CLOEXEC;
 192                   flags = fcntl (fd, F_SETFD, flags);
 193                 }
 194             }
 195
 196           if (flags == -1)
 197             {
 198               /* Something went wrong.  We cannot return the descriptor.  */
 199               int save_errno = errno;
 200               close (fd);
 201               fd = -1;
 202               __set_errno (save_errno);
 203             }
 204         }
 205 #endif
 206     }
 207   else if (__builtin_expect (errno == EISDIR, 0))
 208     /* It might be better to fold this error with EINVAL since
 209        directory names are just another example for unsuitable shared
 210        object names and the standard does not mention EISDIR.  */
 211     __set_errno (EINVAL);
 212
 213   return fd;
 214 }
 215
 216
 217 /* Unlink a shared memory object.  */
 218 int
 219 shm_unlink (const char *name)
 220 {
 221   size_t namelen;
 222   char *fname;
 223
 224   /* Determine where the shmfs is mounted.  */
 225   __libc_once (once, where_is_shmfs);
 226
 227   if (mountpoint.dir == NULL)
 228     {
 229       /* We cannot find the shmfs.  If `name' is really a shared
 230          memory object it must have been created by another process
 231          and we have no idea where that process found the mountpoint.  */
 232       __set_errno (ENOENT);
 233       return -1;
 234     }
 235
 236   /* Construct the filename.  */
 237   while (name[0] == '/')
 238     ++name;
 239
 240   if (name[0] == '\0')
 241     {
 242       /* The name "/" is not supported.  */
 243       __set_errno (ENOENT);
 244       return -1;
 245     }
 246
 247   namelen = strlen (name);
 248   fname = (char *) alloca (mountpoint.dirlen + namelen + 1);
 249   __mempcpy (__mempcpy (fname, mountpoint.dir, mountpoint.dirlen),
 250              name, namelen + 1);
 251
 252   /* And remove the file.  */
 253   int ret = unlink (fname);
 254   if (ret < 0 && errno == EPERM)
 255     __set_errno (EACCES);
 256   return ret;
 257 }
 258
 259
 260 /* Make sure the table is freed if we want to free everything before
 261    exiting.  */
 262 libc_freeres_fn (freeit)
 263 {
 264   if (mountpoint.dir != defaultdir)
 265     free (mountpoint.dir);
 266 }