sysdeps/unix/sysv/linux/shm_open.c

   1 /* Copyright (C) 2000,2001,2002,2003,2004,2006 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, write to the Free
  16    Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
  17    02111-1307 USA.  */
  18
  19 #include <errno.h>
  20 #include <fcntl.h>
  21 #include <mntent.h>
  22 #include <paths.h>
  23 #include <stdio.h>
  24 #include <stdlib.h>
  25 #include <string.h>
  26 #include <unistd.h>
  27 #include <sys/mman.h>
  28 #include <sys/statfs.h>
  29 #include <bits/libc-lock.h>
  30 #include "linux_fsinfo.h"
  31
  32 #include <kernel-features.h>
  33
  34
  35 /* Mount point of the shared memory filesystem.  */
  36 static struct
  37 {
  38   char *dir;
  39   size_t dirlen;
  40 } mountpoint;
  41
  42 /* This is the default directory.  */
  43 static const char defaultdir[] = "/dev/shm/";
  44
  45 /* Protect the `mountpoint' variable above.  */
  46 __libc_once_define (static, once);
  47
  48
  49 /* Determine where the shmfs is mounted (if at all).  */
  50 static void
  51 where_is_shmfs (void)
  52 {
  53   char buf[512];
  54   struct statfs f;
  55   struct mntent resmem;
  56   struct mntent *mp;
  57   FILE *fp;
  58
  59   /* The canonical place is /dev/shm.  This is at least what the
  60      documentation tells everybody to do.  */
  61   if (__statfs (defaultdir, &f) == 0 && f.f_type == SHMFS_SUPER_MAGIC)
  62     {
  63       /* It is in the normal place.  */
  64       mountpoint.dir = (char *) defaultdir;
  65       mountpoint.dirlen = sizeof (defaultdir) - 1;
  66
  67       return;
  68     }
  69
  70   /* OK, do it the hard way.  Look through the /proc/mounts file and if
  71      this does not exist through /etc/fstab to find the mount point.  */
  72   fp = __setmntent ("/proc/mounts", "r");
  73   if (__builtin_expect (fp == NULL, 0))
  74     {
  75       fp = __setmntent (_PATH_MNTTAB, "r");
  76       if (__builtin_expect (fp == NULL, 0))
  77         /* There is nothing we can do.  Blind guesses are not helpful.  */
  78         return;
  79     }
  80
  81   /* Now read the entries.  */
  82   while ((mp = __getmntent_r (fp, &resmem, buf, sizeof buf)) != NULL)
  83     /* The original name is "shm" but this got changed in early Linux
  84        2.4.x to "tmpfs".  */
  85     if (strcmp (mp->mnt_type, "tmpfs") == 0
  86 #ifndef __ASSUME_TMPFS_NAME
  87         || strcmp (mp->mnt_type, "shm") == 0
  88 #endif
  89         )
  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   /* And get the file descriptor.
 164      XXX Maybe we should test each descriptor whether it really is for a
 165      file on the shmfs.  If this is what should be done the whole function
 166      should be revamped since we can determine whether shmfs is available
 167      while trying to open the file, all in one turn.  */
 168   fd = open (fname, oflag | O_NOFOLLOW, mode);
 169   if (fd != -1)
 170     {
 171       /* We got a descriptor.  Now set the FD_CLOEXEC bit.  */
 172       int flags = fcntl (fd, F_GETFD, 0);
 173
 174       if (__builtin_expect (flags, 0) >= 0)
 175         {
 176           flags |= FD_CLOEXEC;
 177           flags = fcntl (fd, F_SETFD, flags);
 178         }
 179
 180       if (flags == -1)
 181         {
 182           /* Something went wrong.  We cannot return the descriptor.  */
 183           int save_errno = errno;
 184           close (fd);
 185           fd = -1;
 186           __set_errno (save_errno);
 187         }
 188     }
 189   else if (__builtin_expect (errno == EISDIR, 0))
 190     /* It might be better to fold this error with EINVAL since
 191        directory names are just another example for unsuitable shared
 192        object names and the standard does not mention EISDIR.  */
 193     __set_errno (EINVAL);
 194
 195   return fd;
 196 }
 197
 198
 199 /* Unlink a shared memory object.  */
 200 int
 201 shm_unlink (const char *name)
 202 {
 203   size_t namelen;
 204   char *fname;
 205
 206   /* Determine where the shmfs is mounted.  */
 207   __libc_once (once, where_is_shmfs);
 208
 209   if (mountpoint.dir == NULL)
 210     {
 211       /* We cannot find the shmfs.  If `name' is really a shared
 212          memory object it must have been created by another process
 213          and we have no idea where that process found the mountpoint.  */
 214       __set_errno (ENOENT);
 215       return -1;
 216     }
 217
 218   /* Construct the filename.  */
 219   while (name[0] == '/')
 220     ++name;
 221
 222   if (name[0] == '\0')
 223     {
 224       /* The name "/" is not supported.  */
 225       __set_errno (ENOENT);
 226       return -1;
 227     }
 228
 229   namelen = strlen (name);
 230   fname = (char *) alloca (mountpoint.dirlen + namelen + 1);
 231   __mempcpy (__mempcpy (fname, mountpoint.dir, mountpoint.dirlen),
 232              name, namelen + 1);
 233
 234   /* And remove the file.  */
 235   int ret = unlink (fname);
 236   if (ret < 0 && errno == EPERM)
 237     __set_errno (EACCES);
 238   return ret;
 239 }
 240
 241
 242 /* Make sure the table is freed if we want to free everything before
 243    exiting.  */
 244 libc_freeres_fn (freeit)
 245 {
 246   if (mountpoint.dir != defaultdir)
 247     free (mountpoint.dir);
 248 }