nptl/sem_open.c

   1 /* Copyright (C) 2002-2013 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3    Contributed by Ulrich Drepper <drepper@redhat.com>, 2002.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <http://www.gnu.org/licenses/>.  */
  18
  19 #include <errno.h>
  20 #include <fcntl.h>
  21 #include <mntent.h>
  22 #include <paths.h>
  23 #include <pthread.h>
  24 #include <search.h>
  25 #include <semaphore.h>
  26 #include <stdarg.h>
  27 #include <stdio.h>
  28 #include <stdlib.h>
  29 #include <string.h>
  30 #include <unistd.h>
  31 #include <sys/mman.h>
  32 #include <sys/stat.h>
  33 #include <sys/statfs.h>
  34 #include <linux_fsinfo.h>
  35 #include "semaphoreP.h"
  36
  37
  38
  39 /* Information about the mount point.  */
  40 struct mountpoint_info mountpoint attribute_hidden;
  41
  42 /* This is the default mount point.  */
  43 static const char defaultmount[] = "/dev/shm";
  44 /* This is the default directory.  */
  45 static const char defaultdir[] = "/dev/shm/sem.";
  46
  47 /* Protect the `mountpoint' variable above.  */
  48 pthread_once_t __namedsem_once attribute_hidden = PTHREAD_ONCE_INIT;
  49
  50
  51 /* Determine where the shmfs is mounted (if at all).  */
  52 void
  53 attribute_hidden
  54 __where_is_shmfs (void)
  55 {
  56   char buf[512];
  57   struct statfs f;
  58   struct mntent resmem;
  59   struct mntent *mp;
  60   FILE *fp;
  61
  62   /* The canonical place is /dev/shm.  This is at least what the
  63      documentation tells everybody to do.  */
  64   if (__statfs (defaultmount, &f) == 0 && f.f_type == SHMFS_SUPER_MAGIC)
  65     {
  66       /* It is in the normal place.  */
  67       mountpoint.dir = (char *) defaultdir;
  68       mountpoint.dirlen = sizeof (defaultdir) - 1;
  69
  70       return;
  71     }
  72
  73   /* OK, do it the hard way.  Look through the /proc/mounts file and if
  74      this does not exist through /etc/fstab to find the mount point.  */
  75   fp = __setmntent ("/proc/mounts", "r");
  76   if (__builtin_expect (fp == NULL, 0))
  77     {
  78       fp = __setmntent (_PATH_MNTTAB, "r");
  79       if (__builtin_expect (fp == NULL, 0))
  80         /* There is nothing we can do.  Blind guesses are not helpful.  */
  81         return;
  82     }
  83
  84   /* Now read the entries.  */
  85   while ((mp = __getmntent_r (fp, &resmem, buf, sizeof buf)) != NULL)
  86     /* The original name is "shm" but this got changed in early Linux
  87        2.4.x to "tmpfs".  */
  88     if (strcmp (mp->mnt_type, "tmpfs") == 0
  89         || strcmp (mp->mnt_type, "shm") == 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 + 4 + 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 = stpcpy (cp, "sem.");
 114             mountpoint.dirlen = cp - mountpoint.dir;
 115           }
 116
 117         break;
 118       }
 119
 120   /* Close the stream.  */
 121   __endmntent (fp);
 122 }
 123
 124
 125 /* Comparison function for search of existing mapping.  */
 126 int
 127 attribute_hidden
 128 __sem_search (const void *a, const void *b)
 129 {
 130   const struct inuse_sem *as = (const struct inuse_sem *) a;
 131   const struct inuse_sem *bs = (const struct inuse_sem *) b;
 132
 133   if (as->ino != bs->ino)
 134     /* Cannot return the difference the type is larger than int.  */
 135     return as->ino < bs->ino ? -1 : (as->ino == bs->ino ? 0 : 1);
 136
 137   if (as->dev != bs->dev)
 138     /* Cannot return the difference the type is larger than int.  */
 139     return as->dev < bs->dev ? -1 : (as->dev == bs->dev ? 0 : 1);
 140
 141   return strcmp (as->name, bs->name);
 142 }
 143
 144
 145 /* The search tree for existing mappings.  */
 146 void *__sem_mappings attribute_hidden;
 147
 148 /* Lock to protect the search tree.  */
 149 int __sem_mappings_lock attribute_hidden = LLL_LOCK_INITIALIZER;
 150
 151
 152 /* Search for existing mapping and if possible add the one provided.  */
 153 static sem_t *
 154 check_add_mapping (const char *name, size_t namelen, int fd, sem_t *existing)
 155 {
 156   sem_t *result = SEM_FAILED;
 157
 158   /* Get the information about the file.  */
 159   struct stat64 st;
 160   if (__fxstat64 (_STAT_VER, fd, &st) == 0)
 161     {
 162       /* Get the lock.  */
 163       lll_lock (__sem_mappings_lock, LLL_PRIVATE);
 164
 165       /* Search for an existing mapping given the information we have.  */
 166       struct inuse_sem *fake;
 167       fake = (struct inuse_sem *) alloca (sizeof (*fake) + namelen);
 168       memcpy (fake->name, name, namelen);
 169       fake->dev = st.st_dev;
 170       fake->ino = st.st_ino;
 171
 172       struct inuse_sem **foundp = tfind (fake, &__sem_mappings, __sem_search);
 173       if (foundp != NULL)
 174         {
 175           /* There is already a mapping.  Use it.  */
 176           result = (*foundp)->sem;
 177           ++(*foundp)->refcnt;
 178         }
 179       else
 180         {
 181           /* We haven't found a mapping.  Install ione.  */
 182           struct inuse_sem *newp;
 183
 184           newp = (struct inuse_sem *) malloc (sizeof (*newp) + namelen);
 185           if (newp != NULL)
 186             {
 187               /* If the caller hasn't provided any map it now.  */
 188               if (existing == SEM_FAILED)
 189                 existing = (sem_t *) mmap (NULL, sizeof (sem_t),
 190                                            PROT_READ | PROT_WRITE, MAP_SHARED,
 191                                            fd, 0);
 192
 193               newp->dev = st.st_dev;
 194               newp->ino = st.st_ino;
 195               newp->refcnt = 1;
 196               newp->sem = existing;
 197               memcpy (newp->name, name, namelen);
 198
 199               /* Insert the new value.  */
 200               if (existing != MAP_FAILED
 201                   && tsearch (newp, &__sem_mappings, __sem_search) != NULL)
 202                 /* Successful.  */
 203                 result = existing;
 204               else
 205                 /* Something went wrong while inserting the new
 206                    value.  We fail completely.  */
 207                 free (newp);
 208             }
 209         }
 210
 211       /* Release the lock.  */
 212       lll_unlock (__sem_mappings_lock, LLL_PRIVATE);
 213     }
 214
 215   if (result != existing && existing != SEM_FAILED && existing != MAP_FAILED)
 216     {
 217       /* Do not disturb errno.  */
 218       INTERNAL_SYSCALL_DECL (err);
 219       INTERNAL_SYSCALL (munmap, err, 2, existing, sizeof (sem_t));
 220     }
 221
 222   return result;
 223 }
 224
 225
 226 sem_t *
 227 sem_open (const char *name, int oflag, ...)
 228 {
 229   char *finalname;
 230   sem_t *result = SEM_FAILED;
 231   int fd;
 232
 233   /* Determine where the shmfs is mounted.  */
 234   __pthread_once (&__namedsem_once, __where_is_shmfs);
 235
 236   /* If we don't know the mount points there is nothing we can do.  Ever.  */
 237   if (mountpoint.dir == NULL)
 238     {
 239       __set_errno (ENOSYS);
 240       return SEM_FAILED;
 241     }
 242
 243   /* Construct the filename.  */
 244   while (name[0] == '/')
 245     ++name;
 246
 247   if (name[0] == '\0')
 248     {
 249       /* The name "/" is not supported.  */
 250       __set_errno (EINVAL);
 251       return SEM_FAILED;
 252     }
 253   size_t namelen = strlen (name) + 1;
 254
 255   /* Create the name of the final file.  */
 256   finalname = (char *) alloca (mountpoint.dirlen + namelen);
 257   __mempcpy (__mempcpy (finalname, mountpoint.dir, mountpoint.dirlen),
 258              name, namelen);
 259
 260   /* If the semaphore object has to exist simply open it.  */
 261   if ((oflag & O_CREAT) == 0 || (oflag & O_EXCL) == 0)
 262     {
 263     try_again:
 264       fd = __libc_open (finalname,
 265                         (oflag & ~(O_CREAT|O_ACCMODE)) | O_NOFOLLOW | O_RDWR);
 266
 267       if (fd == -1)
 268         {
 269           /* If we are supposed to create the file try this next.  */
 270           if ((oflag & O_CREAT) != 0 && errno == ENOENT)
 271             goto try_create;
 272
 273           /* Return.  errno is already set.  */
 274         }
 275       else
 276         /* Check whether we already have this semaphore mapped and
 277            create one if necessary.  */
 278         result = check_add_mapping (name, namelen, fd, SEM_FAILED);
 279     }
 280   else
 281     {
 282       /* We have to open a temporary file first since it must have the
 283          correct form before we can start using it.  */
 284       char *tmpfname;
 285       mode_t mode;
 286       unsigned int value;
 287       va_list ap;
 288
 289     try_create:
 290       va_start (ap, oflag);
 291
 292       mode = va_arg (ap, mode_t);
 293       value = va_arg (ap, unsigned int);
 294
 295       va_end (ap);
 296
 297       if (value > SEM_VALUE_MAX)
 298         {
 299           __set_errno (EINVAL);
 300           return SEM_FAILED;
 301         }
 302
 303       /* Create the initial file content.  */
 304       union
 305       {
 306         sem_t initsem;
 307         struct new_sem newsem;
 308       } sem;
 309
 310       sem.newsem.value = value;
 311       sem.newsem.private = 0;
 312       sem.newsem.nwaiters = 0;
 313
 314       /* Initialize the remaining bytes as well.  */
 315       memset ((char *) &sem.initsem + sizeof (struct new_sem), '\0',
 316               sizeof (sem_t) - sizeof (struct new_sem));
 317
 318       tmpfname = (char *) alloca (mountpoint.dirlen + 6 + 1);
 319       char *xxxxxx = __mempcpy (tmpfname, mountpoint.dir, mountpoint.dirlen);
 320
 321       int retries = 0;
 322 #define NRETRIES 50
 323       while (1)
 324         {
 325           /* Add the suffix for mktemp.  */
 326           strcpy (xxxxxx, "XXXXXX");
 327
 328           /* We really want to use mktemp here.  We cannot use mkstemp
 329              since the file must be opened with a specific mode.  The
 330              mode cannot later be set since then we cannot apply the
 331              file create mask.  */
 332           if (__mktemp (tmpfname) == NULL)
 333             return SEM_FAILED;
 334
 335           /* Open the file.  Make sure we do not overwrite anything.  */
 336           fd = __libc_open (tmpfname, O_RDWR | O_CREAT | O_EXCL, mode);
 337           if (fd == -1)
 338             {
 339               if (errno == EEXIST)
 340                 {
 341                   if (++retries < NRETRIES)
 342                     continue;
 343
 344                   __set_errno (EAGAIN);
 345                 }
 346
 347               return SEM_FAILED;
 348             }
 349
 350           /* We got a file.  */
 351           break;
 352         }
 353
 354       if (TEMP_FAILURE_RETRY (__libc_write (fd, &sem.initsem, sizeof (sem_t)))
 355           == sizeof (sem_t)
 356           /* Map the sem_t structure from the file.  */
 357           && (result = (sem_t *) mmap (NULL, sizeof (sem_t),
 358                                        PROT_READ | PROT_WRITE, MAP_SHARED,
 359                                        fd, 0)) != MAP_FAILED)
 360         {
 361           /* Create the file.  Don't overwrite an existing file.  */
 362           if (link (tmpfname, finalname) != 0)
 363             {
 364               /* Undo the mapping.  */
 365               (void) munmap (result, sizeof (sem_t));
 366
 367               /* Reinitialize 'result'.  */
 368               result = SEM_FAILED;
 369
 370               /* This failed.  If O_EXCL is not set and the problem was
 371                  that the file exists, try again.  */
 372               if ((oflag & O_EXCL) == 0 && errno == EEXIST)
 373                 {
 374                   /* Remove the file.  */
 375                   (void) unlink (tmpfname);
 376
 377                   /* Close the file.  */
 378                   (void) __libc_close (fd);
 379
 380                   goto try_again;
 381                 }
 382             }
 383           else
 384             /* Insert the mapping into the search tree.  This also
 385                determines whether another thread sneaked by and already
 386                added such a mapping despite the fact that we created it.  */
 387             result = check_add_mapping (name, namelen, fd, result);
 388         }
 389
 390       /* Now remove the temporary name.  This should never fail.  If
 391          it fails we leak a file name.  Better fix the kernel.  */
 392       (void) unlink (tmpfname);
 393     }
 394
 395   /* Map the mmap error to the error we need.  */
 396   if (MAP_FAILED != (void *) SEM_FAILED && result == MAP_FAILED)
 397     result = SEM_FAILED;
 398
 399   /* We don't need the file descriptor anymore.  */
 400   if (fd != -1)
 401     {
 402       /* Do not disturb errno.  */
 403       INTERNAL_SYSCALL_DECL (err);
 404       INTERNAL_SYSCALL (close, err, 1, fd);
 405     }
 406
 407   return result;
 408 }