nptl/sem_open.c

   1 /* Copyright (C) 2002-2015 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 <pthread.h>
  22 #include <search.h>
  23 #include <semaphore.h>
  24 #include <stdarg.h>
  25 #include <stdio.h>
  26 #include <stdlib.h>
  27 #include <string.h>
  28 #include <unistd.h>
  29 #include <sys/mman.h>
  30 #include <sys/stat.h>
  31 #include "semaphoreP.h"
  32 #include <shm-directory.h>
  33
  34
  35 /* Comparison function for search of existing mapping.  */
  36 int
  37 attribute_hidden
  38 __sem_search (const void *a, const void *b)
  39 {
  40   const struct inuse_sem *as = (const struct inuse_sem *) a;
  41   const struct inuse_sem *bs = (const struct inuse_sem *) b;
  42
  43   if (as->ino != bs->ino)
  44     /* Cannot return the difference the type is larger than int.  */
  45     return as->ino < bs->ino ? -1 : (as->ino == bs->ino ? 0 : 1);
  46
  47   if (as->dev != bs->dev)
  48     /* Cannot return the difference the type is larger than int.  */
  49     return as->dev < bs->dev ? -1 : (as->dev == bs->dev ? 0 : 1);
  50
  51   return strcmp (as->name, bs->name);
  52 }
  53
  54
  55 /* The search tree for existing mappings.  */
  56 void *__sem_mappings attribute_hidden;
  57
  58 /* Lock to protect the search tree.  */
  59 int __sem_mappings_lock attribute_hidden = LLL_LOCK_INITIALIZER;
  60
  61
  62 /* Search for existing mapping and if possible add the one provided.  */
  63 static sem_t *
  64 check_add_mapping (const char *name, size_t namelen, int fd, sem_t *existing)
  65 {
  66   sem_t *result = SEM_FAILED;
  67
  68   /* Get the information about the file.  */
  69   struct stat64 st;
  70   if (__fxstat64 (_STAT_VER, fd, &st) == 0)
  71     {
  72       /* Get the lock.  */
  73       lll_lock (__sem_mappings_lock, LLL_PRIVATE);
  74
  75       /* Search for an existing mapping given the information we have.  */
  76       struct inuse_sem *fake;
  77       fake = (struct inuse_sem *) alloca (sizeof (*fake) + namelen);
  78       memcpy (fake->name, name, namelen);
  79       fake->dev = st.st_dev;
  80       fake->ino = st.st_ino;
  81
  82       struct inuse_sem **foundp = tfind (fake, &__sem_mappings, __sem_search);
  83       if (foundp != NULL)
  84         {
  85           /* There is already a mapping.  Use it.  */
  86           result = (*foundp)->sem;
  87           ++(*foundp)->refcnt;
  88         }
  89       else
  90         {
  91           /* We haven't found a mapping.  Install ione.  */
  92           struct inuse_sem *newp;
  93
  94           newp = (struct inuse_sem *) malloc (sizeof (*newp) + namelen);
  95           if (newp != NULL)
  96             {
  97               /* If the caller hasn't provided any map it now.  */
  98               if (existing == SEM_FAILED)
  99                 existing = (sem_t *) mmap (NULL, sizeof (sem_t),
 100                                            PROT_READ | PROT_WRITE, MAP_SHARED,
 101                                            fd, 0);
 102
 103               newp->dev = st.st_dev;
 104               newp->ino = st.st_ino;
 105               newp->refcnt = 1;
 106               newp->sem = existing;
 107               memcpy (newp->name, name, namelen);
 108
 109               /* Insert the new value.  */
 110               if (existing != MAP_FAILED
 111                   && tsearch (newp, &__sem_mappings, __sem_search) != NULL)
 112                 /* Successful.  */
 113                 result = existing;
 114               else
 115                 /* Something went wrong while inserting the new
 116                    value.  We fail completely.  */
 117                 free (newp);
 118             }
 119         }
 120
 121       /* Release the lock.  */
 122       lll_unlock (__sem_mappings_lock, LLL_PRIVATE);
 123     }
 124
 125   if (result != existing && existing != SEM_FAILED && existing != MAP_FAILED)
 126     {
 127       /* Do not disturb errno.  */
 128       int save = errno;
 129       munmap (existing, sizeof (sem_t));
 130       errno = save;
 131     }
 132
 133   return result;
 134 }
 135
 136
 137 sem_t *
 138 sem_open (const char *name, int oflag, ...)
 139 {
 140   int fd;
 141   sem_t *result;
 142
 143   /* Create the name of the final file in local variable SHM_NAME.  */
 144   SHM_GET_NAME (EINVAL, SEM_FAILED, SEM_SHM_PREFIX);
 145
 146   /* If the semaphore object has to exist simply open it.  */
 147   if ((oflag & O_CREAT) == 0 || (oflag & O_EXCL) == 0)
 148     {
 149     try_again:
 150       fd = __libc_open (shm_name,
 151                         (oflag & ~(O_CREAT|O_ACCMODE)) | O_NOFOLLOW | O_RDWR);
 152
 153       if (fd == -1)
 154         {
 155           /* If we are supposed to create the file try this next.  */
 156           if ((oflag & O_CREAT) != 0 && errno == ENOENT)
 157             goto try_create;
 158
 159           /* Return.  errno is already set.  */
 160         }
 161       else
 162         /* Check whether we already have this semaphore mapped and
 163            create one if necessary.  */
 164         result = check_add_mapping (name, namelen, fd, SEM_FAILED);
 165     }
 166   else
 167     {
 168       /* We have to open a temporary file first since it must have the
 169          correct form before we can start using it.  */
 170       char *tmpfname;
 171       mode_t mode;
 172       unsigned int value;
 173       va_list ap;
 174
 175     try_create:
 176       va_start (ap, oflag);
 177
 178       mode = va_arg (ap, mode_t);
 179       value = va_arg (ap, unsigned int);
 180
 181       va_end (ap);
 182
 183       if (value > SEM_VALUE_MAX)
 184         {
 185           __set_errno (EINVAL);
 186           return SEM_FAILED;
 187         }
 188
 189       /* Create the initial file content.  */
 190       union
 191       {
 192         sem_t initsem;
 193         struct new_sem newsem;
 194       } sem;
 195
 196       sem.newsem.value = value;
 197       sem.newsem.private = 0;
 198       sem.newsem.nwaiters = 0;
 199
 200       /* Initialize the remaining bytes as well.  */
 201       memset ((char *) &sem.initsem + sizeof (struct new_sem), '\0',
 202               sizeof (sem_t) - sizeof (struct new_sem));
 203
 204       tmpfname = __alloca (shm_dirlen + sizeof SEM_SHM_PREFIX + 6);
 205       char *xxxxxx = __mempcpy (tmpfname, shm_dir, shm_dirlen);
 206
 207       int retries = 0;
 208 #define NRETRIES 50
 209       while (1)
 210         {
 211           /* Add the suffix for mktemp.  */
 212           strcpy (xxxxxx, "XXXXXX");
 213
 214           /* We really want to use mktemp here.  We cannot use mkstemp
 215              since the file must be opened with a specific mode.  The
 216              mode cannot later be set since then we cannot apply the
 217              file create mask.  */
 218           if (__mktemp (tmpfname) == NULL)
 219             return SEM_FAILED;
 220
 221           /* Open the file.  Make sure we do not overwrite anything.  */
 222           fd = __libc_open (tmpfname, O_RDWR | O_CREAT | O_EXCL, mode);
 223           if (fd == -1)
 224             {
 225               if (errno == EEXIST)
 226                 {
 227                   if (++retries < NRETRIES)
 228                     continue;
 229
 230                   __set_errno (EAGAIN);
 231                 }
 232
 233               return SEM_FAILED;
 234             }
 235
 236           /* We got a file.  */
 237           break;
 238         }
 239
 240       if (TEMP_FAILURE_RETRY (__libc_write (fd, &sem.initsem, sizeof (sem_t)))
 241           == sizeof (sem_t)
 242           /* Map the sem_t structure from the file.  */
 243           && (result = (sem_t *) mmap (NULL, sizeof (sem_t),
 244                                        PROT_READ | PROT_WRITE, MAP_SHARED,
 245                                        fd, 0)) != MAP_FAILED)
 246         {
 247           /* Create the file.  Don't overwrite an existing file.  */
 248           if (link (tmpfname, shm_name) != 0)
 249             {
 250               /* Undo the mapping.  */
 251               (void) munmap (result, sizeof (sem_t));
 252
 253               /* Reinitialize 'result'.  */
 254               result = SEM_FAILED;
 255
 256               /* This failed.  If O_EXCL is not set and the problem was
 257                  that the file exists, try again.  */
 258               if ((oflag & O_EXCL) == 0 && errno == EEXIST)
 259                 {
 260                   /* Remove the file.  */
 261                   (void) unlink (tmpfname);
 262
 263                   /* Close the file.  */
 264                   (void) __libc_close (fd);
 265
 266                   goto try_again;
 267                 }
 268             }
 269           else
 270             /* Insert the mapping into the search tree.  This also
 271                determines whether another thread sneaked by and already
 272                added such a mapping despite the fact that we created it.  */
 273             result = check_add_mapping (name, namelen, fd, result);
 274         }
 275
 276       /* Now remove the temporary name.  This should never fail.  If
 277          it fails we leak a file name.  Better fix the kernel.  */
 278       (void) unlink (tmpfname);
 279     }
 280
 281   /* Map the mmap error to the error we need.  */
 282   if (MAP_FAILED != (void *) SEM_FAILED && result == MAP_FAILED)
 283     result = SEM_FAILED;
 284
 285   /* We don't need the file descriptor anymore.  */
 286   if (fd != -1)
 287     {
 288       /* Do not disturb errno.  */
 289       int save = errno;
 290       __libc_close (fd);
 291       errno = save;
 292     }
 293
 294   return result;
 295 }