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,
  83                                            __sem_search);
  84       if (foundp != NULL)
  85         {
  86           /* There is already a mapping.  Use it.  */
  87           result = (*foundp)->sem;
  88           ++(*foundp)->refcnt;
  89         }
  90       else
  91         {
  92           /* We haven't found a mapping.  Install ione.  */
  93           struct inuse_sem *newp;
  94
  95           newp = (struct inuse_sem *) malloc (sizeof (*newp) + namelen);
  96           if (newp != NULL)
  97             {
  98               /* If the caller hasn't provided any map it now.  */
  99               if (existing == SEM_FAILED)
 100                 existing = (sem_t *) mmap (NULL, sizeof (sem_t),
 101                                            PROT_READ | PROT_WRITE, MAP_SHARED,
 102                                            fd, 0);
 103
 104               newp->dev = st.st_dev;
 105               newp->ino = st.st_ino;
 106               newp->refcnt = 1;
 107               newp->sem = existing;
 108               memcpy (newp->name, name, namelen);
 109
 110               /* Insert the new value.  */
 111               if (existing != MAP_FAILED
 112                   && __tsearch (newp, &__sem_mappings, __sem_search) != NULL)
 113                 /* Successful.  */
 114                 result = existing;
 115               else
 116                 /* Something went wrong while inserting the new
 117                    value.  We fail completely.  */
 118                 free (newp);
 119             }
 120         }
 121
 122       /* Release the lock.  */
 123       lll_unlock (__sem_mappings_lock, LLL_PRIVATE);
 124     }
 125
 126   if (result != existing && existing != SEM_FAILED && existing != MAP_FAILED)
 127     {
 128       /* Do not disturb errno.  */
 129       int save = errno;
 130       munmap (existing, sizeof (sem_t));
 131       errno = save;
 132     }
 133
 134   return result;
 135 }
 136
 137
 138 sem_t *
 139 sem_open (const char *name, int oflag, ...)
 140 {
 141   int fd;
 142   sem_t *result;
 143
 144   /* Create the name of the final file in local variable SHM_NAME.  */
 145   SHM_GET_NAME (EINVAL, SEM_FAILED, SEM_SHM_PREFIX);
 146
 147   /* If the semaphore object has to exist simply open it.  */
 148   if ((oflag & O_CREAT) == 0 || (oflag & O_EXCL) == 0)
 149     {
 150     try_again:
 151       fd = __libc_open (shm_name,
 152                         (oflag & ~(O_CREAT|O_ACCMODE)) | O_NOFOLLOW | O_RDWR);
 153
 154       if (fd == -1)
 155         {
 156           /* If we are supposed to create the file try this next.  */
 157           if ((oflag & O_CREAT) != 0 && errno == ENOENT)
 158             goto try_create;
 159
 160           /* Return.  errno is already set.  */
 161         }
 162       else
 163         /* Check whether we already have this semaphore mapped and
 164            create one if necessary.  */
 165         result = check_add_mapping (name, namelen, fd, SEM_FAILED);
 166     }
 167   else
 168     {
 169       /* We have to open a temporary file first since it must have the
 170          correct form before we can start using it.  */
 171       char *tmpfname;
 172       mode_t mode;
 173       unsigned int value;
 174       va_list ap;
 175
 176     try_create:
 177       va_start (ap, oflag);
 178
 179       mode = va_arg (ap, mode_t);
 180       value = va_arg (ap, unsigned int);
 181
 182       va_end (ap);
 183
 184       if (value > SEM_VALUE_MAX)
 185         {
 186           __set_errno (EINVAL);
 187           return SEM_FAILED;
 188         }
 189
 190       /* Create the initial file content.  */
 191       union
 192       {
 193         sem_t initsem;
 194         struct new_sem newsem;
 195       } sem;
 196
 197 #if __HAVE_64B_ATOMICS
 198       sem.newsem.data = value;
 199 #else
 200       sem.newsem.value = value << SEM_VALUE_SHIFT;
 201       sem.newsem.nwaiters = 0;
 202 #endif
 203       /* This always is a shared semaphore.  */
 204       sem.newsem.private = LLL_SHARED;
 205
 206       /* Initialize the remaining bytes as well.  */
 207       memset ((char *) &sem.initsem + sizeof (struct new_sem), '\0',
 208               sizeof (sem_t) - sizeof (struct new_sem));
 209
 210       tmpfname = __alloca (shm_dirlen + sizeof SEM_SHM_PREFIX + 6);
 211       char *xxxxxx = __mempcpy (tmpfname, shm_dir, shm_dirlen);
 212
 213       int retries = 0;
 214 #define NRETRIES 50
 215       while (1)
 216         {
 217           /* Add the suffix for mktemp.  */
 218           strcpy (xxxxxx, "XXXXXX");
 219
 220           /* We really want to use mktemp here.  We cannot use mkstemp
 221              since the file must be opened with a specific mode.  The
 222              mode cannot later be set since then we cannot apply the
 223              file create mask.  */
 224           if (__mktemp (tmpfname) == NULL)
 225             return SEM_FAILED;
 226
 227           /* Open the file.  Make sure we do not overwrite anything.  */
 228           fd = __libc_open (tmpfname, O_RDWR | O_CREAT | O_EXCL, mode);
 229           if (fd == -1)
 230             {
 231               if (errno == EEXIST)
 232                 {
 233                   if (++retries < NRETRIES)
 234                     continue;
 235
 236                   __set_errno (EAGAIN);
 237                 }
 238
 239               return SEM_FAILED;
 240             }
 241
 242           /* We got a file.  */
 243           break;
 244         }
 245
 246       if (TEMP_FAILURE_RETRY (__libc_write (fd, &sem.initsem, sizeof (sem_t)))
 247           == sizeof (sem_t)
 248           /* Map the sem_t structure from the file.  */
 249           && (result = (sem_t *) mmap (NULL, sizeof (sem_t),
 250                                        PROT_READ | PROT_WRITE, MAP_SHARED,
 251                                        fd, 0)) != MAP_FAILED)
 252         {
 253           /* Create the file.  Don't overwrite an existing file.  */
 254           if (link (tmpfname, shm_name) != 0)
 255             {
 256               /* Undo the mapping.  */
 257               (void) munmap (result, sizeof (sem_t));
 258
 259               /* Reinitialize 'result'.  */
 260               result = SEM_FAILED;
 261
 262               /* This failed.  If O_EXCL is not set and the problem was
 263                  that the file exists, try again.  */
 264               if ((oflag & O_EXCL) == 0 && errno == EEXIST)
 265                 {
 266                   /* Remove the file.  */
 267                   (void) unlink (tmpfname);
 268
 269                   /* Close the file.  */
 270                   (void) __libc_close (fd);
 271
 272                   goto try_again;
 273                 }
 274             }
 275           else
 276             /* Insert the mapping into the search tree.  This also
 277                determines whether another thread sneaked by and already
 278                added such a mapping despite the fact that we created it.  */
 279             result = check_add_mapping (name, namelen, fd, result);
 280         }
 281
 282       /* Now remove the temporary name.  This should never fail.  If
 283          it fails we leak a file name.  Better fix the kernel.  */
 284       (void) unlink (tmpfname);
 285     }
 286
 287   /* Map the mmap error to the error we need.  */
 288   if (MAP_FAILED != (void *) SEM_FAILED && result == MAP_FAILED)
 289     result = SEM_FAILED;
 290
 291   /* We don't need the file descriptor anymore.  */
 292   if (fd != -1)
 293     {
 294       /* Do not disturb errno.  */
 295       int save = errno;
 296       __libc_close (fd);
 297       errno = save;
 298     }
 299
 300   return result;
 301 }