nptl/sem_open.c

   1 /* Copyright (C) 2002-2016 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 #include <futex-internal.h>
  34
  35
  36 /* Comparison function for search of existing mapping.  */
  37 int
  38 attribute_hidden
  39 __sem_search (const void *a, const void *b)
  40 {
  41   const struct inuse_sem *as = (const struct inuse_sem *) a;
  42   const struct inuse_sem *bs = (const struct inuse_sem *) b;
  43
  44   if (as->ino != bs->ino)
  45     /* Cannot return the difference the type is larger than int.  */
  46     return as->ino < bs->ino ? -1 : (as->ino == bs->ino ? 0 : 1);
  47
  48   if (as->dev != bs->dev)
  49     /* Cannot return the difference the type is larger than int.  */
  50     return as->dev < bs->dev ? -1 : (as->dev == bs->dev ? 0 : 1);
  51
  52   return strcmp (as->name, bs->name);
  53 }
  54
  55
  56 /* The search tree for existing mappings.  */
  57 void *__sem_mappings attribute_hidden;
  58
  59 /* Lock to protect the search tree.  */
  60 int __sem_mappings_lock attribute_hidden = LLL_LOCK_INITIALIZER;
  61
  62
  63 /* Search for existing mapping and if possible add the one provided.  */
  64 static sem_t *
  65 check_add_mapping (const char *name, size_t namelen, int fd, sem_t *existing)
  66 {
  67   sem_t *result = SEM_FAILED;
  68
  69   /* Get the information about the file.  */
  70   struct stat64 st;
  71   if (__fxstat64 (_STAT_VER, fd, &st) == 0)
  72     {
  73       /* Get the lock.  */
  74       lll_lock (__sem_mappings_lock, LLL_PRIVATE);
  75
  76       /* Search for an existing mapping given the information we have.  */
  77       struct inuse_sem *fake;
  78       fake = (struct inuse_sem *) alloca (sizeof (*fake) + namelen);
  79       memcpy (fake->name, name, namelen);
  80       fake->dev = st.st_dev;
  81       fake->ino = st.st_ino;
  82
  83       struct inuse_sem **foundp = __tfind (fake, &__sem_mappings,
  84                                            __sem_search);
  85       if (foundp != NULL)
  86         {
  87           /* There is already a mapping.  Use it.  */
  88           result = (*foundp)->sem;
  89           ++(*foundp)->refcnt;
  90         }
  91       else
  92         {
  93           /* We haven't found a mapping.  Install ione.  */
  94           struct inuse_sem *newp;
  95
  96           newp = (struct inuse_sem *) malloc (sizeof (*newp) + namelen);
  97           if (newp != NULL)
  98             {
  99               /* If the caller hasn't provided any map it now.  */
 100               if (existing == SEM_FAILED)
 101                 existing = (sem_t *) mmap (NULL, sizeof (sem_t),
 102                                            PROT_READ | PROT_WRITE, MAP_SHARED,
 103                                            fd, 0);
 104
 105               newp->dev = st.st_dev;
 106               newp->ino = st.st_ino;
 107               newp->refcnt = 1;
 108               newp->sem = existing;
 109               memcpy (newp->name, name, namelen);
 110
 111               /* Insert the new value.  */
 112               if (existing != MAP_FAILED
 113                   && __tsearch (newp, &__sem_mappings, __sem_search) != NULL)
 114                 /* Successful.  */
 115                 result = existing;
 116               else
 117                 /* Something went wrong while inserting the new
 118                    value.  We fail completely.  */
 119                 free (newp);
 120             }
 121         }
 122
 123       /* Release the lock.  */
 124       lll_unlock (__sem_mappings_lock, LLL_PRIVATE);
 125     }
 126
 127   if (result != existing && existing != SEM_FAILED && existing != MAP_FAILED)
 128     {
 129       /* Do not disturb errno.  */
 130       int save = errno;
 131       munmap (existing, sizeof (sem_t));
 132       errno = save;
 133     }
 134
 135   return result;
 136 }
 137
 138
 139 sem_t *
 140 sem_open (const char *name, int oflag, ...)
 141 {
 142   int fd;
 143   sem_t *result;
 144
 145   /* Check that shared futexes are supported.  */
 146   int err = futex_supports_pshared (PTHREAD_PROCESS_SHARED);
 147   if (err != 0)
 148     {
 149       __set_errno (err);
 150       return SEM_FAILED;
 151     }
 152
 153   /* Create the name of the final file in local variable SHM_NAME.  */
 154   SHM_GET_NAME (EINVAL, SEM_FAILED, SEM_SHM_PREFIX);
 155
 156   /* If the semaphore object has to exist simply open it.  */
 157   if ((oflag & O_CREAT) == 0 || (oflag & O_EXCL) == 0)
 158     {
 159     try_again:
 160       fd = __libc_open (shm_name,
 161                         (oflag & ~(O_CREAT|O_ACCMODE)) | O_NOFOLLOW | O_RDWR);
 162
 163       if (fd == -1)
 164         {
 165           /* If we are supposed to create the file try this next.  */
 166           if ((oflag & O_CREAT) != 0 && errno == ENOENT)
 167             goto try_create;
 168
 169           /* Return.  errno is already set.  */
 170         }
 171       else
 172         /* Check whether we already have this semaphore mapped and
 173            create one if necessary.  */
 174         result = check_add_mapping (name, namelen, fd, SEM_FAILED);
 175     }
 176   else
 177     {
 178       /* We have to open a temporary file first since it must have the
 179          correct form before we can start using it.  */
 180       char *tmpfname;
 181       mode_t mode;
 182       unsigned int value;
 183       va_list ap;
 184
 185     try_create:
 186       va_start (ap, oflag);
 187
 188       mode = va_arg (ap, mode_t);
 189       value = va_arg (ap, unsigned int);
 190
 191       va_end (ap);
 192
 193       if (value > SEM_VALUE_MAX)
 194         {
 195           __set_errno (EINVAL);
 196           return SEM_FAILED;
 197         }
 198
 199       /* Create the initial file content.  */
 200       union
 201       {
 202         sem_t initsem;
 203         struct new_sem newsem;
 204       } sem;
 205
 206 #if __HAVE_64B_ATOMICS
 207       sem.newsem.data = value;
 208 #else
 209       sem.newsem.value = value << SEM_VALUE_SHIFT;
 210       sem.newsem.nwaiters = 0;
 211 #endif
 212       /* This always is a shared semaphore.  */
 213       sem.newsem.private = FUTEX_SHARED;
 214
 215       /* Initialize the remaining bytes as well.  */
 216       memset ((char *) &sem.initsem + sizeof (struct new_sem), '\0',
 217               sizeof (sem_t) - sizeof (struct new_sem));
 218
 219       tmpfname = __alloca (shm_dirlen + sizeof SEM_SHM_PREFIX + 6);
 220       char *xxxxxx = __mempcpy (tmpfname, shm_dir, shm_dirlen);
 221
 222       int retries = 0;
 223 #define NRETRIES 50
 224       while (1)
 225         {
 226           /* Add the suffix for mktemp.  */
 227           strcpy (xxxxxx, "XXXXXX");
 228
 229           /* We really want to use mktemp here.  We cannot use mkstemp
 230              since the file must be opened with a specific mode.  The
 231              mode cannot later be set since then we cannot apply the
 232              file create mask.  */
 233           if (__mktemp (tmpfname) == NULL)
 234             return SEM_FAILED;
 235
 236           /* Open the file.  Make sure we do not overwrite anything.  */
 237           fd = __libc_open (tmpfname, O_RDWR | O_CREAT | O_EXCL, mode);
 238           if (fd == -1)
 239             {
 240               if (errno == EEXIST)
 241                 {
 242                   if (++retries < NRETRIES)
 243                     continue;
 244
 245                   __set_errno (EAGAIN);
 246                 }
 247
 248               return SEM_FAILED;
 249             }
 250
 251           /* We got a file.  */
 252           break;
 253         }
 254
 255       if (TEMP_FAILURE_RETRY (__libc_write (fd, &sem.initsem, sizeof (sem_t)))
 256           == sizeof (sem_t)
 257           /* Map the sem_t structure from the file.  */
 258           && (result = (sem_t *) mmap (NULL, sizeof (sem_t),
 259                                        PROT_READ | PROT_WRITE, MAP_SHARED,
 260                                        fd, 0)) != MAP_FAILED)
 261         {
 262           /* Create the file.  Don't overwrite an existing file.  */
 263           if (link (tmpfname, shm_name) != 0)
 264             {
 265               /* Undo the mapping.  */
 266               (void) munmap (result, sizeof (sem_t));
 267
 268               /* Reinitialize 'result'.  */
 269               result = SEM_FAILED;
 270
 271               /* This failed.  If O_EXCL is not set and the problem was
 272                  that the file exists, try again.  */
 273               if ((oflag & O_EXCL) == 0 && errno == EEXIST)
 274                 {
 275                   /* Remove the file.  */
 276                   (void) unlink (tmpfname);
 277
 278                   /* Close the file.  */
 279                   (void) __libc_close (fd);
 280
 281                   goto try_again;
 282                 }
 283             }
 284           else
 285             /* Insert the mapping into the search tree.  This also
 286                determines whether another thread sneaked by and already
 287                added such a mapping despite the fact that we created it.  */
 288             result = check_add_mapping (name, namelen, fd, result);
 289         }
 290
 291       /* Now remove the temporary name.  This should never fail.  If
 292          it fails we leak a file name.  Better fix the kernel.  */
 293       (void) unlink (tmpfname);
 294     }
 295
 296   /* Map the mmap error to the error we need.  */
 297   if (MAP_FAILED != (void *) SEM_FAILED && result == MAP_FAILED)
 298     result = SEM_FAILED;
 299
 300   /* We don't need the file descriptor anymore.  */
 301   if (fd != -1)
 302     {
 303       /* Do not disturb errno.  */
 304       int save = errno;
 305       __libc_close (fd);
 306       errno = save;
 307     }
 308
 309   return result;
 310 }