nptl/sem_open.c

   1 /* Copyright (C) 2002-2020 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    <https://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 #include <libc-lock.h>
  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   /* Disable asynchronous cancellation.  */
 157 #ifdef __libc_ptf_call
 158   int state;
 159   __libc_ptf_call (__pthread_setcancelstate,
 160                    (PTHREAD_CANCEL_DISABLE, &state), 0);
 161 #endif
 162
 163   /* If the semaphore object has to exist simply open it.  */
 164   if ((oflag & O_CREAT) == 0 || (oflag & O_EXCL) == 0)
 165     {
 166     try_again:
 167       fd = __libc_open (shm_name,
 168                         (oflag & ~(O_CREAT|O_ACCMODE)) | O_NOFOLLOW | O_RDWR);
 169
 170       if (fd == -1)
 171         {
 172           /* If we are supposed to create the file try this next.  */
 173           if ((oflag & O_CREAT) != 0 && errno == ENOENT)
 174             goto try_create;
 175
 176           /* Return.  errno is already set.  */
 177         }
 178       else
 179         /* Check whether we already have this semaphore mapped and
 180            create one if necessary.  */
 181         result = check_add_mapping (name, namelen, fd, SEM_FAILED);
 182     }
 183   else
 184     {
 185       /* We have to open a temporary file first since it must have the
 186          correct form before we can start using it.  */
 187       char *tmpfname;
 188       mode_t mode;
 189       unsigned int value;
 190       va_list ap;
 191
 192     try_create:
 193       va_start (ap, oflag);
 194
 195       mode = va_arg (ap, mode_t);
 196       value = va_arg (ap, unsigned int);
 197
 198       va_end (ap);
 199
 200       if (value > SEM_VALUE_MAX)
 201         {
 202           __set_errno (EINVAL);
 203           result = SEM_FAILED;
 204           goto out;
 205         }
 206
 207       /* Create the initial file content.  */
 208       union
 209       {
 210         sem_t initsem;
 211         struct new_sem newsem;
 212       } sem;
 213
 214 #if __HAVE_64B_ATOMICS
 215       sem.newsem.data = value;
 216 #else
 217       sem.newsem.value = value << SEM_VALUE_SHIFT;
 218       sem.newsem.nwaiters = 0;
 219 #endif
 220       /* pad is used as a mutex on pre-v9 sparc and ignored otherwise.  */
 221       sem.newsem.pad = 0;
 222
 223       /* This always is a shared semaphore.  */
 224       sem.newsem.private = FUTEX_SHARED;
 225
 226       /* Initialize the remaining bytes as well.  */
 227       memset ((char *) &sem.initsem + sizeof (struct new_sem), '\0',
 228               sizeof (sem_t) - sizeof (struct new_sem));
 229
 230       tmpfname = __alloca (shm_dirlen + sizeof SEM_SHM_PREFIX + 6);
 231       char *xxxxxx = __mempcpy (tmpfname, shm_dir, shm_dirlen);
 232
 233       int retries = 0;
 234 #define NRETRIES 50
 235       while (1)
 236         {
 237           /* Add the suffix for mktemp.  */
 238           strcpy (xxxxxx, "XXXXXX");
 239
 240           /* We really want to use mktemp here.  We cannot use mkstemp
 241              since the file must be opened with a specific mode.  The
 242              mode cannot later be set since then we cannot apply the
 243              file create mask.  */
 244           if (__mktemp (tmpfname) == NULL)
 245             {
 246               result = SEM_FAILED;
 247               goto out;
 248             }
 249
 250           /* Open the file.  Make sure we do not overwrite anything.  */
 251           fd = __libc_open (tmpfname, O_RDWR | O_CREAT | O_EXCL, mode);
 252           if (fd == -1)
 253             {
 254               if (errno == EEXIST)
 255                 {
 256                   if (++retries < NRETRIES)
 257                     continue;
 258
 259                   __set_errno (EAGAIN);
 260                 }
 261
 262               result = SEM_FAILED;
 263               goto out;
 264             }
 265
 266           /* We got a file.  */
 267           break;
 268         }
 269
 270       if (TEMP_FAILURE_RETRY (__libc_write (fd, &sem.initsem, sizeof (sem_t)))
 271           == sizeof (sem_t)
 272           /* Map the sem_t structure from the file.  */
 273           && (result = (sem_t *) mmap (NULL, sizeof (sem_t),
 274                                        PROT_READ | PROT_WRITE, MAP_SHARED,
 275                                        fd, 0)) != MAP_FAILED)
 276         {
 277           /* Create the file.  Don't overwrite an existing file.  */
 278           if (link (tmpfname, shm_name) != 0)
 279             {
 280               /* Undo the mapping.  */
 281               (void) munmap (result, sizeof (sem_t));
 282
 283               /* Reinitialize 'result'.  */
 284               result = SEM_FAILED;
 285
 286               /* This failed.  If O_EXCL is not set and the problem was
 287                  that the file exists, try again.  */
 288               if ((oflag & O_EXCL) == 0 && errno == EEXIST)
 289                 {
 290                   /* Remove the file.  */
 291                   (void) unlink (tmpfname);
 292
 293                   /* Close the file.  */
 294                   (void) __libc_close (fd);
 295
 296                   goto try_again;
 297                 }
 298             }
 299           else
 300             /* Insert the mapping into the search tree.  This also
 301                determines whether another thread sneaked by and already
 302                added such a mapping despite the fact that we created it.  */
 303             result = check_add_mapping (name, namelen, fd, result);
 304         }
 305
 306       /* Now remove the temporary name.  This should never fail.  If
 307          it fails we leak a file name.  Better fix the kernel.  */
 308       (void) unlink (tmpfname);
 309     }
 310
 311   /* Map the mmap error to the error we need.  */
 312   if (MAP_FAILED != (void *) SEM_FAILED && result == MAP_FAILED)
 313     result = SEM_FAILED;
 314
 315   /* We don't need the file descriptor anymore.  */
 316   if (fd != -1)
 317     {
 318       /* Do not disturb errno.  */
 319       int save = errno;
 320       __libc_close (fd);
 321       errno = save;
 322     }
 323
 324 out:
 325 #ifdef __libc_ptf_call
 326   __libc_ptf_call (__pthread_setcancelstate, (state, NULL), 0);
 327 #endif
 328
 329   return result;
 330 }