summaryrefslogtreecommitdiff
path: root/nptl/sem_open.c
blob: 83b2fdf4721695bef37ccc3f2264356938cbe052 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
/* Copyright (C) 2002-2013 Free Software Foundation, Inc.
   This file is part of the GNU C Library.
   Contributed by Ulrich Drepper <drepper@redhat.com>, 2002.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <http://www.gnu.org/licenses/>.  */

#include <errno.h>
#include <fcntl.h>
#include <mntent.h>
#include <paths.h>
#include <pthread.h>
#include <search.h>
#include <semaphore.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/statfs.h>
#include <linux_fsinfo.h>
#include "semaphoreP.h"



/* Information about the mount point.  */
struct mountpoint_info mountpoint attribute_hidden;

/* This is the default mount point.  */
static const char defaultmount[] = "/dev/shm";
/* This is the default directory.  */
static const char defaultdir[] = "/dev/shm/sem.";

/* Protect the `mountpoint' variable above.  */
pthread_once_t __namedsem_once attribute_hidden = PTHREAD_ONCE_INIT;


/* Determine where the shmfs is mounted (if at all).  */
void
attribute_hidden
__where_is_shmfs (void)
{
  char buf[512];
  struct statfs f;
  struct mntent resmem;
  struct mntent *mp;
  FILE *fp;

  /* The canonical place is /dev/shm.  This is at least what the
     documentation tells everybody to do.  */
  if (__statfs (defaultmount, &f) == 0 && f.f_type == SHMFS_SUPER_MAGIC)
    {
      /* It is in the normal place.  */
      mountpoint.dir = (char *) defaultdir;
      mountpoint.dirlen = sizeof (defaultdir) - 1;

      return;
    }

  /* OK, do it the hard way.  Look through the /proc/mounts file and if
     this does not exist through /etc/fstab to find the mount point.  */
  fp = __setmntent ("/proc/mounts", "r");
  if (__builtin_expect (fp == NULL, 0))
    {
      fp = __setmntent (_PATH_MNTTAB, "r");
      if (__builtin_expect (fp == NULL, 0))
	/* There is nothing we can do.  Blind guesses are not helpful.  */
	return;
    }

  /* Now read the entries.  */
  while ((mp = __getmntent_r (fp, &resmem, buf, sizeof buf)) != NULL)
    /* The original name is "shm" but this got changed in early Linux
       2.4.x to "tmpfs".  */
    if (strcmp (mp->mnt_type, "tmpfs") == 0
	|| strcmp (mp->mnt_type, "shm") == 0)
      {
	/* Found it.  There might be more than one place where the
           filesystem is mounted but one is enough for us.  */
	size_t namelen;

	/* First make sure this really is the correct entry.  At least
	   some versions of the kernel give wrong information because
	   of the implicit mount of the shmfs for SysV IPC.  */
	if (__statfs (mp->mnt_dir, &f) != 0 || f.f_type != SHMFS_SUPER_MAGIC)
	  continue;

	namelen = strlen (mp->mnt_dir);

	if (namelen == 0)
	  /* Hum, maybe some crippled entry.  Keep on searching.  */
	  continue;

	mountpoint.dir = (char *) malloc (namelen + 4 + 2);
	if (mountpoint.dir != NULL)
	  {
	    char *cp = __mempcpy (mountpoint.dir, mp->mnt_dir, namelen);
	    if (cp[-1] != '/')
	      *cp++ = '/';
	    cp = stpcpy (cp, "sem.");
	    mountpoint.dirlen = cp - mountpoint.dir;
	  }

	break;
      }

  /* Close the stream.  */
  __endmntent (fp);
}


/* Comparison function for search of existing mapping.  */
int
attribute_hidden
__sem_search (const void *a, const void *b)
{
  const struct inuse_sem *as = (const struct inuse_sem *) a;
  const struct inuse_sem *bs = (const struct inuse_sem *) b;

  if (as->ino != bs->ino)
    /* Cannot return the difference the type is larger than int.  */
    return as->ino < bs->ino ? -1 : (as->ino == bs->ino ? 0 : 1);

  if (as->dev != bs->dev)
    /* Cannot return the difference the type is larger than int.  */
    return as->dev < bs->dev ? -1 : (as->dev == bs->dev ? 0 : 1);

  return strcmp (as->name, bs->name);
}


/* The search tree for existing mappings.  */
void *__sem_mappings attribute_hidden;

/* Lock to protect the search tree.  */
int __sem_mappings_lock attribute_hidden = LLL_LOCK_INITIALIZER;


/* Search for existing mapping and if possible add the one provided.  */
static sem_t *
check_add_mapping (const char *name, size_t namelen, int fd, sem_t *existing)
{
  sem_t *result = SEM_FAILED;

  /* Get the information about the file.  */
  struct stat64 st;
  if (__fxstat64 (_STAT_VER, fd, &st) == 0)
    {
      /* Get the lock.  */
      lll_lock (__sem_mappings_lock, LLL_PRIVATE);

      /* Search for an existing mapping given the information we have.  */
      struct inuse_sem *fake;
      fake = (struct inuse_sem *) alloca (sizeof (*fake) + namelen);
      memcpy (fake->name, name, namelen);
      fake->dev = st.st_dev;
      fake->ino = st.st_ino;

      struct inuse_sem **foundp = tfind (fake, &__sem_mappings, __sem_search);
      if (foundp != NULL)
	{
	  /* There is already a mapping.  Use it.  */
	  result = (*foundp)->sem;
	  ++(*foundp)->refcnt;
	}
      else
	{
	  /* We haven't found a mapping.  Install ione.  */
	  struct inuse_sem *newp;

	  newp = (struct inuse_sem *) malloc (sizeof (*newp) + namelen);
	  if (newp != NULL)
	    {
	      /* If the caller hasn't provided any map it now.  */
	      if (existing == SEM_FAILED)
		existing = (sem_t *) mmap (NULL, sizeof (sem_t),
					   PROT_READ | PROT_WRITE, MAP_SHARED,
					   fd, 0);

	      newp->dev = st.st_dev;
	      newp->ino = st.st_ino;
	      newp->refcnt = 1;
	      newp->sem = existing;
	      memcpy (newp->name, name, namelen);

	      /* Insert the new value.  */
	      if (existing != MAP_FAILED
		  && tsearch (newp, &__sem_mappings, __sem_search) != NULL)
		/* Successful.  */
		result = existing;
	      else
		/* Something went wrong while inserting the new
		   value.  We fail completely.  */
		free (newp);
	    }
	}

      /* Release the lock.  */
      lll_unlock (__sem_mappings_lock, LLL_PRIVATE);
    }

  if (result != existing && existing != SEM_FAILED && existing != MAP_FAILED)
    {
      /* Do not disturb errno.  */
      INTERNAL_SYSCALL_DECL (err);
      INTERNAL_SYSCALL (munmap, err, 2, existing, sizeof (sem_t));
    }

  return result;
}


sem_t *
sem_open (const char *name, int oflag, ...)
{
  char *finalname;
  sem_t *result = SEM_FAILED;
  int fd;

  /* Determine where the shmfs is mounted.  */
  __pthread_once (&__namedsem_once, __where_is_shmfs);

  /* If we don't know the mount points there is nothing we can do.  Ever.  */
  if (mountpoint.dir == NULL)
    {
      __set_errno (ENOSYS);
      return SEM_FAILED;
    }

  /* Construct the filename.  */
  while (name[0] == '/')
    ++name;

  if (name[0] == '\0')
    {
      /* The name "/" is not supported.  */
      __set_errno (EINVAL);
      return SEM_FAILED;
    }
  size_t namelen = strlen (name) + 1;

  /* Create the name of the final file.  */
  finalname = (char *) alloca (mountpoint.dirlen + namelen);
  __mempcpy (__mempcpy (finalname, mountpoint.dir, mountpoint.dirlen),
	     name, namelen);

  /* If the semaphore object has to exist simply open it.  */
  if ((oflag & O_CREAT) == 0 || (oflag & O_EXCL) == 0)
    {
    try_again:
      fd = __libc_open (finalname,
			(oflag & ~(O_CREAT|O_ACCMODE)) | O_NOFOLLOW | O_RDWR);

      if (fd == -1)
	{
	  /* If we are supposed to create the file try this next.  */
	  if ((oflag & O_CREAT) != 0 && errno == ENOENT)
	    goto try_create;

	  /* Return.  errno is already set.  */
	}
      else
	/* Check whether we already have this semaphore mapped and
	   create one if necessary.  */
	result = check_add_mapping (name, namelen, fd, SEM_FAILED);
    }
  else
    {
      /* We have to open a temporary file first since it must have the
	 correct form before we can start using it.  */
      char *tmpfname;
      mode_t mode;
      unsigned int value;
      va_list ap;

    try_create:
      va_start (ap, oflag);

      mode = va_arg (ap, mode_t);
      value = va_arg (ap, unsigned int);

      va_end (ap);

      if (value > SEM_VALUE_MAX)
	{
	  __set_errno (EINVAL);
	  return SEM_FAILED;
	}

      /* Create the initial file content.  */
      union
      {
	sem_t initsem;
	struct new_sem newsem;
      } sem;

      sem.newsem.value = value;
      sem.newsem.private = 0;
      sem.newsem.nwaiters = 0;

      /* Initialize the remaining bytes as well.  */
      memset ((char *) &sem.initsem + sizeof (struct new_sem), '\0',
	      sizeof (sem_t) - sizeof (struct new_sem));

      tmpfname = (char *) alloca (mountpoint.dirlen + 6 + 1);
      char *xxxxxx = __mempcpy (tmpfname, mountpoint.dir, mountpoint.dirlen);

      int retries = 0;
#define NRETRIES 50
      while (1)
	{
	  /* Add the suffix for mktemp.  */
	  strcpy (xxxxxx, "XXXXXX");

	  /* We really want to use mktemp here.  We cannot use mkstemp
	     since the file must be opened with a specific mode.  The
	     mode cannot later be set since then we cannot apply the
	     file create mask.  */
	  if (__mktemp (tmpfname) == NULL)
	    return SEM_FAILED;

	  /* Open the file.  Make sure we do not overwrite anything.  */
	  fd = __libc_open (tmpfname, O_RDWR | O_CREAT | O_EXCL, mode);
	  if (fd == -1)
	    {
	      if (errno == EEXIST)
		{
		  if (++retries < NRETRIES)
		    continue;

		  __set_errno (EAGAIN);
		}

	      return SEM_FAILED;
	    }

	  /* We got a file.  */
	  break;
	}

      if (TEMP_FAILURE_RETRY (__libc_write (fd, &sem.initsem, sizeof (sem_t)))
	  == sizeof (sem_t)
	  /* Map the sem_t structure from the file.  */
	  && (result = (sem_t *) mmap (NULL, sizeof (sem_t),
				       PROT_READ | PROT_WRITE, MAP_SHARED,
				       fd, 0)) != MAP_FAILED)
	{
	  /* Create the file.  Don't overwrite an existing file.  */
	  if (link (tmpfname, finalname) != 0)
	    {
	      /* Undo the mapping.  */
	      (void) munmap (result, sizeof (sem_t));

	      /* Reinitialize 'result'.  */
	      result = SEM_FAILED;

	      /* This failed.  If O_EXCL is not set and the problem was
		 that the file exists, try again.  */
	      if ((oflag & O_EXCL) == 0 && errno == EEXIST)
		{
		  /* Remove the file.  */
		  (void) unlink (tmpfname);

		  /* Close the file.  */
		  (void) __libc_close (fd);

		  goto try_again;
		}
	    }
	  else
	    /* Insert the mapping into the search tree.  This also
	       determines whether another thread sneaked by and already
	       added such a mapping despite the fact that we created it.  */
	    result = check_add_mapping (name, namelen, fd, result);
	}

      /* Now remove the temporary name.  This should never fail.  If
	 it fails we leak a file name.  Better fix the kernel.  */
      (void) unlink (tmpfname);
    }

  /* Map the mmap error to the error we need.  */
  if (MAP_FAILED != (void *) SEM_FAILED && result == MAP_FAILED)
    result = SEM_FAILED;

  /* We don't need the file descriptor anymore.  */
  if (fd != -1)
    {
      /* Do not disturb errno.  */
      INTERNAL_SYSCALL_DECL (err);
      INTERNAL_SYSCALL (close, err, 1, fd);
    }

  return result;
}