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sem_open.c

/* Copyright (C) 2002, 2003 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, write to the Free
   Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
   02111-1307 USA.  */

#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.  */
lll_lock_t __sem_mappings_lock = 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);

      /* 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);
    }

  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.  */
  INTUSE(__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.  */
      sem_t initsem;

      struct sem *iinitsem = (struct sem *) &initsem;
      iinitsem->count = value;

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

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