path: root/src/lib/Bcfg2/Client/Tools/POSIX/base.py

""" Base class for tools that handle POSIX (Path) entries """

import os
import sys
import pwd
import grp
import stat
import copy
import shutil
import Bcfg2.Client.Tools
import Bcfg2.Client.XML
from Bcfg2.Compat import oct_mode

try:
    import selinux
    HAS_SELINUX = selinux.is_selinux_enabled()
except ImportError:
    HAS_SELINUX = False

try:
    import posix1e
    HAS_ACLS = True

    # map between permissions characters and numeric ACL constants
    ACL_MAP = dict(r=posix1e.ACL_READ,
                   w=posix1e.ACL_WRITE,
                   x=posix1e.ACL_EXECUTE)
except ImportError:
    HAS_ACLS = False
    ACL_MAP = dict(r=4, w=2, x=1)

# map between dev_type attribute and stat constants
device_map = dict(block=stat.S_IFBLK,  # pylint: disable=C0103
                  char=stat.S_IFCHR,
                  fifo=stat.S_IFIFO)


class POSIXTool(Bcfg2.Client.Tools.Tool):
    """ Base class for tools that handle POSIX (Path) entries """
    def fully_specified(self, entry):  # pylint: disable=W0613
        """ return True if the entry is fully specified """
        # checking is done by __req__
        return True

    def verify(self, entry, modlist):  # pylint: disable=W0613
        """ return True if the entry is correct on disk """
        if not self._verify_metadata(entry):
            return False
        if entry.get('recursive', 'false').lower() == 'true':
            # verify ownership information recursively
            for root, dirs, files in os.walk(entry.get('name')):
                for path in dirs + files:
                    if not self._verify_metadata(entry,
                                                 path=os.path.join(root,
                                                                   path)):
                        return False
        return True

    def install(self, entry):
        """ Install the given entry.  Return True on success. """
        rv = True
        rv &= self._set_perms(entry)
        if entry.get('recursive', 'false').lower() == 'true':
            # set metadata recursively
            for root, dirs, files in os.walk(entry.get('name')):
                for path in dirs + files:
                    rv &= self._set_perms(entry, path=os.path.join(root, path))
        return rv

    def _remove(self, entry, recursive=True):
        """ Remove a Path entry, whatever that takes """
        if os.path.islink(entry.get('name')):
            os.unlink(entry.get('name'))
        elif os.path.isdir(entry.get('name')):
            if recursive:
                shutil.rmtree(entry.get('name'))
            else:
                os.rmdir(entry.get('name'))
        else:
            os.unlink(entry.get('name'))

    def _exists(self, entry, remove=False):
        """ check for existing paths and optionally remove them.  if
        the path exists, return the lstat of it """
        try:
            ondisk = os.lstat(entry.get('name'))
            if remove:
                try:
                    self._remove(entry)
                    return None
                except OSError:
                    err = sys.exc_info()[1]
                    self.logger.warning('POSIX: Failed to unlink %s: %s' %
                                        (entry.get('name'), err))
                    return ondisk  # probably still exists
            else:
                return ondisk
        except OSError:
            return None

    def _set_perms(self, entry, path=None):
        """ set permissions on the given entry, or on the given path
        according to the given entry """
        if path is None:
            path = entry.get("name")

        rv = True
        if entry.get("owner") and entry.get("group"):
            try:
                self.logger.debug("POSIX: Setting ownership of %s to %s:%s" %
                                  (path,
                                   self._norm_entry_uid(entry),
                                   self._norm_entry_gid(entry)))
                os.chown(path, self._norm_entry_uid(entry),
                         self._norm_entry_gid(entry))
            except KeyError:
                self.logger.error('POSIX: Failed to change ownership of %s' %
                                  path)
                rv = False
                os.chown(path, 0, 0)
            except OSError:
                self.logger.error('POSIX: Failed to change ownership of %s' %
                                  path)
                rv = False

        if entry.get("mode"):
            wanted_mode = int(entry.get('mode'), 8)
            if entry.get('dev_type'):
                wanted_mode |= device_map[entry.get('dev_type')]
            try:
                self.logger.debug("POSIX: Setting mode on %s to %s" %
                                  (path, oct_mode(wanted_mode)))
                os.chmod(path, wanted_mode)
            except (OSError, KeyError):
                self.logger.error('POSIX: Failed to change mode on %s' %
                                  path)
                rv = False

        if entry.get('mtime'):
            try:
                os.utime(entry.get('name'), (int(entry.get('mtime')),
                                             int(entry.get('mtime'))))
            except OSError:
                self.logger.error("POSIX: Failed to set mtime of %s" % path)
                rv = False

        rv &= self._set_secontext(entry, path=path)
        rv &= self._set_acls(entry, path=path)
        return rv

    def _apply_acl(self, acl, path, atype=None):
        """ Apply the given ACL to the given path """
        if atype is None:
            # the default value for atype is set this way (rather than
            # in the argument list) because posix1e libs may not be
            # installed, and this code is executed at run-time (and
            # thus will never be reached if ACLs aren't supported),
            # but argument lists are parsed at compile-time
            atype = posix1e.ACL_TYPE_ACCESS
        if atype == posix1e.ACL_TYPE_ACCESS:
            atype_str = "access"
        else:
            atype_str = "default"
        if acl.valid():
            self.logger.debug("POSIX: Applying %s ACL to %s:" % (atype_str,
                                                                 path))
            for line in str(acl).splitlines():
                self.logger.debug("  " + line)
            try:
                acl.applyto(path, atype)
                return True
            except OSError:
                err = sys.exc_info()[1]
                self.logger.error("POSIX: Failed to set ACLs on %s: %s" %
                                  (path, err))
                return False
        else:
            self.logger.warning("POSIX: %s ACL created for %s was invalid:"
                                % (atype_str.title(), path))
            for line in str(acl).splitlines():
                self.logger.warning("  " + line)
            return False

    def _set_acls(self, entry, path=None):  # pylint: disable=R0912
        """ set POSIX ACLs on the file on disk according to the config """
        if not HAS_ACLS:
            if entry.findall("ACL"):
                self.logger.debug("POSIX: ACLs listed for %s but no pylibacl "
                                  "library installed" % entry.get('name'))
            return True
        acls = self._list_entry_acls(entry)

        if path is None:
            path = entry.get("name")

        try:
            acl = posix1e.ACL(file=path)
        except IOError:
            err = sys.exc_info()[1]
            if err.errno == 95:
                # fs is mounted noacl
                if acls:
                    self.logger.error("POSIX: Cannot set ACLs on filesystem "
                                      "mounted without ACL support: %s" % path)
                else:
                    # no ACLs on the entry, no ACLs on the filesystem.
                    # all is well in the world.
                    return True
            else:
                self.logger.error("POSIX: Error getting current ACLS on %s: %s"
                                  % (path, err))
            return False
        # clear ACLs out so we start fresh -- way easier than trying
        # to add/remove/modify ACLs
        for aclentry in acl:
            if aclentry.tag_type in [posix1e.ACL_USER, posix1e.ACL_GROUP]:
                acl.delete_entry(aclentry)
        if os.path.isdir(path):
            defacl = posix1e.ACL(filedef=path)
            if not defacl.valid():
                # when a default ACL is queried on a directory that
                # has no default ACL entries at all, you get an empty
                # ACL, which is not valid.  in this circumstance, we
                # just copy the access ACL to get a base valid ACL
                # that we can add things to.
                defacl = posix1e.ACL(acl=acl)
            else:
                for aclentry in defacl:
                    if aclentry.tag_type in [posix1e.ACL_USER,
                                             posix1e.ACL_GROUP]:
                        defacl.delete_entry(aclentry)
        else:
            defacl = None

        if not acls:
            self.logger.debug("POSIX: Removed ACLs from %s" %
                              entry.get("name"))
            return True

        for aclkey, perms in acls.items():
            atype, scope, qualifier = aclkey
            if atype == "default":
                if defacl is None:
                    self.logger.warning("POSIX: Cannot set default ACLs on "
                                        "non-directory %s" % path)
                    continue
                aclentry = posix1e.Entry(defacl)
            else:
                aclentry = posix1e.Entry(acl)
            for perm in ACL_MAP.values():
                if perm & perms:
                    aclentry.permset.add(perm)
            aclentry.tag_type = scope
            try:
                if scope == posix1e.ACL_USER:
                    scopename = "user"
                    aclentry.qualifier = self._norm_uid(qualifier)
                elif scope == posix1e.ACL_GROUP:
                    scopename = "group"
                    aclentry.qualifier = self._norm_gid(qualifier)
            except (OSError, KeyError):
                err = sys.exc_info()[1]
                self.logger.error("POSIX: Could not resolve %s %s: %s" %
                                  (scopename, qualifier, err))
                continue
        acl.calc_mask()

        rv = self._apply_acl(acl, path)
        if defacl:
            defacl.calc_mask()
            rv &= self._apply_acl(defacl, path, posix1e.ACL_TYPE_DEFAULT)
        return rv

    def _set_secontext(self, entry, path=None):
        """ set the SELinux context of the file on disk according to the
        config"""
        if not HAS_SELINUX:
            return True

        if path is None:
            path = entry.get("name")
        context = entry.get("secontext")
        if not context:
            # no context listed
            return True

        if context == '__default__':
            try:
                selinux.restorecon(path)
                rv = True
            except OSError:
                err = sys.exc_info()[1]
                self.logger.error("POSIX: Failed to restore SELinux context "
                                  "for %s: %s" % (path, err))
                rv = False
        else:
            try:
                rv = selinux.lsetfilecon(path, context) == 0
            except OSError:
                err = sys.exc_info()[1]
                self.logger.error("POSIX: Failed to restore SELinux context "
                                  "for %s: %s" % (path, err))
                rv = False
        return rv

    def _norm_gid(self, gid):
        """ This takes a group name or gid and returns the
        corresponding gid. """
        try:
            return int(gid)
        except ValueError:
            return int(grp.getgrnam(gid)[2])

    def _norm_entry_gid(self, entry):
        """ Given an entry, return the GID number of the desired group """
        try:
            return self._norm_gid(entry.get('group'))
        except (OSError, KeyError):
            err = sys.exc_info()[1]
            self.logger.error('POSIX: GID normalization failed for %s on %s: '
                              '%s' % (entry.get('group'),
                                      entry.get('name'),
                                      err))
            return 0

    def _norm_uid(self, uid):
        """ This takes a username or uid and returns the
        corresponding uid. """
        try:
            return int(uid)
        except ValueError:
            return int(pwd.getpwnam(uid)[2])

    def _norm_entry_uid(self, entry):
        """ Given an entry, return the UID number of the desired owner """
        try:
            return self._norm_uid(entry.get("owner"))
        except (OSError, KeyError):
            err = sys.exc_info()[1]
            self.logger.error('POSIX: UID normalization failed for %s on %s: '
                              '%s' % (entry.get('owner'),
                                      entry.get('name'),
                                      err))
            return 0

    def _norm_acl_perms(self, perms):
        """ takes a representation of an ACL permset and returns a digit
        representing the permissions entailed by it.  representations can
        either be a single octal digit, a string of up to three 'r',
        'w', 'x', or '-' characters, or a posix1e.Permset object"""
        if perms is None:
            return 0
        elif hasattr(perms, 'test'):
            # Permset object
            return sum([p for p in ACL_MAP.values()
                        if perms.test(p)])

        try:
            # single octal digit
            rv = int(perms)
            if rv > 0 and rv < 8:
                return rv
            else:
                self.logger.error("POSIX: Permissions digit out of range in "
                                  "ACL: %s" % perms)
                return 0
        except ValueError:
            # couldn't be converted to an int; process as a string
            if len(perms) > 3:
                self.logger.error("POSIX: Permissions string too long in ACL: "
                                  "%s" % perms)
                return 0
            rv = 0
            for char in perms:
                if char == '-':
                    continue
                elif char not in ACL_MAP:
                    self.logger.warning("POSIX: Unknown permissions character "
                                        "in ACL: %s" % char)
                elif rv & ACL_MAP[char]:
                    self.logger.warning("POSIX: Duplicate permissions "
                                        "character in ACL: %s" % perms)
                else:
                    rv |= ACL_MAP[char]
            return rv

    def _acl2string(self, aclkey, perms):
        """ Get a string representation of the given ACL.  aclkey must
        be a tuple of (<acl type>, <acl scope>, <qualifier>) """
        atype, scope, qualifier = aclkey
        acl_str = []
        if atype == 'default':
            acl_str.append(atype)
        if scope == posix1e.ACL_USER:
            acl_str.append("user")
        elif scope == posix1e.ACL_GROUP:
            acl_str.append("group")
        acl_str.append(qualifier)
        acl_str.append(self._acl_perm2string(perms))
        return ":".join(acl_str)

    def _acl_perm2string(self, perm):
        """ Turn an octal permissions integer into a string suitable
        for use with ACLs """
        rv = []
        for char in 'rwx':
            if ACL_MAP[char] & perm:
                rv.append(char)
            else:
                rv.append('-')
        return ''.join(rv)

    def _gather_data(self, path):
        """ Get data on the existing state of <path> -- e.g., whether
        or not it exists, owner, group, permissions, etc. """
        try:
            ondisk = os.stat(path)
        except OSError:
            self.logger.debug("POSIX: %s does not exist" % path)
            return (False, None, None, None, None, None)

        try:
            owner = str(ondisk[stat.ST_UID])
        except OSError:
            err = sys.exc_info()[1]
            self.logger.debug("POSIX: Could not get current owner of %s: %s" %
                              (path, err))
            owner = None
        except KeyError:
            self.logger.error('POSIX: User resolution failed for %s' % path)
            owner = None

        try:
            group = str(ondisk[stat.ST_GID])
        except (OSError, KeyError):
            err = sys.exc_info()[1]
            self.logger.debug("POSIX: Could not get current group of %s: %s" %
                              (path, err))
            group = None
        except KeyError:
            self.logger.error('POSIX: Group resolution failed for %s' % path)
            group = None

        try:
            mode = oct_mode(ondisk[stat.ST_MODE])[-4:]
        except (OSError, KeyError, TypeError):
            err = sys.exc_info()[1]
            self.logger.debug("POSIX: Could not get current permissions of "
                              "%s: %s" % (path, err))
            mode = None

        if HAS_SELINUX:
            try:
                secontext = selinux.getfilecon(path)[1].split(":")[2]
            except (OSError, KeyError):
                err = sys.exc_info()[1]
                self.logger.debug("POSIX: Could not get current SELinux "
                                  "context of %s: %s" % (path, err))
                secontext = None
        else:
            secontext = None

        if HAS_ACLS:
            acls = self._list_file_acls(path)
        else:
            acls = None
        return (ondisk, owner, group, mode, secontext, acls)

    def _verify_metadata(self, entry, path=None):  # pylint: disable=R0912
        """ generic method to verify mode, owner, group, secontext, acls,
        and mtime """
        # allow setting an alternate path for recursive permissions checking
        if path is None:
            path = entry.get('name')
        attrib = dict()
        ondisk, attrib['current_owner'], attrib['current_group'], \
            attrib['current_mode'], attrib['current_secontext'] = \
            self._gather_data(path)[0:5]

        if not ondisk:
            entry.set('current_exists', 'false')
            return False

        # we conditionally verify every bit of metadata only if it's
        # specified on the entry.  consequently, canVerify() and
        # fully_specified() are preconditions of _verify_metadata(),
        # since they will ensure that everything that needs to be
        # specified actually is.  this lets us gracefully handle
        # symlink and hardlink entries, which have SELinux contexts
        # but not other permissions, optional secontext and mtime
        # attrs, and so on.
        wanted_owner, wanted_group, wanted_mode, mtime = None, None, None, -1
        if entry.get('mtime', '-1') != '-1':
            mtime = str(ondisk[stat.ST_MTIME])
        if entry.get("owner"):
            wanted_owner = str(self._norm_entry_uid(entry))
        if entry.get("group"):
            wanted_group = str(self._norm_entry_gid(entry))
        if entry.get("mode"):
            while len(entry.get('mode', '')) < 4:
                entry.set('mode', '0' + entry.get('mode', ''))
            wanted_mode = int(entry.get('mode'), 8)

        errors = []
        if wanted_owner and attrib['current_owner'] != wanted_owner:
            errors.append("Owner for path %s is incorrect. "
                          "Current owner is %s but should be %s" %
                          (path, attrib['current_owner'], entry.get('owner')))

        if wanted_group and attrib['current_group'] != wanted_group:
            errors.append("Group for path %s is incorrect. "
                          "Current group is %s but should be %s" %
                          (path, attrib['current_group'], entry.get('group')))

        if (wanted_mode and
            oct_mode(int(attrib['current_mode'], 8)) != oct_mode(wanted_mode)):
            errors.append("Permissions for path %s are incorrect. "
                          "Current permissions are %s but should be %s" %
                          (path, attrib['current_mode'], entry.get('mode')))

        if entry.get('mtime'):
            attrib['current_mtime'] = mtime
            if mtime != entry.get('mtime', '-1'):
                errors.append("mtime for path %s is incorrect. "
                              "Current mtime is %s but should be %s" %
                              (path, mtime, entry.get('mtime')))

        if HAS_SELINUX:
            wanted_secontext = None
            if entry.get("secontext") == "__default__":
                try:
                    wanted_secontext = \
                        selinux.matchpathcon(
                            path, ondisk[stat.ST_MODE])[1].split(":")[2]
                except OSError:
                    errors.append("%s has no default SELinux context" %
                                  entry.get("name"))
            else:
                wanted_secontext = entry.get("secontext")
            if (wanted_secontext and
                attrib['current_secontext'] != wanted_secontext):
                errors.append("SELinux context for path %s is incorrect. "
                              "Current context is %s but should be %s" %
                              (path, attrib['current_secontext'],
                               wanted_secontext))

        if errors:
            for error in errors:
                self.logger.debug("POSIX: " + error)
            entry.set('qtext', "\n".join([entry.get('qtext', '')] + errors))
        if path == entry.get("name"):
            for attr, val in attrib.items():
                if val is not None:
                    entry.set(attr, str(val))

        return self._verify_acls(entry, path=path) and len(errors) == 0

    def _list_entry_acls(self, entry):
        """ Given an entry, get a dict of POSIX ACLs described in that
        entry. """
        wanted = dict()
        for acl in entry.findall("ACL"):
            if acl.get("scope") == "user":
                scope = posix1e.ACL_USER
            elif acl.get("scope") == "group":
                scope = posix1e.ACL_GROUP
            else:
                self.logger.error("POSIX: Unknown ACL scope %s" %
                                  acl.get("scope"))
                continue
            if acl.get('perms') is None:
                self.logger.error("POSIX: No permissions set for ACL: %s" %
                                  Bcfg2.Client.XML.tostring(acl))
                continue
            wanted[(acl.get("type"), scope, acl.get(acl.get("scope")))] = \
                self._norm_acl_perms(acl.get('perms'))
        return wanted

    def _list_file_acls(self, path):
        """ Given a path, get a dict of existing POSIX ACLs on that
        path.  The dict keys are a tuple of (<acl type (access or
        default)>, <acl scope (user or group)>, <acl qualifer (the
        user or group it applies to)>.  values are the permissions of
        the described ACL. """
        def _process_acl(acl, atype):
            """ Given an ACL object, process it appropriately and add
            it to the return value """
            try:
                if acl.tag_type == posix1e.ACL_USER:
                    qual = pwd.getpwuid(acl.qualifier)[0]
                elif acl.tag_type == posix1e.ACL_GROUP:
                    qual = grp.getgrgid(acl.qualifier)[0]
                else:
                    return
            except (OSError, KeyError):
                err = sys.exc_info()[1]
                self.logger.error("POSIX: Lookup of %s %s failed: %s" %
                                  (atype, acl.qualifier, err))
                qual = acl.qualifier
            existing[(atype, acl.tag_type, qual)] = \
                self._norm_acl_perms(acl.permset)

        existing = dict()
        try:
            for acl in posix1e.ACL(file=path):
                _process_acl(acl, "access")
        except IOError:
            err = sys.exc_info()[1]
            if err.errno == 95:
                # fs is mounted noacl
                self.logger.debug("POSIX: Filesystem mounted without ACL "
                                  "support: %s" % path)
            else:
                self.logger.error("POSIX: Error getting current ACLS on %s: %s"
                                  % (path, err))
            return existing

        if os.path.isdir(path):
            for acl in posix1e.ACL(filedef=path):
                _process_acl(acl, "default")
        return existing

    def _verify_acls(self, entry, path=None):
        """ verify POSIX ACLs on the given entry.  return True if all
        ACLS are correct, false otherwise """
        if not HAS_ACLS:
            if entry.findall("ACL"):
                self.logger.debug("POSIX: ACLs listed for %s but no pylibacl "
                                  "library installed" % entry.get('name'))
            return True

        if path is None:
            path = entry.get("name")

        # create lists of normalized representations of the ACLs we want
        # and the ACLs we have.  this will make them easier to compare
        # than trying to mine that data out of the ACL objects and XML
        # objects and compare it at the same time.
        wanted = self._list_entry_acls(entry)
        existing = self._list_file_acls(path)

        missing = []
        extra = []
        wrong = []
        for aclkey, perms in wanted.items():
            if aclkey not in existing:
                missing.append(self._acl2string(aclkey, perms))
            elif existing[aclkey] != perms:
                wrong.append((self._acl2string(aclkey, perms),
                              self._acl2string(aclkey, existing[aclkey])))
            if path == entry.get("name"):
                atype, scope, qual = aclkey
                aclentry = Bcfg2.Client.XML.Element("ACL", type=atype,
                                                    perms=str(perms))
                if scope == posix1e.ACL_USER:
                    aclentry.set("scope", "user")
                elif scope == posix1e.ACL_GROUP:
                    aclentry.set("scope", "group")
                else:
                    self.logger.debug("POSIX: Unknown ACL scope %s on %s" %
                                      (scope, path))
                    continue
                aclentry.set(aclentry.get("scope"), qual)
                entry.append(aclentry)

        for aclkey, perms in existing.items():
            if aclkey not in wanted:
                extra.append(self._acl2string(aclkey, perms))

        msg = []
        if missing:
            msg.append("%s ACLs are missing: %s" % (len(missing),
                                                    ", ".join(missing)))
        if wrong:
            msg.append("%s ACLs are wrong: %s" %
                       (len(wrong),
                        "; ".join(["%s should be %s" % (e, w)
                                   for w, e in wrong])))
        if extra:
            msg.append("%s extra ACLs: %s" % (len(extra), ", ".join(extra)))

        if msg:
            msg.insert(0, "POSIX: ACLs for %s are incorrect." % path)
            self.logger.debug(msg[0])
            for line in msg[1:]:
                self.logger.debug("  " + line)
            entry.set('qtext', "\n".join([entry.get("qtext", '')] + msg))
            return False
        return True

    def _makedirs(self, entry, path=None):
        """ os.makedirs helpfully creates all parent directories for
        us, but it sets permissions according to umask, which is
        probably wrong.  we need to find out which directories were
        created and try to set permissions on those
        (http://trac.mcs.anl.gov/projects/bcfg2/ticket/1125 and
        http://trac.mcs.anl.gov/projects/bcfg2/ticket/1134) """
        created = []
        if path is None:
            path = entry.get("name")
        cur = path
        while cur and cur != '/':
            if not os.path.exists(cur):
                created.append(cur)
            cur = os.path.dirname(cur)
        rv = True
        try:
            os.makedirs(path)
        except OSError:
            err = sys.exc_info()[1]
            self.logger.error('POSIX: Failed to create directory %s: %s' %
                              (path, err))
            rv = False

        # set auto-created directories to mode 755 and use best effort for
        # permissions.  If you need something else, you should specify it in
        # your config.
        tmpentry = copy.deepcopy(entry)
        tmpentry.set('mode', '0755')
        for acl in tmpentry.findall('ACL'):
            acl.set('perms',
                    oct_mode(self._norm_acl_perms(acl.get('perms')) |
                             ACL_MAP['x']))
        for cpath in created:
            self._set_perms(tmpentry, path=cpath)
        return rv


class POSIXLinkTool(POSIXTool):
    """ Base handler for link (symbolic and hard) entries """
    __req__ = ['name', 'to']
    __linktype__ = None

    def verify(self, entry, modlist):
        rv = True

        try:
            if not self._verify(entry):
                msg = "%s %s is incorrect" % (self.__linktype__.title(),
                                              entry.get('name'))
                self.logger.debug("POSIX: " + msg)
                entry.set('qtext', "\n".join([entry.get('qtext', ''), msg]))
                rv = False
        except OSError:
            self.logger.debug("POSIX: %s %s does not exist" %
                              (entry.tag, entry.get("name")))
            entry.set('current_exists', 'false')
            return False

        return POSIXTool.verify(self, entry, modlist) and rv

    def _verify(self, entry):
        """ perform actual verification of the link entry """
        raise NotImplementedError

    def install(self, entry):
        ondisk = self._exists(entry, remove=True)
        if ondisk:
            self.logger.info("POSIX: %s %s cleanup failed" %
                             (self.__linktype__.title(), entry.get('name')))
        try:
            self._link(entry)
            rv = True
        except OSError:
            err = sys.exc_info()[1]
            self.logger.error("POSIX: Failed to create %s %s to %s: %s" %
                              (self.__linktype__, entry.get('name'),
                               entry.get('to'), err))
            rv = False
        return POSIXTool.install(self, entry) and rv

    def _link(self, entry):
        """ create the link """
        raise NotImplementedError