On (03/08/16 18:56), Lukas Slebodnik wrote:
On (29/07/16 16:41), Jakub Hrozek wrote:
>On Thu, Jul 28, 2016 at 01:56:50PM +0200, Jakub Hrozek wrote:
>> On Thu, Jul 28, 2016 at 01:33:32PM +0200, Lukas Slebodnik wrote:
>> > On (28/07/16 12:06), thierry bordaz wrote:
>> > >On 07/28/2016 09:39 AM, Jakub Hrozek wrote:
>> > >> On Wed, Jul 27, 2016 at 04:09:07PM +0200, thierry bordaz wrote:
>> > >> >
>> > >> > On 07/27/2016 03:36 PM, Jakub Hrozek wrote:
>> > >> > > On Wed, Jul 27, 2016 at 02:55:37PM +0200, thierry bordaz
wrote:
>> > >> > > > On 07/27/2016 01:56 PM, Jakub Hrozek wrote:
>> > >> > > > > On Wed, Jul 27, 2016 at 01:03:59PM +0200, Jakub
Hrozek wrote:
>> > >> > > > > > On Wed, Jul 27, 2016 at 12:22:46PM +0200,
Lukas Slebodnik wrote:
>> > >> > > > > > > On (27/07/16 12:08), Jakub Hrozek
wrote:
>> > >> > > > > > > > On Wed, Jul 27, 2016 at
12:02:24PM +0200, Jakub Hrozek wrote:
>> > >> > > > > > > > > On Wed, Jul 27, 2016 at
11:54:16AM +0200, Lukas Slebodnik wrote:
>> > >> > > > > > > > > > ehlo,
>> > >> > > > > > > > > >
>> > >> > > > > > > > > > attached patch fixes
acces denied after activating user in 389ds.
>> > >> > > > > > > > > > Jakub had some
comments/ideas in ticket but I think it's better to discuss
>> > >> > > > > > > > > > about virtual
attributes and timestamp cache on mailing list.
>> > >> > > > > > > > > Yes, so the comment I have
is that while this works, it might break some
>> > >> > > > > > > > > strange LDAP servers.
>> > >> > > > > > > > >
>> > >> > > > > > > > > We use modifyTimestamp as a
'positive' indicator that the entry has not
>> > >> > > > > > > > > changed -- if the
modifyTimestamp didn't change, we consider the cached
>> > >> > > > > > > > > entry the same as what is
on the server and only bump the timestamp
>> > >> > > > > > > > > cache. If the timestamp is
different, we do a deep-comparison of cached
>> > >> > > > > > > > > attribute values with what
is on the LDAP server and write the sysdb
>> > >> > > > > > > > > cache entry only if the
attributes differ.
>> > >> > > > > > > > >
>> > >> > > > > > > > > I was wondering if we can
use the modifyTimestamp at all, then, because
>> > >> > > > > > > > > even if it's the same,
we might want to check the attributes to see if
>> > >> > > > > > > > > some of the values are
different because some of the attributes might be
>> > >> > > > > > > > > this operational/virtual
attribute..
>> > >> > > > > > > > Sorry, sent too soon.
>> > >> > > > > > > >
>> > >> > > > > > > > I think the questions are -- 1)
can we enumerate the virtual attributes?
>> > >> > > > > > > That might be a question for 389-ds
developers.
>> > >> > > > > > > But it's very likely it will be
different on other LDAP servers.
>> > >> > > > > > >
>> > >> > > > > > > > 2) Would different LDAP servers
have different virtual attributes.
>> > >> > > > > > > >
>> > >> > > > > > > > For 2) maybe a possible solution
might be to set a non-existing
>> > >> > > > > > > > modifyTimestamp attribute value,
but I would consider that only a
>> > >> > > > > > > > kludge, we shouldn't break
existing setups..
>> > >> > > > > > > I am not satisfied with this POC
solution either.
>> > >> > > > > > >
>> > >> > > > > > > So should we remove usage of
modifyTimestamp for detecting changes?
>> > >> > > > > > I would prefer to ask the DS developers
before removing it completely.
>> > >> > > > > >
>> > >> > > > > > At least for large groups it might take a
long time to compare all attribute
>> > >> > > > > > values and IIRC we don't depend on any
virtual attributes for groups. Maybe
>> > >> > > > > > we could parametrize that part of the code
and enable the fast way with
>> > >> > > > > > modifyTimestamps for 'known'
server types, that is for setups with AD and
>> > >> > > > > > IPA providers.
>> > >> > > > > >
>> > >> > > > > > For users, there is typically not as many
attributes so we should be
>> > >> > > > > > fine deep-comparing all attributes.
>> > >> > > > > I'm adding Thierry (so please reply-to-all
to keep him in the thread).
>> > >> > > > >
>> > >> > > > > Thierry, in the latest sssd version we tried to
add a performance
>> > >> > > > > improvement related to how we store SSSD
entries in the cache. The short
>> > >> > > > > version is that we store the modifyTimestamp
attribute in the cache and
>> > >> > > > > when we fetch an entry, we compare the entry
modifyTimestamp with what
>> > >> > > > > is on the server. When the two are the same, we
say that the entry did
>> > >> > > > > not change and don't update the cache.
>> > >> > > > >
>> > >> > > > > This works fine for most attributes, but not
for attributes like
>> > >> > > > > nsAccountLock which do not change
modifyTimestamp when they are
>> > >> > > > > modified. So when an entry was already cached
but then nsAccountLock
>> > >> > > > > changed, we treated the entry as the same and
never read the new
>> > >> > > > > nsAccountLock value.
>> > >> > > > >
>> > >> > > > > To fix this, I think we have several options:
>> > >> > > > > 1) special-case the nsAccountLock. This
seems a bit dangerous,
>> > >> > > > > because I'm not sure we can say that
some other attribute we are
>> > >> > > > > interested in behaves the same as
nsAccountLock.
>> > >> > > > > 2) drop the modifyTimestamp optimization
completely. Then we fall
>> > >> > > > > back to comparing the attribute values,
which might work, but for
>> > >> > > > > huge objects like groups with thousands
of members, this might be
>> > >> > > > > too expensive.
>> > >> > > > > 3) only use the modifyTimestamp
optimization for cases where we know
>> > >> > > > > we don't read any virtual
attributes.
>> > >> > > > >
>> > >> > > > > And my question is -- can we, in general, know
if the modifyTimestamp
>> > >> > > > > way of detecting changes is realiable for all
LDAP servers? Or do you
>> > >> > > > > think it should only be used for cases where we
know we are not
>> > >> > > > > interested in any virtual attributes (that
would mostly be storing
>> > >> > > > > groups from servers where we know exactly what
is on the server side,
>> > >> > > > > like IPA or AD).
>> > >> > > > Hello,
>> > >> > > >
>> > >> > > > Relying on modifytimestamp looks a good idea. Any
MOD/MODRDN will update it,
>> > >> > > > except I think it is unchanged when updating some
password policy
>> > >> > > > attributes.
>> > >> > > >
>> > >> > > > Regarding virtual attribute, the only one I know in
IPA is nsaccountlock.
>> > >> > > > nsaccountlock is an operational attribute (you need
to request it to see it)
>> > >> > > > and is also a virtual attribute BUT only for
'staged' and 'deleted' users.
>> > >> > > > It is a stored attribute for regular users and we
should update
>> > >> > > > modifytimestamp when it is set.
>> > >> > > >
>> > >> > > > thanks
>> > >> > > > thierry
>> > >> > > OK, in that case it seems like we can special-case it.
But do you know
>> > >> > > about any other attributes in any other LDAP servers?
>> > >> > Any LDAP server following standard should provide
modifytimestamp that
>> > >> > reflect the last update of the entry. Now virtual attribute
values may be
>> > >> > "attached" to the entry and its value change without
modification of
>> > >> > modifytimestamp.
>> > >> > For 389-ds and IPA it is fine as virtual value of
nsaccountlock is changed
>> > >> > only when the DN change.
>> > >> > For others LDAP servers I suppose it exists the same ability
to define
>> > >> > service providers that return virtual attribute values. The
difficulty is
>> > >> > that the schema may not give any hint if the retrieved
attributes values
>> > >> > were stored or computed and consequently trust modifytimestamp
to know if
>> > >> > the values changed or not.
>> > >> > For example in ODSEE, memberof is a virtual attribute.
>> > >> Thank you, for the explanation Thierry.
>> > >>
>> > >> Then to be on the safe side I propose:
>> > >> 1) We add an (probably undocumented?) flag that says whether
to use
>> > >> modifyTimestamp to detect entry changes or not
>> > >> 2) for the generic LDAP provider we always really compare the
>> > >> attribute values, in other words the option would be set
to
>> > >> false. If there is anyone with performance issues with a
generic
>> > >> setup, we tell them to flip the option.
>> > >> 3) For the IPA and AD providers, we set this option to true
and use
>> > >> the modifyTimestamp value to detect changes
>> > >> 4) We special case nsAccountLock
>> > >>
>> > >> Lukas, do you agree?
>> > >Hi Jakub,
>> > >
>> > >digging further into the server, it appears that a DS plugin
'acctpolicy'
>> > >updates an entry without changing the mofidytimestamp.
>> > >The updated attribute is 'lastlogintime' (by default) but i
think can be any
>> > >attribute configured in the entry account policy.
>> > >I need to do further tests to confirm this.
>> > >
>> >
>> > IMHO, the problems with nsAccountLock just revealed the fact that
>> > it might happen that the attribute modifyTimestamp/whenChanged needn't
be
>> > a reliable way how to determine change in entry for any LDAP Server.
>> >
>> > We improved the performance but there might be other corner cases with
>> > other LDAP servers.
>> >
>> > Jakub proosed in 2) that we should always really compare the the attibutes
>> > from sssd cache and from LDAP search. But it would mean that we would not
>> > improve a performance for generic LDAP providers.
>>
>> We would still avoid the cache writes, "only" after comparing the
>> attribute values. So essentially by using the modifyTimestamp we save a
>> single LDB base search and a number of attribute-value comparisons,
>> depending on how large the object is.
>>
>> >
>> > We cannot generally detect virtual attributes for any LDAP server.
>> > What about adding an option where user could list virtual attributes.
>> > It would be a kind of proposed solution 4) but it would not be a
>> > special case for nsAccountLock but for more attributes which could be
changed
>> > in configuration.
>>
>> OK, so your proposal is to keep the more aggressive cache optimization?
>> I think I'm mostly afraid about the more exotic LDAP servers, like IBM
>> Tivoli or Novell eDirectory which I already know from experience don't
>> follow the established standards closely. I guess I'm fine with that, we
>> can always flip the default back. Worst case for people who start using
>> 1.14, the admin can always define modifyTimestamp to some non-existing
>> attribute and force the attribute value comparison check.
>>
>> Yes, we can make the list configurable. We also need to be
>> very careful about printing the reason for (not) updating the cache to
>> the admin (#3060).
>
>OK, so after some discussion with Simo on IRC, there is a different
>proposal - let responder control the optimization level, so that PAM
>responder would always trigger a full cache write, or at least
>deep-compare the attributes and NSS responder would rely on
>modifyTimestamp.
>
>But that's a larger fix, so for short-term fix I propose to only use
>modifyTimestamp for group objects and always compare attributes for
>users. Then later, as another patch we can let the responder send a flag
>to control the optimization (would probably be done as a flag for a
>sysdb transaction).
>
>If you agree, I would file a ticket for the second part and you can
>instead write a patch to disable modifyTimestamp checks for users.
As you wish.
The updated patch is attached.
LS
From ae01ffdbbc74c5b43c2b644f8847d856cd2bf997 Mon Sep 17 00:00:00
2001
From: Lukas Slebodnik <lslebodn(a)redhat.com>
Date: Wed, 3 Aug 2016 18:48:04 +0200
Subject: [PATCH] SYSDB: Avoid optimisation with modifyTimestamp for users
The usage of modifyTimestamp needn't be a reliable way
for detecting of changes in user entry in LDAP.
The authorisation need to rely current data from LDAP
and therefore we will temporary disable optimisation with
modifyTimestamp and we will rather rely on deep comparison
of attributes. In he future, it might be changed and
responders might control the optimization level.
And now with version witout failures in unit test and without compiler warnings
:-)
LS