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?
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