Re: [PATCH] cifs: Support for an upcall to map SID to an uid and a gid

[Andrew Bartlett <abartlet-eUNUBHrolfbYtjvyW6yDsg@public.gmane.org>]

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On Sun, 2010-12-12 at 06:39 -0500, Jeff Layton wrote:
> On Sun, 12 Dec 2010 14:48:04 +1100
> Andrew Bartlett <abartlet-eUNUBHrolfbYtjvyW6yDsg@public.gmane.org> wrote:
> 
> > On Sat, 2010-12-11 at 22:11 -0500, Jeff Layton wrote:
> > > On Sat, 11 Dec 2010 19:57:11 -0500
> > > Richard Sharpe <realrichardsharpe-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> > > 
> > > > On Sat, Dec 11, 2010 at 7:30 PM, Jeff Layton <jlayton-eUNUBHrolfbYtjvyW6yDsg@public.gmane.org> wrote:
> > > > >>
> > > > >> Will look into this.  One thing that concerns me is if a cached etnry
> > > > >> for a SID with its name and an id (either an uid or a gid), if that SID
> > > > >> now represents a different object and has differernt name, would
> > > > >> not cached info be incorrect?  Not sure if this can ever happen
> > > > >> or how would it happen and if it does, what would be a trigger
> > > > >> for a cache revalidation and purges!
> > > > >>
> > > > >
> > > > > Sure, mappings can change. But, you still have the same problem with
> > > > > what you're proposing in these patches. The userspace program isn't
> > > > > setting a timeout on the key. Once a mapping is put in the keyring,
> > > > > it's there until it's revoked. You probably want to set a max TTL for
> > > > > the entries in the cache regardless of what scheme is used.
> > > > 
> > > > I was under the impression that SIDs are never reused. Perhaps I am mistaken.
> > > > 
> > > 
> > > That may be, but the mapping of a SID is dependent upon settings in
> > > config files that could change. It seems reasonable to me to only cache
> > > these mappings for a period of time in the event that they do. That
> > > period of time could default to being rather long and be tunable.
> > 
> > I think that instead some explicit signal should be made to indicate
> > that a mapping has changed, so you don't have to worry about cache
> > times.  It should change *very* rarely and only on specific
> > administrator intervention.  We do a lot of things to avoid this
> > happening in the normal course of events. 
> > 
> 
> What would provide this signal? winbindd? I suppose we could add a knob
> or something under /sys that tells cifs to dump the idmap cache.

I think a /sys knob seems appropriate, perhaps easily sent a command
option on the same utility used for the upcall?

> We would also have to consider however how to deal with someone running
> an old winbindd that doesn't signal the kernel properly.

That's a very interesting question, as after a manual reconfiguration
perhaps even winbind might not know it changed.  It depends how deeply
the administrator changed things (changing the idmap_rid config settings
might matter for example).  I'll let others who deal with idmap more
often comment. 

Andrew Bartlett

-- 
Andrew Bartlett                                http://samba.org/~abartlet/
Authentication Developer, Samba Team           http://samba.org
Samba Developer, Cisco Inc.

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