RFC: New VOP to translate vnode to its component name

[Robert Watson]

On Mon, 8 Dec 2008, Joe Marcus Clarke wrote:

>> Just to give a general vote of "we need to do something here, whether the 
>> details are exactly these or not" -- having better object->path resolution 
>> is quite important for audit, as well as if we want to adopt a file system 
>> notification services along the lines of Apple's fsevents (which is 
>> path-centric and operates from close() events rather than open() events). 
>> I don't think we should run in the Linux 'dentry' direction, but having a 
>> more robust translation service would be quite valuable.  One comment: I 
>> think we should continue to have a KPI which does a sleep-free translation 
>> to call, but with weaker semantics than one that is sleepable and can query 
>> for more robust reverse lookup.
>
> Okay, what about a name?

Oh, I do love a good bikeshed.  I'm actually fine with any of these, but 
vn_fullpath_cache() sounds good to me.  One of the cases I have in mind is 
path-based MAC policies, which will convert from a vnode to a path while 
holding the vnode lock -- if something is going to run around locking vnodes 
and doing VOP_READDIR's, that is not the time to do it.

Robert N M Watson
Computer Laboratory
University of Cambridge

>
> vn_fullpath_cache
> vn_fullpath_quick
> vn_fullpath_fast
> vn_fullpath_nosleep
> ...
>
> Joe
>
>>
>> Robert N M Watson
>> Computer Laboratory
>> University of Cambridge
>>
>>>
>>> Using procfs (on 4.x and 6.x) or the kinfo stuff on 7.x+ sort of
>>> works, but it quickly becomes unusable if any paths involve NFS.  nfs
>>> times out its cached items very quickly.
>>>
>>> Anyway, I see this as a good first step.  I very much want to see a
>>> real vop_default implementation that does the readdir("..") method to
>>> fill in the gaps. It isn't particularly important to me if
>>> vn_fullpath() recovers the original pathname or not, so long as it can
>>> find *a* valid pathname that will work.
>>>
>>> As for names.. VOP_CNP doesn't tell me what it does at a glance.  Ideas:
>>> VOP_VPTOCNP  (vnode to component name, or VOP_VNTOCNP)
>>> VOP_RLOOKUP (reverse lookup)
>>>
>>> We have precedent for the first form.  VOP_FHTOVP().
>>>
>>> I don't think VOP_VPTOCNP() is too unwieldy and I think it would be a
>>> little more intuitive to a casual observer.  I don't want to get
>>> trapped in a bikeshed sized To:/CC: list over it though.  I'd rather
>>> see it committed to head and get some progress.
>>>
>>> BTW: at work we do extensive open-by-filehandle stuff on NFS.  For the
>>> vast majority of vnodes on those machines, there never was a name
>>> cache entry.  It would be priceless if the vop_default readdir(..)
>>> method could discover those names in procstat etc.
>>>
>>> --
>>> Peter Wemm - peter at wemm.org; peter at FreeBSD.org; peter at yahoo-inc.com; KI6FJV
>>> "All of this is for nothing if we don't go to the stars" - JMS/B5
>>> "If Java had true garbage collection, most programs would delete
>>> themselves upon execution." -- Robert Sewell
>>> _______________________________________________
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>>>
>>
> -- 
> Joe Marcus Clarke
> FreeBSD GNOME Team      ::      gnome at FreeBSD.org
> FreeNode / #freebsd-gnome
> http://www.FreeBSD.org/gnome
>