Thoughts on correct support for Windows Previous versions

[Richard Sharpe]

On Mon, Jan 26, 2015 at 4:40 PM, Jeremy Allison <jra at samba.org> wrote:
> On Sun, Jan 25, 2015 at 05:49:35PM -0800, Richard Sharpe wrote:
>> When we implemented support for Windows Previous Versions at Panzura
>> we started with one of the existing Shadow Copy modules and adapted it
>> to the SnapShots provided by the file system (ZFS-based), and life
>> seemed good.
>>
>> Then in came the bug reports from QA and we were forced to check what
>> Windows actually did.
>>
>> At first I assumed that Windows would return an error like
>> STATUS_ACCESS_DENIED if one tried to open a previous version for
>> write-like access, eg, opening a previous version in Notepad or
>> wordpad, but this turned out to be incorrect. In fact, Windows allows
>> you to do that, at least according to our tests, but returns
>> STATUS_MEDIA_WRITE_PROTECTED.
>>
>> We also found that Windows will ask for FS_INFO on open files,
>> presumably to check that the volume is write protected.
>>
>> This does lead to some interesting problems, because it means that
>> when the client asks for WRITE access to a file and the ACL allows it,
>> you then try to open the file for WRITE access, but the file system
>> denies it (or at least ours did) and returns EROFS. You have to turn
>> around and open the file RO and pretend that you opened it for WRITE
>> to get the correct behavior.
>>
>> Unfortunately, at Panzura we modified Samba too much, and I am now
>> convinced that most of the changes could have been done in the VFS
>> module we had.
>>
>> However, there are a few small changes I think are needed. One is to
>> add a field to the FSP to record whether or not a file is a previous
>> version. This would only ever be set by a VFS module providing
>> previous versions functionality, but it seems simpler than adorning
>> the FSP with an extension and is needed in one place in Samba.
>>
>> The other one is to pass an FSP into one function and to return the
>> correct value when Windows asks for FS_INFO on an open file.
>>
>> Attached is a patch for discussion that makes those changes.
>>
>> However, this might not be enough because VFS calls like ntimes does
>> not get passed an FSP (something I want to fix at some time as well,
>> as soon as 4.2 is finally cut.) (The only way to return
>> STATUS_MEDIA_WRITE_PROTECTED from VFS modules is if all functions that
>> implement WRITE-like behavior get access to the FSP.)
>>
>> Over the next few weeks I will try to get a capture of the behavior of
>> Windows with respect to getting FS_INFO on an open file to document
>> this.
>
> Yep, that's the next step. We need to know exactly what
> behavior we need to emulate to make the Windows Previous Versions
> work like a Windows server, then I'll be happy to add tests
> and make any needed VFS changes.

OK, I will try to bring this to the top of my priority queue. I would
also like to get changes to the VFS in such that we provide an FSP to
all VFS functions that deal with files. There are some that don't as
yet get an FSP.

However, I think this change has to wait until 4.3.

-- 
Regards,
Richard Sharpe
(何以解憂？唯有杜康。--曹操)