Thoughts on correct support for Windows Previous versions

[ronnie sahlberg]

-Wall -Werror is what the doctor orders

On Tue, Jan 27, 2015 at 4:17 PM, Satyajit Deshmukh
<satyaji.deshmuk at nutanix.com> wrote:
> A minor correction is needed in the patch attached previously-
> diff --git a/source3/smbd/trans2.c b/source3/smbd/trans2.c
> index 16498b3..57d7bde 100644
> --- a/source3/smbd/trans2.c
> +++ b/source3/smbd/trans2.c
> .....
> +                       /*
> +                        * If the file is a previous version, Windows expects
> +                        * FS_INFO to indicate that it is on an ROFS.
> +                        */
> +                       if (fsp && fsp->is_previous_version &&
> +                           !fsp->is_directory)
> +                               additional_flags != FILE_READ_ONLY_VOLUME;
>
> I believe it should be-
> additional_flags |= FILE_READ_ONLY_VOLUME;
>
> Thanks,
> Satyajit
>
> On Mon, Jan 26, 2015 at 4:57 PM, Richard Sharpe
> <realrichardsharpe at gmail.com> wrote:
>> 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)