Re: [PATCH v3 0/7] Add O_DENY* support for VFS and CIFS/NFS

["J. Bruce Fields" <bfields@fieldses.org>]

On Mon, Mar 11, 2013 at 11:18:15AM -0700, Andy Lutomirski wrote:
> On Tue, Mar 5, 2013 at 11:07 AM, Simo <simo@samba.org> wrote:
> > On 03/05/2013 01:13 PM, J. Bruce Fields wrote:
> >>
> >> On Mon, Mar 04, 2013 at 05:49:46PM -0500, Simo wrote:
> >>>
> >>> On 03/04/2013 04:19 PM, J. Bruce Fields wrote:
> >>>>
> >>>> On Thu, Feb 28, 2013 at 01:53:25PM -0800, Andy Lutomirski wrote:
> >>>>>
> >>>>> [possible resend -- sorry]
> >>>>>
> >>>>> On 02/28/2013 07:25 AM, Pavel Shilovsky wrote:
> >>>>>>
> >>>>>> This patchset adds support of O_DENY* flags for Linux fs layer. These
> >>>>>> flags can be used by any application that needs share reservations to
> >>>>>> organize a file access. VFS already has some sort of this capability - now
> >>>>>> it's done through flock/LOCK_MAND mechanis, but that approach is non-atomic.
> >>>>>> This patchset build new capabilities on top of the existing one but doesn't
> >>>>>> bring any changes into the flock call semantic.
> >>>>>>
> >>>>>> These flags can be used by NFS (built-in-kernel) and CIFS (Samba)
> >>>>>> servers and Wine applications through VFS (for local filesystems) or
> >>>>>> CIFS/NFS modules. This will help when e.g. Samba and NFS server share the
> >>>>>> same directory for Windows and Linux users or Wine applications use
> >>>>>> Samba/NFS share to access the same data from different clients.
> >>>>>>
> >>>>>> According to the previous discussions the most problematic question is
> >>>>>> how to prevent situations like DoS attacks where e.g /lib/liba.so file can
> >>>>>> be open with DENYREAD, or smth like this. That's why one extra flag
> >>>>>> O_DENYMAND is added. It indicates to underlying layer that an application
> >>>>>> want to use O_DENY* flags semantic. It allows us not affect native Linux
> >>>>>> applications (that don't use O_DENYMAND flag) - so, these flags (and the
> >>>>>> semantic of open syscall that they bring) are used only for those
> >>>>>> applications that really want it proccessed that way.
> >>>>>>
> >>>>>> So, we have four new flags:
> >>>>>> O_DENYREAD - to prevent other opens with read access,
> >>>>>> O_DENYWRITE - to prevent other opens with write access,
> >>>>>> O_DENYDELETE - to prevent delete operations (this flag is not
> >>>>>> implemented in VFS and NFS part and only suitable for CIFS module),
> >>>>>> O_DENYMAND - to switch on/off three flags above.
> >>>>>
> >>>>> O_DENYMAND doesn't deny anything.  Would a name like O_RESPECT_DENY be
> >>>>> better?
> >>>>>
> >>>>> Other than that, this seems like a sensible mechanism.
> >>>>
> >>>> I'm a little more worried: these are mandatory locks, and applications
> >>>> that use them are used to the locks being enforced correctly.  Are we
> >>>> sure that an application that opens a file O_DENYWRITE won't crash if it
> >>>> sees the file data change while it holds the open?
> >>>
> >>> The redirector may simply assume it has full control of that part of
> >>> the file and not read nor send data until the lock is released too,
> >>> so you get conflicting views of the file contents between different
> >>> clients if you let a mandatory lock not be mandatory.
> >>>
> >>>> In general the idea of making a mandatory lock opt-in makes me nervous.
> >>>> I'd prefer something like a mount option, so that we know that everyone
> >>>> on that one filesystem is playing by the same rules, but we can still
> >>>> mount filesystems like / without the option.
> >>>
> >>> +1
> >>>
> >>>> But I'll admit I'm definitely not an expert on Windows locking and may
> >>>> be missing something about how these locks are meant to work.
> >>>
> >>> Mandatory locks really are mandatory in Windows.
> >>> That may not be nice to a Unix system but what can you do ?
> >>
> >> I wonder if we could repurpose the existing -omand mount option?
> >>
> >> That would be a problem for anyone that wants to allow mandatory fcntl
> >> locks without allowing share locks.  I doubt anyone sane actually uses
> >> mandatory fcntl locks, but still I suppose it would probably be better
> >> to play it safe and use a new mount option.
> >
> >
> > Maybe we should have a -o win_semantics option :-)
> >
> 
> It's not entirely obvious to me that allowing programs to bypass this
> kind of locking is a bad idea.  It's hard to do on Windows, but
> presumably network filesystems on Windows already have this issue.

Could be, but I'd like to see evidence of that.

--b.