On Mon, Mar 11, 2013 at 11:18:15AM -0700, Andy Lutomirski wrote: > On Tue, Mar 5, 2013 at 11:07 AM, Simo <simo@xxxxxxxxx> 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. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
