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. --Andy -- 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
