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. --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
