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? 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. 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. --b. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
