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

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Date:	Mon, 4 Mar 2013 16:19:24 -0500
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Pavel Shilovsky <piastry@...rsoft.ru>,
	linux-kernel@...r.kernel.org, linux-cifs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-nfs@...r.kernel.org,
	wine-devel@...ehq.org
Subject: Re: [PATCH v3 0/7] Add O_DENY* support for VFS and CIFS/NFS

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.
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