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

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Date:	Tue, 05 Mar 2013 14:07:49 -0500
From:	Simo <simo@...ba.org>
To:	"J. Bruce Fields" <bfields@...ldses.org>
CC:	Andy Lutomirski <luto@...capital.net>,
	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 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 :-)

/me runs

Simo.
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