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

From: Andy Lutomirski
Date: Mon Mar 11 2013 - 14:18:43 EST

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/ 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.

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