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Subject: Re: [RFC PATCH v2 0/3] ceph: add support for snapshot names
 encryption
To:     Jeff Layton <jlayton@kernel.org>,
        =?UTF-8?Q?Lu=c3=ads_Henriques?= <lhenriques@suse.de>
Cc:     Ilya Dryomov <idryomov@gmail.com>,
        Ceph Development <ceph-devel@vger.kernel.org>,
        linux-kernel@vger.kernel.org
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From:   Xiubo Li <xiubli@redhat.com>
Message-ID: <83db0781-2e9a-55e1-fb0b-ee0923f0b11a@redhat.com>
Date:   Thu, 17 Mar 2022 20:44:36 +0800
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On 3/17/22 8:41 PM, Jeff Layton wrote:
> On Thu, 2022-03-17 at 20:31 +0800, Xiubo Li wrote:
>> On 3/17/22 8:01 PM, Jeff Layton wrote:
>>> On Thu, 2022-03-17 at 11:11 +0000, Lu?s Henriques wrote:
>>>> Xiubo Li <xiubli@redhat.com> writes:
>>>>
>>>>> On 3/17/22 6:01 PM, Jeff Layton wrote:
>>>>>> I'm not sure we want to worry about .snap directories here since they
>>>>>> aren't "real". IIRC, snaps are inherited from parents too, so you could
>>>>>> do something like
>>>>>>
>>>>>>        mkdir dir1
>>>>>>        mkdir dir1/.snap/snap1
>>>>>>        mkdir dir1/dir2
>>>>>>        fscrypt encrypt dir1/dir2
>>>>>>
>>>>>> There should be nothing to prevent encrypting dir2, but I'm pretty sure
>>>>>> dir2/.snap will not be empty at that point.
>>>>> If we don't take care of this. Then we don't know which snapshots should do
>>>>> encrypt/dencrypt and which shouldn't when building the path in lookup and when
>>>>> reading the snapdir ?
>>>> In my patchset (which I plan to send a new revision later today, I think I
>>>> still need to rebase it) this is handled by using the *real* snapshot
>>>> parent inode.  If we're decrypting/encrypting a name for a snapshot that
>>>> starts with a '_' character, we first find the parent inode for that
>>>> snapshot and only do the operation if that parent is encrypted.
>>>>
>>>> In the other email I suggested that we could prevent enabling encryption
>>>> in a directory when there are snapshots above in the hierarchy.  But now
>>>> that I think more about it, it won't solve any problem because you could
>>>> create those snapshots later and then you would still need to handle these
>>>> (non-encrypted) "_name_xxxx" snapshots anyway.
>>>>
>>> Yeah, that sounds about right.
>>>
>>> What happens if you don't have the snapshot parent's inode in cache?
>>> That can happen if you (e.g.) are running NFS over ceph, or if you get
>>> crafty with name_to_handle_at() and open_by_handle_at().
>>>
>>> Do we have to do a LOOKUPINO in that case or does the trace contain that
>>> info? If it doesn't then that could really suck in a big hierarchy if
>>> there are a lot of different snapshot parent inodes to hunt down.
>>>
>>> I think this is a case where the client just doesn't have complete
>>> control over the dentry name. It may be better to just not encrypt them
>>> if it's too ugly.
>>>
>>> Another idea might be to just use the same parent inode (maybe the
>>> root?) for all snapshot names. It's not as secure, but it's probably
>>> better than nothing.
>> Does it allow to have different keys for the subdirs in the hierarchy ?
>>   From my test it doesn't.
>>
> No. Once you set a key on directory you can't set another on a subtree
> of it.
If so. Yeah, I think your approach mentioned above is the best.
>> If so we can always use the same oldest ancestor in the hierarchy, who
>> has encryption key, to encyrpt/decrypt all the .snap in the hierarchy,
>> then just need to lookup oldest ancestor inode only once.
>>
Just like this.

-- Xiubo

> That's a possibility.