linux-kernel - Re: [PATCH v5 00/14] ceph: support idmapped mounts

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Date:   Tue, 13 Jun 2023 09:43:07 +0800
From:   Xiubo Li <xiubli@...hat.com>
To:     Aleksandr Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>,
        Christian Brauner <brauner@...nel.org>,
        Gregory Farnum <gfarnum@...hat.com>
Cc:     stgraber@...ntu.com, linux-fsdevel@...r.kernel.org,
        Ilya Dryomov <idryomov@...il.com>,
        Jeff Layton <jlayton@...nel.org>, ceph-devel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 00/14] ceph: support idmapped mounts


On 6/9/23 18:12, Aleksandr Mikhalitsyn wrote:
> On Fri, Jun 9, 2023 at 12:00 PM Christian Brauner <brauner@...nel.org> wrote:
>> On Fri, Jun 09, 2023 at 10:59:19AM +0200, Aleksandr Mikhalitsyn wrote:
>>> On Fri, Jun 9, 2023 at 3:57 AM Xiubo Li <xiubli@...hat.com> wrote:
>>>>
>>>> On 6/8/23 23:42, Alexander Mikhalitsyn wrote:
>>>>> Dear friends,
>>>>>
>>>>> This patchset was originally developed by Christian Brauner but I'll continue
>>>>> to push it forward. Christian allowed me to do that :)
>>>>>
>>>>> This feature is already actively used/tested with LXD/LXC project.
>>>>>
>>>>> Git tree (based on https://github.com/ceph/ceph-client.git master):
>>> Hi Xiubo!
>>>
>>>> Could you rebase these patches to 'testing' branch ?
>>> Will do in -v6.
>>>
>>>> And you still have missed several places, for example the following cases:
>>>>
>>>>
>>>>      1    269  fs/ceph/addr.c <<ceph_netfs_issue_op_inline>>
>>>>                req = ceph_mdsc_create_request(mdsc, CEPH_MDS_OP_GETATTR,
>>>> mode);
>>> +
>>>
>>>>      2    389  fs/ceph/dir.c <<ceph_readdir>>
>>>>                req = ceph_mdsc_create_request(mdsc, op, USE_AUTH_MDS);
>>> +
>>>
>>>>      3    789  fs/ceph/dir.c <<ceph_lookup>>
>>>>                req = ceph_mdsc_create_request(mdsc, op, USE_ANY_MDS);
>>> We don't have an idmapping passed to lookup from the VFS layer. As I
>>> mentioned before, it's just impossible now.
>> ->lookup() doesn't deal with idmappings and really can't otherwise you
>> risk ending up with inode aliasing which is really not something you
>> want. IOW, you can't fill in inode->i_{g,u}id based on a mount's
>> idmapping as inode->i_{g,u}id absolutely needs to be a filesystem wide
>> value. So better not even risk exposing the idmapping in there at all.
> Thanks for adding, Christian!
>
> I agree, every time when we use an idmapping we need to be careful with
> what we map. AFAIU, inode->i_{g,u}id should be based on the filesystem
> idmapping (not mount),
> but in this case, Xiubo want's current_fs{u,g}id to be mapped
> according to an idmapping.
> Anyway, it's impossible at now and IMHO, until we don't have any
> practical use case where
> UID/GID-based path restriction is used in combination with idmapped
> mounts it's not worth to
> make such big changes in the VFS layer.
>
> May be I'm not right, but it seems like UID/GID-based path restriction
> is not a widespread
> feature and I can hardly imagine it to be used with the container
> workloads (for instance),
> because it will require to always keep in sync MDS permissions
> configuration with the
> possible UID/GID ranges on the client. It looks like a nightmare for sysadmin.
> It is useful when cephfs is used as an external storage on the host, but if you
> share cephfs with a few containers with different user namespaces idmapping...

Hmm, while this will break the MDS permission check in cephfs then in 
lookup case. If we really couldn't support it we should make it to 
escape the check anyway or some OPs may fail and won't work as expected.

@Greg

For the lookup requests the idmapping couldn't get the mapped UID/GID 
just like all the other requests, which is needed by the MDS permission 
check. Is that okay to make it disable the check for this case ? I am 
afraid this will break the MDS permssions logic.

Any idea ?

Thanks

- Xiubo


> Kind regards,
> Alex
>