Re: [PATCH] fs/ceph/super: add mount options "snapdir{mode,uid,gid}"

[Jeff Layton]

On Mon, 2022-10-10 at 10:02 +0800, Xiubo Li wrote:
> On 09/10/2022 18:27, Max Kellermann wrote:
> > On Sun, Oct 9, 2022 at 10:43 AM Xiubo Li <xiubli@xxxxxxxxxx> wrote:
> > > I mean CEPHFS CLIENT CAPABILITIES [1].
> > I know that, but that's suitable for me. This is client-specific, not
> > user (uid/gid) specific.
> > 
> > In my use case, a server can run unprivileged user processes which
> > should not be able create snapshots for their own home directory, and
> > ideally they should not even be able to traverse into the ".snap"
> > directory and access the snapshots created of their home directory.
> > Other (non-superuser) system processes however should be able to
> > manage snapshots. It should be possible to bind-mount snapshots into
> > the user's mount namespace.
> > 
> > All of that is possible with my patch, but impossible with your
> > suggestion. The client-specific approach is all-or-nothing (unless I
> > miss something vital).
> > 
> > > The snapdir name is a different case.
> > But this is only about the snapdir. The snapdir does not exist on the
> > server, it is synthesized on the client (in the Linux kernel cephfs
> > code).
> 
> This could be applied to it's parent dir instead as one metadata in mds 
> side and in client side it will be transfer to snapdir's metadata, just 
> like what the snapshots.
> 
> But just ignore this approach.
> 
> > > But your current approach will introduce issues when an UID/GID is reused after an user/groud is deleted ?
> > The UID I would specify is one which exists on the client, for a
> > dedicated system user whose purpose is to manage cephfs snapshots of
> > all users. The UID is created when the machine is installed, and is
> > never deleted.
> 
> This is an ideal use case IMO.
> 
> I googled about reusing the UID/GID issues and found someone has hit a 
> similar issue in their use case.
> 

This is always a danger and not just with ceph. The solution to that is
good sysadmin practices (i.e. don't reuse uid/gid values without
sanitizing the filesystems first).

> > > Maybe the proper approach is the posix acl. Then by default the .snap dir will inherit the permission from its parent and you can change it as you wish. This permission could be spread to all the other clients too ?
> > No, that would be impractical and unreliable.
> > Impractical because it would require me to walk the whole filesystem
> > tree and let the kernel synthesize the snapdir inode for all
> > directories and change its ACL;
> 
> No, it don't have to. This could work simply as the snaprealm hierarchy 
> thing in kceph.
> 
> Only the up top directory need to record the ACL and all the descendants 
> will point and use it if they don't have their own ACLs.
> 
> >   impractical because walking millions
> > of directories takes longer than I am willing to wait.
> > Unreliable because there would be race problems when another client
> > (or even the local client) creates a new directory. Until my local
> > "snapdir ACL daemon" learns about the existence of the new directory
> > and is able to update its ACL, the user can already have messed with
> > it.
> 
> For multiple clients case I think the cephfs capabilities [3] could 
> guarantee the consistency of this. While for the single client case if 
> before the user could update its ACL just after creating it someone else 
> has changed it or messed it up, then won't the existing ACLs have the 
> same issue ?
> 
> [3] https://docs.ceph.com/en/quincy/cephfs/capabilities/
> 
> 
> > Both of that is not a problem with my patch.
> > 
> Jeff,
> 
> Any idea ?
> 

I tend to agree with Max here. The .snap dir is a client-side fiction,
so trying to do something on the MDS to govern its use seems a bit odd.
cephx is really about authenticating clients. I know we do things like
enforce root squashing on the MDS, but this is a little different.

Now, all of that said, snapshot handling is an area where I'm just not
that knowledgeable. Feel free to ignore my opinion here as uninformed.
-- 
Jeff Layton <jlayton@xxxxxxxxxx>