Re: [RFC 1/1] shiftfs: uid/gid shifting bind mount

[Amir Goldstein]

On Tue, Feb 7, 2017 at 10:05 PM, James Bottomley
<James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, 2017-02-07 at 11:49 -0800, Christoph Hellwig wrote:
>> On Tue, Feb 07, 2017 at 11:02:03AM -0800, James Bottomley wrote:
>> > >   Another option would be to require something like a project as
>> > > used
>> > > for project quotas as the root.  This would also be conveniant as
>> > > it
>> > > could storge the used remapping tables.
>> >
>> > So this would be like the current project quota except set on a
>> > subtree?  I could see it being done that way but I don't see what
>> > advantage it has over using flags in the subtree itself (the
>> > mapping is
>> > known based on the mount namespace, so there's really only a single
>> > bit
>> > of information to store).
>>
>> projects (which are the underling concept for project quotas) are
>> per-subtree in practice - the flag is set on an inode and then
>> all directories and files underneath inherit the project ID,
>> hardlinking outside a project is prohinited.
>
> OK, this is what I don't understand: how is something that's inode
> based limited to be per-subtree?  The way I've seen the VFS operate it
> seems that any given inode (and indeed dentry) can appear in many
> subtrees so how do I limit them to just one?
>

Project id's are not exactly "subtree" semantic, but inheritance semantics,
which is not the same when non empty directories get their project id changed.
Here is a recap:
https://lwn.net/Articles/623835/

So if you created an empty directory and "marked" it for shiftuid and all
descendants inherited this property you would be able to check that property
on a per inode basis. Not sure that is what you are looking for?
I guess we should define the semantics for the required sub-tree marking,
before we can talk about solutions.