Re: [RFC 0/2] fuse: introduce fuse server recovery mechanism

[Gao Xiang]

Hi Christian,

On 2024/5/28 16:43, Christian Brauner wrote:
> On Tue, May 28, 2024 at 12:02:46PM +0800, Gao Xiang wrote:
> > On 2024/5/28 11:08, Jingbo Xu wrote:
> > > On 5/28/24 10:45 AM, Jingbo Xu wrote:
> > > > On 5/27/24 11:16 PM, Miklos Szeredi wrote:
> > > > > On Fri, 24 May 2024 at 08:40, Jingbo Xu <jefflexu@xxxxxxxxxxxxxxxxx> wrote:
> > > > >
> > > > > > 3. I don't know if a kernel based recovery mechanism is welcome on the
> > > > > > community side.  Any comment is welcome.  Thanks!
> > > > >
> > > > > I'd prefer something external to fuse.
> > > >
> > > > Okay, understood.
> > > >
> > > > > Maybe a kernel based fdstore (lifetime connected to that of the
> > > > > container) would a useful service more generally?
> > > >
> > > > Yeah I indeed had considered this, but I'm afraid VFS guys would be
> > > > concerned about why we do this on kernel side rather than in user space.
> >
> > Just from my own perspective, even if it's in FUSE, the concern is
> > almost the same.
> >
> > I wonder if on-demand cachefiles can keep fds too in the future
> > (thus e.g. daemonless feature could even be implemented entirely
> > with kernel fdstore) but it still has the same concern or it's
> > a source of duplication.
> >
> > Thanks,
> > Gao Xiang
> >
> > > > I'm not sure what the VFS guys think about this and if the kernel side
> > > > shall care about this.
>
> Fwiw, I'm not convinced and I think that's a big can of worms security
> wise and semantics wise. I have discussed whether a kernel-side fdstore
> would be something that systemd would use if available multiple times
> and they wouldn't use it because it provides them with no benefits over
> having it in userspace.

As far as I know, currently there are approximately two ways to do
failover mechanisms in kernel.

The first model much like a fuse-like model: in this mode, we should
keep and pass fd to maintain the active state.  And currently,
userspace should be responsible for the permission/security issues
when doing something like passing fds.

The second model is like one device-one instance model, for example
ublk (If I understand correctly): each active instance (/dev/ublkbX)
has their own unique control device (/dev/ublkcX).  Users could
assign/change DAC/MAC for each control device.  And failover
recovery just needs to reopen the control device with proper
permission and do recovery.

So just my own thought, kernel-side fdstore pseudo filesystem may
provide a DAC/MAC mechanism for the first model.  That is a much
cleaner way than doing some similar thing independently in each
subsystem which may need DAC/MAC-like mechanism.  But that is
just my own thought.

Thanks,
Gao Xiang

> Especially since it implements a lot of special semantics and policy
> that we really don't want in the kernel. I think that's just not
> something we should do. We should give userspace all the means to
> implement fdstores in userspace but not hold fds ourselves.