Message-ID: <CA+CK2bBrCd8t_BUeE-sVPGjsJwmtk3mCSVhTMGbseTi_Wk+4yQ@mail.gmail.com>
Date: Tue, 26 Aug 2025 15:02:13 +0000
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Pratyush Yadav <pratyush@...nel.org>, jasonmiu@...gle.com, graf@...zon.com, 
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Subject: Re: [PATCH v3 00/30] Live Update Orchestrator

On Tue, Aug 26, 2025 at 2:24 PM Jason Gunthorpe <jgg@...dia.com> wrote:
>
> On Tue, Aug 26, 2025 at 01:54:31PM +0000, Pasha Tatashin wrote:
> > > > https://github.com/googleprodkernel/linux-liveupdate/tree/luo/v3
> > > >
> > > > Changelog from v2:
> > > > - Addressed comments from Mike Rapoport and Jason Gunthorpe
> > > > - Only one user agent (LiveupdateD) can open /dev/liveupdate
> > > > - With the above changes, sessions are not needed, and should be
> > > >   maintained by the user-agent itself, so removed support for
> > > >   sessions.
> > >
> > > If all the FDs are restored in the agent's context, this assigns all the
> > > resources to the agent. For example, if the agent restores a memfd, all
> > > the memory gets charged to the agent's cgroup, and the client gets none
> > > of it. This makes it impossible to do any kind of resource limits.
> > >
> > > This was one of the advantages of being able to pass around sessions
> > > instead of FDs. The agent can pass on the right session to the right
> > > client, and then the client does the restore, getting all the resources
> > > charged to it.
> > >
> > > If we don't allow this, I think we will make LUO/LiveupdateD unsuitable
> > > for many kinds of workloads. Do you have any ideas on how to do proper
> > > resource attribution with the current patches? If not, then perhaps we
> > > should reconsider this change?
> >
> > Hi Pratyush,
> >
> > That's an excellent point, and you're right that we must have a
> > solution for correct resource charging.
> >
> > I'd prefer to keep the session logic in the userspace agent (luod
> > https://tinyurl.com/luoddesign).
> >
> > For the charging problem, I believe there's a clear path forward with
> > the current ioctl-based API. The design of the ioctl commands (with a
> > size field in each struct) is intentionally extensible. In a follow-up
> > patch, we can extend the liveupdate_ioctl_fd_restore struct to include
> > a target pid field. The luod agent, would then be able to restore an
> > FD on behalf of a client and instruct the kernel to charge the
> > associated resources to that client's PID.
>
> This wasn't quite the idea though..
>
> The sessions sub FD were intended to be passed directly to other
> processes though unix sockets and fd passing so they could run their
> own ioctls in their own context for both save and restore. The ioctls
> available on the sessions should be specifically narrowed to be safe
> for this.
>
> I can understand not implementing session FDs in the first version,
> but when sessions FD are available they should work like this and
> solve the namespace/cgroup/etc issues.
>
> Passing some PID in an ioctl is not a great idea...

Hi Jason,

I'm trying to understand the drawbacks of the PID-based approach.
Could you elaborate on why passing a PID in the RESTORE_FD ioctl is
not a good idea?

>From my perspective, luod would have a live, open socket to the client
process requesting the restore. It can use SO_PEERCRED to securely
identify the client's PID at that moment. The flow would be:

1. Client connects and resumes its session with luod.
2. Client requests to restore TOKEN_X.
3. luod verifies the client owns TOKEN_X for its session.
4. luod calls the RESTORE_FD ioctl, telling the kernel: "Please
restore TOKEN_X and charge the resources to PID Y (which I just
verified is on the other end of this socket)."
5. The kernel performs the action.
6. luod receives the new FD from the kernel and passes it back to the
client over the socket.

In this flow, the client isn't providing an arbitrary PID; the trusted
luod agent is providing the PID of a process it has an active
connection with.

The idea was to let luod handle the session/security story, and the
kernel handle the core preservation mechanism. Adding sessions to the
kernel, delegates the management and part of the security model into
the kernel. I am not sure if it is necessary, what can be cleanly
managed in userspace should stay in userspace.

Thanks,
Pasha


>
> Jason

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