linux-kernel - Re: [PATCH v6 05/20] liveupdate: luo

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CALzav=dmFQr+BrqzRDgio0q68MPRVnZPK4-wUXVj47o1FObgNg@mail.gmail.com>
Date: Thu, 20 Nov 2025 11:42:47 -0800
From: David Matlack <dmatlack@...gle.com>
To: Pasha Tatashin <pasha.tatashin@...een.com>
Cc: pratyush@...nel.org, jasonmiu@...gle.com, graf@...zon.com, rppt@...nel.org, 
	rientjes@...gle.com, corbet@....net, rdunlap@...radead.org, 
	ilpo.jarvinen@...ux.intel.com, kanie@...ux.alibaba.com, ojeda@...nel.org, 
	aliceryhl@...gle.com, masahiroy@...nel.org, akpm@...ux-foundation.org, 
	tj@...nel.org, yoann.congal@...le.fr, mmaurer@...gle.com, 
	roman.gushchin@...ux.dev, chenridong@...wei.com, axboe@...nel.dk, 
	mark.rutland@....com, jannh@...gle.com, vincent.guittot@...aro.org, 
	hannes@...xchg.org, dan.j.williams@...el.com, david@...hat.com, 
	joel.granados@...nel.org, rostedt@...dmis.org, anna.schumaker@...cle.com, 
	song@...nel.org, linux@...ssschuh.net, linux-kernel@...r.kernel.org, 
	linux-doc@...r.kernel.org, linux-mm@...ck.org, gregkh@...uxfoundation.org, 
	tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, 
	dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, rafael@...nel.org, 
	dakr@...nel.org, bartosz.golaszewski@...aro.org, cw00.choi@...sung.com, 
	myungjoo.ham@...sung.com, yesanishhere@...il.com, Jonathan.Cameron@...wei.com, 
	quic_zijuhu@...cinc.com, aleksander.lobakin@...el.com, ira.weiny@...el.com, 
	andriy.shevchenko@...ux.intel.com, leon@...nel.org, lukas@...ner.de, 
	bhelgaas@...gle.com, wagi@...nel.org, djeffery@...hat.com, 
	stuart.w.hayes@...il.com, ptyadav@...zon.de, lennart@...ttering.net, 
	brauner@...nel.org, linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	saeedm@...dia.com, ajayachandra@...dia.com, jgg@...dia.com, parav@...dia.com, 
	leonro@...dia.com, witu@...dia.com, hughd@...gle.com, skhawaja@...gle.com, 
	chrisl@...nel.org
Subject: Re: [PATCH v6 05/20] liveupdate: luo_ioctl: add user interface

On Thu, Nov 20, 2025 at 11:23 AM Pasha Tatashin
<pasha.tatashin@...een.com> wrote:
>
> On Thu, Nov 20, 2025 at 1:38 PM David Matlack <dmatlack@...gle.com> wrote:
> >
> > On Sat, Nov 15, 2025 at 3:34 PM Pasha Tatashin
> > <pasha.tatashin@...een.com> wrote:
> > > The idea is that there is going to be a single userspace agent driving
> > > the live update, therefore, only a single process can ever hold this
> > > device opened at a time.
> > ...
> > > +static int luo_open(struct inode *inodep, struct file *filep)
> > > +{
> > > +       struct luo_device_state *ldev = container_of(filep->private_data,
> > > +                                                    struct luo_device_state,
> > > +                                                    miscdev);
> > > +
> > > +       if (atomic_cmpxchg(&ldev->in_use, 0, 1))
> > > +               return -EBUSY;
> >
> > Can you remind me why the kernel needs to enforce this? What would be
> > wrong or unsafe from the kernel perspective if there were multiple
> > userspace agents holding open files for /dev/liveupdate, each with
> > their own sessions?
>
> By enforcing a singleton, we will ensure a consistent view for tooling
> like luoadm (which will track incoming/outgoing sessions, UUIDs, etc.)
> and prevent conflicting commands regarding the transition state.
>
> This is not a bottleneck because the vast majority of the work
> (preserving devicse/memory) is handled via the individual Session FDs.
> Also, since sessions persist even if /dev/liveupdate is closed, we
> allow the agent upgrade, or crashing without requiring concurrent
> access.

Yeah, I'm not concerned about bottlenecking. It just seems like an
artificial constraint to impose on userspace at this point. The only
ioctls on /dev/liveupdate are to create a session and retreive a
session. Neither of those will conflict with having multiple open
files for /dev/liveupdate.