Re: [PATCH v3 1/5] liveupdate: block outgoing session updates during reboot

[Pasha Tatashin]

On Sat, Mar 21, 2026 at 6:28 PM Pasha Tatashin
<pasha.tatashin@xxxxxxxxxx> wrote:
>
> On Sat, Mar 21, 2026 at 10:38 AM Oskar Gerlicz Kowalczuk
> <oskar@gerlicz.space> wrote:
> >
> > kernel_kexec() serializes outgoing sessions before the reboot path
> > freezes tasks, so close() and session ioctls can still mutate a
> > session while handover state is being prepared. The original v2 code
> > also let incoming lookups keep a bare session pointer after dropping
> > the list lock.
> >
> > That leaves two correctness problems in the reboot path: outgoing state
> > can change after serialization starts, and incoming sessions can be
> > freed while another thread still holds a pointer to them.
> >
> > Add refcounted session lifetime management, track in-flight outgoing
> > close() paths with an atomic closing counter, and make serialization
> > wait for closing to drain before setting rebooting. Reject phase-invalid
> > ioctls, keep incoming release on a common cleanup path, and make the
> > release wait freezable without spinning.
> >
> > Fixes: fc5acd5c89fe ("liveupdate: block outgoing session updates during reboot")
> > Signed-off-by: Oskar Gerlicz Kowalczuk <oskar@gerlicz.space>
> > ---
> >  kernel/liveupdate/luo_internal.h |  12 +-
> >  kernel/liveupdate/luo_session.c  | 236 +++++++++++++++++++++++++++----
> >  2 files changed, 221 insertions(+), 27 deletions(-)
>
> Hi Oskar,
>
> Thank you for sending this series and finding these bugs in LUO. I
> agree with Andrew that a cover letter would help to understand the
> summary of the overall effort.
>
> I have not reviewed the other patches yet, but for this patch, my
> understanding is that it solves two specific races during reboot()
> syscalls: session closure after serialization, and the addition of new
> sessions or preserving new files after serialization.
>
> Given that KHO is now stateless, and liveupdate_reboot() is
> specifically placed at the last point where we can still return an
> error to userspace, we should simply return an error if a userspace is
> doing something unexpected.
>
> Instead of creating a new state machine, let's just reuse the file
> references and simply take them for each session at the beginning of
> serialization. This ensures that no session closes will happen later.
> For file preservation and session addition, we can block them by
> simply adding a new boolean.
>
> Please take a look at the two patches below and see if this approach
> would work. It is a much smaller change compared to the proposed state
> machine in this patch.
>
> https://git.kernel.org/pub/scm/linux/kernel/git/tatashin/linux.git/log/?h=luo-reboot-sync/rfc/1

Oskar, I made a few more changes to avoid returning an error if
get_file_active() fails. This prevents a race condition where the user
might call close(session_fd) right before calling reboot(). I
force-updated the above branch. Please let me know if you want to take
these changes and use them to in the next version.

Pasha