Re: [PATCH v7 03/12] PCI: liveupdate: Track incoming preserved PCI devices

[David Matlack]

On Fri, Jul 17, 2026 at 2:47 PM Pasha Tatashin
<pasha.tatashin@xxxxxxxxxx> wrote:
>
> On Fri, 10 Jul 2026 21:26:06 +0000, David Matlack <dmatlack@xxxxxxxxxx> wrote:
> > diff --git a/drivers/pci/liveupdate.c b/drivers/pci/liveupdate.c
> > index 03075ce06ac9..df6a02240aa4 100644
> > --- a/drivers/pci/liveupdate.c
> > +++ b/drivers/pci/liveupdate.c
> > @@ -298,6 +377,87 @@ void pci_liveupdate_unpreserve(struct pci_dev *dev)
> > [ ... skip 76 lines ... ]
> > +     /*
> > +      * Hold the ref on the incoming FLB until pci_liveupdate_finish() so
> > +      * that dev->liveupdate.incoming cannot get freed while the PCI core
> > +      * has a pointer to it. It's better to leak the incoming FLB than do a
> > +      * use-after-free if driver does not call pci_liveupdate_finish().
> > +      */
>
> I am confused by this. Each preserved PCI device must have an associated
> FD preserved with it via LUO. I.e., vfiofd would need to be preserved. If
> the vfiofd was not reclaimed, and finish is not possible, that vfiofd
> would still be owned by LUO, and therefore PCI FLB refcount would stay
> positive.
>
> However, if finish is possible, and this is the last vfiofd that is
> finished, FLB will be freed as soon as the reference count reaches zero,
> which I would think is the expected behavior.
>
> What is the point of holding a reference here, instead of only for the
> duration of FLB access, i.e. to make sure we are accessing a valid data?

The duration of the access is from here until pci_liveupdate_finish()
because that it when the pointer (dev->liveupdate.incoming) is
cleared. So that is why the PCI core holds the reference from here
until pci_liveupdate_finish().

We could avoid this by deleteing dev->liveupdate.incoming and,
instead, fetching the incoming FLB and doing the xarray lookup every
atime the PCI core needs to access the device's incoming ser struct,
but that would be inefficient.