On Thu, 12 Mar 2026 at 10:26, Alexandr Moshkov <[email protected]> wrote: > > When loading a subset, its name is checked for the parent prefix. The > following bug may occur here: > > Let's say there is a vmstate named "virtio-blk", it has a subsection > named "virtio-blk/subsection", and it also has another vmstate named > "virtio" in the fields. > Then, during the migration, when trying to load this subsection for > "virtio", the prefix condition will pass for "virtio-blk/subsection" and > then the migration will break, because this vmstate does not have such a > subsection. > > In other words, if a field inside vmstate1 is set via vmstate2 with a > name that is a prefix of the parent vmstate, then the field can "steal" > a subsection belonging to the parent state. > > Looks like it happens because migration stream for "virtio-blk" looks > like this: > > [virtio-blk header] [virtio-blk fields] [virtio-blk subsections] > > "virtio-blk" contains "virtio" field, so migration stream is: > > [virtio-blk header] [virtio header] [virtio fields] [virtio > subsections] [virtio-blk subsections] > > And when we load the subsections of the "virtio" device, > vmstate_subsection_load() uses qemu_peek_byte() to try to figure out if > this is his subsection. This is where we encounter an error. > > Thus, the error occurs due to the fact that vmsd does not know how many > subsections it has when loading (this does not appear anywhere in the > migration stream), so it tries to load all the appropriate ones by > names. > > To fix this issue, we call vmstate_get_subsection() and, in case of > fail, ignore it, pop the VMSD stack and will try to define this > subsection for outer VMSD > > Signed-off-by: Alexandr Moshkov <[email protected]> > --- > migration/vmstate.c | 4 +--- > 1 file changed, 1 insertion(+), 3 deletions(-) > > diff --git a/migration/vmstate.c b/migration/vmstate.c > index 4d28364f7b..8f3a69c26e 100644 > --- a/migration/vmstate.c > +++ b/migration/vmstate.c > @@ -639,9 +639,7 @@ static int vmstate_subsection_load(QEMUFile *f, const > VMStateDescription *vmsd, > sub_vmsd = vmstate_get_subsection(vmsd->subsections, idstr); > if (sub_vmsd == NULL) { > trace_vmstate_subsection_load_bad(vmsd->name, idstr, "(lookup)"); > - error_setg(errp, "VM subsection '%s' in '%s' does not exist", > - idstr, vmsd->name); > - return -ENOENT; > + return 0; > }
Does this result in a less useful error message for the case where the source sent us a non-existent subsection ? What do we report once this patch is applied ? -- PMM
