On 3/12/26 16:19, Peter Maydell wrote:
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 ?
I think it might lead to, but in general, it will work correctly.
