Re: Endless loop in qcow2_alloc_cluster_offset

Jan Kiszka
2009-12-07T16:10:00Z

Kevin Wolf wrote:
> Am 07.12.2009 15:16, schrieb Jan Kiszka:
>>> Likely not. What I did was nothing special, and I did not noticed such a
>>> crash in the last months.
>> And now it happened again (qemu-kvm head, during kernel installation
>> from network onto local qcow2-disk). Any clever idea how to proceed with
>> this?
>
> I still haven't seen this and I still have no theory on what could be
> happening here. I'm just trying to write down what I think must happen
> to get into this situation. Maybe you can point at something I'm missing
> or maybe it helps you to have a sudden inspiration.
>
> The crash happens because we have a loop in the s->cluster_allocs list.
> A loop can only be created by inserting an object twice. The only insert
> to this list happens in qcow2_alloc_cluster_offset (though an earlier
> call than that of the stack trace).
>
> There is only one relevant caller of this function, qcow_aio_write_cb.
> Part of it is a call to run_dependent_requests which removes the request
> from s->cluster_allocs. So after the QLIST_REMOVE in
> run_dependent_requests the request can't be contained in the list, but
> at the call of qcow2_alloc_cluster_offset it must be contained again. It
> must be added somewhere in between these two calls.
>
> In qcow_aio_write_cb there isn't much happening between these calls. The
> only thing that could somehow become dangerous is the
> qcow_aio_write_cb(req, 0); for queued requests in run_dependent_requests.

If m->nb_clusters is not, the entry won't be removed from the list. And
of something corrupted nb_clusters so that it became 0 although it's
still enqueued, we would see the deadly loop I faced, right?
Unfortunately, any arbitrary memory corruption that generates such zeros
can cause this...

Jan

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