On 2014-10-12 15:34, Kevin Wolf wrote:
Am 11.10.2014 um 09:14 hat Zhang Haoyu geschrieben:
In qcow2_update_snapshot_refcount -> qcow2_process_discards() -> bdrv_discard()
may free the Qcow2DiscardRegion which is referenced by "next" pointer in
qcow2_process_discards() now, in next iteration, d = next, so g_free(d)
will double-free this Qcow2DiscardRegion.
qcow2_snapshot_delete
|- qcow2_update_snapshot_refcount
|-- qcow2_process_discards
|--- bdrv_discard
|---- aio_poll
|----- aio_dispatch
|------ bdrv_co_io_em_complete
|------- qemu_coroutine_enter(co->coroutine, NULL); <=== coroutine entry is bdrv_co_do_rw
|--- g_free(d) <== free first Qcow2DiscardRegion is okay
|--- d = next; <== this set is done in QTAILQ_FOREACH_SAFE() macro.
|--- g_free(d); <== double-free will happen if during previous iteration, bdrv_discard had free this object.
Do you have a reproducer for this or did code review lead you to this?
This problem can be reproduced with loop of savevm -> delvm -> savem ->
delvm ..., about 4 hours.
When I delete the vm snapshot, qemu crashed with a core file,
I debug the core file and find the double-free and the stack.
So I add a breakpoint at g_free(d);, and find that indeed a double-free
happened, twice free with the same address.
And only the first discard region have not happened with double-free.
At the moment I can't see how bdrv_discard(bs->file) could ever free a
Qcow2DiscardRegion of bs, as it's working on a completely different
BlockDriverState (which usually won't even be a qcow2 one).
I think the "aio_context" in bdrv_discard -> aio_poll(aio_context, true)
is the qemu_aio_context,
no matter the bs or bs->file passed to bdrv_discard, so
aio_poll(aio_context) will poll all of the aio.
bdrv_co_do_rw
|- bdrv_co_do_writev
|-- bdrv_co_do_pwritev
|--- bdrv_aligned_pwritev
|---- qcow2_co_writev
|----- qcow2_alloc_cluster_link_l2
|------ qcow2_free_any_clusters
|------- qcow2_free_clusters
|-------- update_refcount
|--------- qcow2_process_discards
|---------- g_free(d) <== In next iteration, this Qcow2DiscardRegion will be double-free.
This shouldn't happen in a nested call either, as s->lock can't be taken
recursively.
Could you detail how s->lock prevent that, above stack is from the gdb,
when I add a breakpoint in g_free(d).
Thanks,
Zhang Haoyu
Kevin
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