Re: [Qemu-devel] KVM Forum block no[td]es

[Alberto Garcia]

On Fri 16 Nov 2018 04:03:12 PM CET, Alberto Garcia wrote:
>> top (T) -> intermediate (I) -> base (B)
>>
>> Now you commit from top to base.  Clearly you don't want the backing
>> chain between top and base to change.  So say you unshare the GRAPH_MOD
>> permission (implying that whenever a parent doesn't care, it just shares
>> it).  But as said above, if someone just drops the backing link from I
>> to B, the permission system won't catch that.
>
> Yes it will, because the block job (either mirror or commit depending on
> the case) is the parent of all three nodes (BlockJob.nodes is where the
> list is stored) and does not alow GRAPH_MOD in any of them[*]. So
> dropping that backing link fails (I also have a test case for that).

I forgot to add the footnote [*] in my previous e-mail:

The mirror block job (in particular when in "commit-active" mode, but
also in other cases) creates a "mirror_top" node and then blocks the
target and all intermediate nodes by calling block_job_add_bdrv() in
each one of them:

   mirror_top (M) -> top (T) -> intermediate (I) -> base (B)

The problem is that those intermediate nodes start from top's
backing_bs(), so top itself is not added to the block job's list of
children and doesn't have its permissions restricted during the job's
lifetime.

This looks like a bug, I'll see if there's a way to reproduce and I'll
prepare a patch.

Berto