Re: [Qemu-block] [Qemu-devel] qcow2 not cleaning up during image create failure

[Daniel P. Berrange]

On Fri, Jul 14, 2017 at 07:57:43PM +0800, Fam Zheng wrote:
> On Fri, 07/14 12:00, Daniel P. Berrange wrote:
> > I've just been looking at the qcow2 image creation code, and found that
> > if any method in qcow2_create2() returns an error, then we'll report that,
> > but leave the newly created image file on disk in some partially initialized
> > state. A user may unwittingly use this file later with undefined behaviour.
> > This is particularly bad if we fail to setup encryption, because the user
> > is left with a file with no encryption enabled.
> > 
> > So I'm wondering how is the best way to clean up after failure ?
> > 
> > Naively I would like to just unlink(filename), but IIUC, filename is
> > not guaranteed to refer to a local file, and AFAIK, there is not
> > bdrv_delete() method todo this portably.
> > 
> > If we can't delete a file (because its a block device or network
> > volume), then we must at least blank out the just-written qcow2
> > header with zeros.
> > 
> > Ideas / suggestions.
> 
> Or just write the header as the last step?

qcow2 has a multi-step creation process - it writes a minimal header,
then opens the file and writes some more metadata.  So delaying write
of the header is impractical with that approach.

Regards,
Daniel
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