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RE: [PATCH] docs/designs: re-work the xenstore migration document...

> -----Original Message-----
> From: Julien Grall <julien@xxxxxxx>
> Sent: 24 April 2020 17:04
> To: Jürgen Groß <jgross@xxxxxxxx>; Paul Durrant <paul@xxxxxxx>; 
> xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: Paul Durrant <pdurrant@xxxxxxxxxx>
> Subject: Re: [PATCH] docs/designs: re-work the xenstore migration document...
> Hi,
> On 24/04/2020 16:59, Jürgen Groß wrote:
> > On 24.04.20 17:44, Julien Grall wrote:
> > If I extend the record and do a downgrade I'm losing the information,
> > too, as the old version won't read it in any case. BTW, extending the
> > record is no problem, as the length is stored in the header. Unknown
> > records (and extensions) will be just ignored when reading.
> That's very much up to the implementation. An implementation may decide
> to bail out if the record is not an exact size.

It won't know. The record will be whatever size it says it is, and if the 
format doesn't match what the implementation was expecting then it'll probably 

> >
> > In your case when reusing the paddings and doing a downgrade you would
> > crash, as the paddings are no longer zero.
> >
> > In case a downgrade should not be done due to vital information loss
> > then you should just not do it.
> Of course, however I don't think a user will necessarily know it should
> not do it. So how do you protect against misuse?

The stream is versioned. If information is vital then I'd expect the version to 
be bumped, which should prevent a downgrade.




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