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

[Paul Durrant <xadimgnik@xxxxxxxxx>]

> -----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 
crash.

> >
> > 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.

  Paul