> -----Original Message----- > From: Julien Grall <jul...@xen.org> > Sent: 24 April 2020 17:04 > To: Jürgen Groß <jgr...@suse.com>; Paul Durrant <p...@xen.org>; > xen-devel@lists.xenproject.org > Cc: Paul Durrant <pdurr...@amazon.com> > 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
