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Re: [Qemu-devel] [PATCH 0/4] Add section footers to detect corrupted mig

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH 0/4] Add section footers to detect corrupted migration streams
Date: Tue, 19 May 2015 07:51:45 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0

On 05/19/2015 05:29 AM, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <address@hidden>
> Badly formatted migration streams can go undetected or produce
> misleading errors due to a lock of checking at the end of sections.
> In particular a section that adds an extra 0x00 at the end
> causes what looks like a normal end of stream and thus doesn't produce
> any errors, and something that ends in a 0x01..0x04 kind of look
> like real section headers and then fail when the section parser tries
> to figure out which section they are.  This is made worse by the
> choice of 0x00..0x04 being small numbers that are particularly common
> in normal section data.
> This patch series adds a section footer consisting of a marker (0x7e - ~)
> followed by the section-id that was also sent in the header.  If
> they mismatch then it throws an error explaining which section was
> being loaded.

Good idea.  Is it redundant with the recent addition of self-describing
json that newer machine types send?  Does it let us detect a corrupted
stream earlier in the process?  Or is the main benefit that it gives
better error messages at the point corruption is first detected?

> The footers are tied to new machine types (on both pc types).

Good that you tied it to machine type, but is it enough?  When we added
the optional section for giving the json representation of the stream,
we ended up having to add a knob to turn off that section, so that
backwards migration from a new qemu to an older one did not send it.
I'm wondering if we'll need to expose a knob to turn off footers, again
for the sake of backwards migration in downstream distros.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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