Re: Let's remove some deprecated stuff

[Eric Blake <eblake@redhat.com>]

On 4/29/21 4:59 AM, Markus Armbruster wrote:
> If you're cc'ed, you added a section to docs/system/deprecated.rst that
> is old enough to permit removal.  This is *not* a demand to remove, it's
> a polite request to consider whether the time for removal has come.
> Extra points for telling us in a reply.  "We should remove, but I can't
> do it myself right now" is a valid answer.  Let's review the file:
> 

[adjusting cc for this response]

> 
> Eric Blake:
> 
>     qemu-img amend to adjust backing file (since 5.1)
>     '''''''''''''''''''''''''''''''''''''''''''''''''
> 
>     The use of ``qemu-img amend`` to modify the name or format of a qcow2
>     backing image is deprecated; this functionality was never fully
>     documented or tested, and interferes with other amend operations that
>     need access to the original backing image (such as deciding whether a
>     v3 zero cluster may be left unallocated when converting to a v2
>     image).  Rather, any changes to the backing chain should be performed
>     with ``qemu-img rebase -u`` either before or after the remaining
>     changes being performed by amend, as appropriate.
> 
>     qemu-img backing file without format (since 5.1)
>     ''''''''''''''''''''''''''''''''''''''''''''''''
> 
>     The use of ``qemu-img create``, ``qemu-img rebase``, or ``qemu-img
>     convert`` to create or modify an image that depends on a backing file
>     now recommends that an explicit backing format be provided.  This is
>     for safety: if QEMU probes a different format than what you thought,
>     the data presented to the guest will be corrupt; similarly, presenting
>     a raw image to a guest allows a potential security exploit if a future
>     probe sees a non-raw image based on guest writes.
> 
>     To avoid the warning message, or even future refusal to create an
>     unsafe image, you must pass ``-o backing_fmt=`` (or the shorthand
>     ``-F`` during create) to specify the intended backing format.  You may
>     use ``qemu-img rebase -u`` to retroactively add a backing format to an
>     existing image.  However, be aware that there are already potential
>     security risks to blindly using ``qemu-img info`` to probe the format
>     of an untrusted backing image, when deciding what format to add into
>     an existing image.

I'm not sure how widely used these were, but I'm game for writing a
patch to drop them.  I'm fairly certain libvirt is not using them.

> 
> Kevin Wolf:
> 
>     ``nbd-server-add`` and ``nbd-server-remove`` (since 5.2)
>     ''''''''''''''''''''''''''''''''''''''''''''''''''''''''
> 
>     Use the more generic commands ``block-export-add`` and ``block-export-del``
>     instead.  As part of this deprecation, where ``nbd-server-add`` used a
>     single ``bitmap``, the new ``block-export-add`` uses a list of ``bitmaps``.

Peter, is libvirt good for this one to go?

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org