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Re: [Qemu-devel] Possible regression

From: Eric Blake
Subject: Re: [Qemu-devel] Possible regression
Date: Thu, 23 Mar 2017 10:48:39 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0

On 03/23/2017 10:37 AM, Richard W.M. Jones wrote:
> On Wed, Mar 22, 2017 at 09:45:24AM -0500, Eric Blake wrote:
>> A regression in commit 15c2f669e caused us to silently ignore
>> excess input to the QemuOpts visitor.  Later, commit ea4641
>> accidentally abused that situation, by removing "qom-type" and
>> "id" from the corresponding QDict but leaving them defined in
>> the QemuOpts, when using the pair of containers to create a
>> user-defined object. Note that since we are already traversing
>> two separate items (a QDict and a QemuOpts), we are already
>> able to flag bogus arguments, as in:
>> $ ./x86_64-softmmu/qemu-system-x86_64 -nodefaults -nographic -qmp stdio 
>> -object memory-backend-ram,id=mem1,size=4k,bogus=huh
>> qemu-system-x86_64: -object memory-backend-ram,id=mem1,size=4k,bogus=huh: 
>> Property '.bogus' not found
>> So the only real concern is that when we re-enable strict checking
>> in the QemuOpts visitor, we do not want to start flagging the two
>> leftover keys as unvisited.  Rearrange the code to clean out the
>> QemuOpts listing in advance, rather than removing items from the
>> QDict.  Since "qom-type" is usually an automatic implicit default,
>> we don't have to restore it (this does mean that once instantiated,
>> QemuOpts is not necessarily an accurate representation of the
>> original command line - but this is not the first place to do that);

Well, my assumption that qom-type was not necessary to restore...

> This commit causes a problem for libguestfs:
>   [02192ms] /home/rjones/d/qemu/x86_64-softmmu/qemu-system-x86_64 \
>   [...]
>       -object rng-random,filename=/dev/urandom,id=rng0 \
>       -device virtio-rng-pci,rng=rng0 \
>   [...]
>   qemu-system-x86_64: -object rng-random,filename=/dev/urandom,id=rng0: 
> Parameter 'qom-type' is missing

...was just proven false :(

I should have been more conservative.  I'll post the obvious follow-up
patch that restores qom-type after the temporary removal from the
QemuOpts that is necessary for the duration of the user_creatable_add.

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

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