Re: [Qemu-devel] [PATCH] monitor: fix object_del for command-line-created objects

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Daniel P. Berrange (berrange@redhat.com) wrote:
> On Wed, Nov 30, 2016 at 05:06:16PM -0600, Michael Roth wrote:
> > Currently objects specified on the command-line are only partially
> > cleaned up when 'object_del' is issued in either HMP or QMP: the
> > object itself is fully finalized, but the QemuOpts are not removed.
> > This results in the following behavior:
> > 
> >   x86_64-softmmu/qemu-system-x86_64 -monitor stdio \
> >     -object memory-backend-ram,id=ram1,size=256M
> > 
> >   QEMU 2.7.91 monitor - type 'help' for more information
> >   (qemu) object_del ram1
> >   (qemu) object_del ram1
> >   object 'ram1' not found
> >   (qemu) object_add memory-backend-ram,id=ram1,size=256M
> >   Duplicate ID 'ram1' for object
> >   Try "help object_add" for more information
> > 
> > which can be an issue for use-cases like memory hotplug.
> > 
> > This happens on the HMP side because hmp_object_add() attempts to
> > create a temporary QemuOpts entry with ID 'ram1', which ends up
> > conflicting with the command-line-created entry, since it was never
> > cleaned up during the previous hmp_object_del() call.
> > 
> > We address this by adding checks in {qmp,hmp}_object_del to determine
> > whether an option group entry matching the object's ID is present,
> > and removing it if it is.
> > 
> > Note that qmp_object_add() never attempts to create a temporary
> > QemuOpts entry, so it does not encounter the duplicate ID error,
> > which is why this isn't generally visible in libvirt.
> > 
> > Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > Cc: Markus Armbruster <armbru@redhat.com>
> > Cc: Eric Blake <eblake@redhat.com>
> > Cc: Daniel Berrange <berrange@redhat.com>
> > Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
> > ---
> >  hmp.c | 10 ++++++++++
> >  qmp.c | 10 ++++++++++
> >  2 files changed, 20 insertions(+)
> > 
> > diff --git a/hmp.c b/hmp.c
> > index b869617..99b8bf3 100644
> > --- a/hmp.c
> > +++ b/hmp.c
> > @@ -2072,6 +2072,16 @@ void hmp_object_del(Monitor *mon, const QDict *qdict)
> >  
> >      user_creatable_del(id, &err);
> >      hmp_handle_error(mon, &err);
> > +
> > +    /* if object was defined on the command-line, remove its corresponding
> > +     * option group entry
> > +     */
> > +    if (err == NULL) {
> > +        QemuOptsList *opt_group = qemu_find_opts_err("object", &err);
> > +        if (opt_group) {
> > +            qemu_opts_del(qemu_opts_find(opt_group, id));
> > +        }
> > +    }
> >  }
> >  
> >  void hmp_info_memdev(Monitor *mon, const QDict *qdict)
> > diff --git a/qmp.c b/qmp.c
> > index 0028f0b..87c545d 100644
> > --- a/qmp.c
> > +++ b/qmp.c
> > @@ -686,6 +686,16 @@ void qmp_object_add(const char *type, const char *id,
> >  void qmp_object_del(const char *id, Error **errp)
> >  {
> >      user_creatable_del(id, errp);
> > +
> > +    /* if object was defined on the command-line, remove its corresponding
> > +     * option group entry
> > +     */
> > +    if (!(errp && *errp)) {
> > +        QemuOptsList *opt_group = qemu_find_opts_err("object", errp);
> > +        if (opt_group) {
> > +            qemu_opts_del(qemu_opts_find(opt_group, id));
> > +        }
> > +    }
> >  }
> 
> I rather feel like this code ought to be in user_creatable_del(), since
> all code paths which call user_creatable_del() need to do deal with
> this scenario.

Yes that seems cleaner;  the cleanup that's being done is not the result
of either of the monitor's actions.

Dave

> Also I'd like to see a unit test added that exposes this problem and
> demonstrates the fix - could att it to tests/check-qom-proplist.c
> 
> Regards,
> Daniel
> -- 
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--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK