[libvirt] [PATCH] conf: fix starting a domain with cpuset=""

[wang.yi59 at zte.com.cn]

> On Wed, Sep 19, 2018 at 08:11:34AM +0800, wang.yi59 at zte.com.cn wrote:
> > > On Mon, Sep 17, 2018 at 06:53:13PM +0800, wang.yi59 at zte.com.cn wrote:
> > > > > On Sat, Sep 15, 2018 at 04:29:24PM +0800, Yi Wang wrote:
> > > > > > Domain fails to start when its config xml including:
> > > > > >   <vcpu cpuset="" current="8">64</vcpu>
> > > > > >
> > > > > >   # virsh create vm.xml
> > > > > >   error: Failed to create domain from vm.xml
> > > > > >   error: invalid argument: Failed to parse bitmap ''
> > > > > >
> > > > > > This patch fixes this.
> > > > > >
> > > > > > Signed-off-by: Yi Wang <wang.yi59 at zte.com.cn>
> > > > > > Reviewed-by: Xi Xu <xu.xi8 at zte.com.cn>
> > > > > > ---
> > > > > >  src/conf/domain_conf.c | 2 +-
> > > > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > > >
> > > > > > diff --git a/src/conf/domain_conf.c b/src/conf/domain_conf.c
> > > > > > index 8619962..bacafb2 100644
> > > > > > --- a/src/conf/domain_conf.c
> > > > > > +++ b/src/conf/domain_conf.c
> > > > > > @@ -18553,7 +18553,7 @@ virDomainVcpuParse(virDomainDefPtr def,
> > > > > >
> > > > > >          if (def->placement_mode != VIR_DOMAIN_CPU_PLACEMENT_MODE_AUTO) {
> > > > > >              tmp = virXMLPropString(vcpuNode, "cpuset");
> > > > > > -            if (tmp) {
> > > > > > +            if (tmp && strlen(tmp) != 0) {
> > > > >
> > > > > .... '&& *tmp' would suffice.
> > > >
> > > > Ok.
> > > >
> > > > >
> > > > > The patch is correct, but I believe there is a number of spots in the massive
> > > > > domain_conf.c file where a similar fix would be needed, it might be worth
> > > > > checking all the spots where no conversion like string-to-int string-to-enum or
> > > > > any other additional parsing like address parsing is performed, those might be
> > > > > good candidates.
> > > > >
> > > > > I understand the file is massive, so let me know how that goes.
> > > >
> > > > In most similar places we have already checked by the return values of function
> > > > like string-to-int string-to-enum, and if failed, libvirt will give explicit report.
> > > >
> > > > To create a domain with iothread="" of disk, for instance:
> > > >
> > > >   # virsh create vm.xml
> > > >   error: Failed to create domain from vm.xml
> > > >   error: XML error: Invalid iothread attribute in disk driver element:
> > > >
> > > > But the cpuset="" config error report may make people confuesd, and in the older
> > > > version like 3.1.0 that's ok to start a domain with this config. So I think this patch
> > > > is enough.
> > >
> > > So, I thought about this quite a bit. The fact that 3.1.0 was okay with that
> > > was because commit b71946af5c7 changed the helper we use during parsing. The
> > > original one considered NULL and foo[0] == '\0' to be the same and always
> > > returned NULL. However, now that I looked at the code more closely, I don't
> > > think we want this patch, in fact, an empty cpuset is an invalid attribute,
> > > what's the point of having an empty cpuset vs not having it at all, if you don't
> > > want static pinning then don't specify one, there's no practical use case in
> > > that. That being said, I also find the error very generic and vague, so if
> > > anyone wants to actually figure out what the part causing problems in the XML
> > > parsing is, they have no chance, we should improve that. Paradoxically, we had
> > > that back in 2013 until commit 106a2ddaa7b changed it with absolutely no
> > > rationale or a corresponding BZ on why decreasing user experience was a good
> > > idea, I know, rather a rare use case what you have here, but still, why is okay
> > > to parse numbers from string with no error, thus giving a clear error (like your
> > > iothread example), but not okay for bitmaps, CC'ing Peter to give more
> > > information about that change.
> >
> > I have checked the commit b71946af5c7 before sending this patch and came up with two solutions:
> >
> > - make cpuset="" passed the check, like this patch does
> > - add specified error report, rather than complaining about parsing bitmap
> >
> > The first method can keep consistence with the older version, so I chose this one.
>
> Well, in this case being consistent with something which wasn't imho entirely
> correct is not the way to go, the consistency should have been with all the
> other cases where the parsing simply fails because it can't parse an empty
> string into anything meaningful. As I said, I don't see a point in using
> cpuset="" in real world. It would be different if the kind of change commit
> b71946af5c7 introduced made some VMs disappear, but that's not the case here,
> you just happen to use an old XML to create a transient VM, so I'd suggest
> updating the XML you use instead.

Ok, I agree with what you said. However, the error report now is not user-friendly after
all, so should I send a new patch to improve the code, such like:

  if (tmp) {
      if (strlen(tmp) == 0) {
          virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
              _("Invalid value of 'cpuset': %s"), tmp);
          goto cleanup;
      }
  }

Thank you, Erik.

---
Best wishes
Yi Wang