[ovs-dev] [PATCH 5/5] [RFC] ofp-actions: Check pre-requisists of set-field actions

[Ben Pfaff]

On Thu, Sep 13, 2012 at 12:01:44PM +0900, Simon Horman wrote:
> On Wed, Sep 12, 2012 at 12:09:41PM -0700, Ben Pfaff wrote:
> > On Wed, Sep 12, 2012 at 05:44:32PM +0900, Simon Horman wrote:
> > > By passing a flow to the action parser the pre-requisites
> > > of set-feild actions will be checked. If the flow is NULL,
> > > for instance in test code such as ofctl_parse_ofp11_instructions(),
> > > then the check is skiped, as it always was before this change.
> > > 
> > > Unfortunately I don't think that this check is correct as
> > > it does not take into account other actions actions that may
> > > be applied before the load action, e.g. vlan push/pop, which may
> > > affect the pre-requisites.
> > > 
> > > I believe that in its current form there is scope for both false positives
> > > (not so bad, perhaps) and false negatives (pretty bad). I would welcome
> > > some input on if these concerns are valid and if so how they may be
> > > overcome.
> > 
> > When I've thought about this problem before, I wasn't able to come up
> > with an example of when the prerequisite check would be wrong.  Do you
> > have an example?
> 
> I would very much like for this not to be a problem.
> But here are examples that I am concerned about.
> 
> 
> # MPLS false negative
> 
> 1. Packet with no MPLS header arrives
> 2. Push MPLS action is applied
> 3. Set-Field:MPLS_LABEL action is applied
> 
> The pre-requisite for 3 is Eth Type 0x8847 or 0x8848,
> but that isn't true until 2 is applied.

I agree that this problem will need to be solved one way or another,
but I don't see how this change helps.  I think that it is redundant,
too, because the same check will be done later in ofpacts_check(),
when the flow is always available.  (That is why ofpacts_check()
exists, since a flow is not always available at parse time.)

Also, this does not solve the whole problem anyway.  When action #2 is
parsed, it does not change the flow, so the check in #3 will still
fail.  (Maybe that is what you meant above when you wrote,
"Unfortunately I don't think this check is correct."  I thought when I
read it earlier that you meant the previously missing check.)