Subject: Re: [PATCH iproute2 -next] ingress, clsact: don't add TCA_OPTIONS to nl msg

From: Jamal Hadi Salim <[email-masked]>
Date: Tue, 17 May 2016 07:54:30 -0400

On 16-05-15 12:36 PM, Daniel Borkmann wrote:
> In ingress and clsact qdisc TCA_OPTIONS are ignored, since it's
> parameterless. In tc, we add an empty addattr_l(... TCA_OPTIONS,
> NULL, 0) to the netlink message nevertheless. This has the
> side effect that when someone tries a 'tc qdisc replace' and
> already an existing such qdisc is present, tc fails with
> EINVAL here.
>
> Reason is that in the kernel, this invokes qdisc_change() when
> such requested qdisc is already present. When TCA_OPTIONS are
> passed to modify parameters, it looks whether qdisc implements
> .change() callback, and if not present (like in both cases here)
> it returns with error. Rather than adding an empty stub to the
> kernel that ignores TCA_OPTIONS again, just don't add TCA_OPTIONS
> to the netlink message in the first place.
>
> Before:
>
>    # tc qdisc replace dev foo clsact    # first try
>    # tc qdisc replace dev foo clsact    # second one
>    RTNETLINK answers: Invalid argument
>
> After:
>
>    # tc qdisc replace dev foo clsact
>    # tc qdisc replace dev foo clsact
>    # tc qdisc replace dev foo clsact
>
> Signed-off-by: Daniel Borkmann <[email-masked]>

Acked-by: Jamal Hadi Salim <[email-masked]>

I see need for correctness but was curious of use case
that made you even look at this ;->

cheers,
jamal