On Tue, 06 Oct 2020 17:10:44 +0200 Johannes Berg wrote:
> Sorry, hat to run out earlier and forgot to comment here.
>
> On Tue, 2020-10-06 at 14:32 +0200, Johannes Berg wrote:
> >
> > + /* the max policy content is currently ~44 bytes for range min/max */
> > + if (err && nlk_has_extack && extack && extack->policy)
> > + tlvlen += 64;
>
> So I'm not really happy with this. I counted 44 bytes content (so 48
> bytes for the nested attribute) for the biggest that we have now, but if
> we ever implement e.g. dumping out the reject string for NLA_REJECT
> (though not sure anyone even uses that?) then it'd be more variable.
I wonder if we should in fact dump the reject string, in this case it
feels like an omission not to have it... although as you say, grep for
reject_message reveals it's completely unused today.
> I couldn't really come up with any better idea, but I believe we do need
> to size the skb fairly well to return the original one ...
>
> The only solution I _could_ think of was to allocate another skb, put
> the attribute into it, check the length, and then later append it to the
> message ... but that seemed kinda ugly.
>
> Any preferences?
It'd feel pretty idiomatic for (rt)netlink to have
netlink_policy_dump_attr_size()
which would calculate the size. That'd cost us probably ~100 LoC?
If that's too much we could at least add a define for this constant,
and WARN_ON_ONCE() in __netlink_policy_dump_write_attr() if the dump
ends up larger?
