netdev - Re: [PATCH v2 net-next 3/5] l2tp: remove l2specific_len dependency in l2tp

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Date:   Tue, 16 Jan 2018 10:40:06 +0100
From:   Guillaume Nault <g.nault@...halink.fr>
To:     Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
Cc:     "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        James Chapman <jchapman@...alix.com>
Subject: Re: [PATCH v2 net-next 3/5] l2tp: remove l2specific_len dependency
 in l2tp_core

On Mon, Jan 15, 2018 at 10:11:04PM +0100, Lorenzo Bianconi wrote:
> > On Mon, Jan 15, 2018 at 07:43:18PM +0100, Lorenzo Bianconi wrote:
> >> > On Sun, Jan 14, 2018 at 03:50:56PM +0100, Lorenzo Bianconi wrote:
> >> >> --- a/net/l2tp/l2tp_core.h
> >> >> +++ b/net/l2tp/l2tp_core.h
> >> >> @@ -302,6 +302,17 @@ static inline void l2tp_session_dec_refcount(struct l2tp_session *session)
> >> >>               l2tp_session_free(session);
> >> >>  }
> >> >>
> >> >> +static inline int l2tp_get_l2specific_len(struct l2tp_session *session)
> >> >> +{
> >> >> +     switch (session->l2specific_type) {
> >> >> +     case L2TP_L2SPECTYPE_NONE:
> >> >> +             return 0;
> >> >> +     case L2TP_L2SPECTYPE_DEFAULT:
> >> >> +     default:
> >> >> +             return 4;
> >> >> +     }
> >> >> +}
> >> >>
> >> > The data path only compares ->l2specific_type to L2SPECTYPE_DEFAULT and
> >> > treats any other value as L2SPECTYPE_NONE. Therefore, we should keep
> >> > this logic here and return 0 for unknown types.
> >>
> >> The data path only compares l2specific_type to L2SPECTYPE_DEFAULT
> >> since in the other supported case (L2SPECTYPE_NONE) there is no action
> >> required. Moreover L2SPECTYPE_DEFAULT is default configured value if
> >> the user does not provide any value for l2specific_type so there are
> >> no 'unknown' types and I thought L2TP_L2SPECTYPE_DEFAULT was a better
> >> choice for default value
> >>
> > Yes, but what I meant is that the data patch treats unknow values as
> > L2SPECTYPE_NONE, while l2tp_get_l2specific_len() now treats them as
> > L2TP_L2SPECTYPE_DEFAULT. I'd just prefer to avoid that inconsistency;
> > it makes it easier to reason about the code.
> >
> > But if you really prefer L2SPECTYPE_DEFAULT, then fine. Unless someone
> > messes with new l2spec types, we should never reach this case.
> 
> Yes, there are now way to hit just default case so there are no
> difference to use L2SPECTYPE_DEFAULT or L2SPECTYPE_NONE as default
> actually.
> Moreover from now on there are no 'unknow' values for l2specific_type.
> 
Everything that isn't L2SPECTYPE_NONE or L2SPECTYPE_DEFAULT is unknown.
The datapath reads them as L2SPECTYPE_NONE and
l2tp_get_l2specific_len() as L2SPECTYPE_DEFAULT. That's inconsistent
and one has to read the rest of the code to figure out that, in fact,
that's not an issue because such values actually can't be configured.
Using "session->l2specific_type == L2TP_L2SPECTYPE_DEFAULT ? 4 : 0"
would have been enough IMO.

> Anyway if you think that change is important I can respin a v3, no
> issue from my side.
> 
It's certainly not worth a v3 on its own. I just give you my
preference, but the patchset is yours and I can live with either
version.