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Date:   Tue, 14 Nov 2017 20:10:36 +0100
From:   Stefano Brivio <sbrivio@...hat.com>
To:     Girish Moodalbail <girish.moodalbail@...cle.com>
Cc:     Nicolas Dichtel <nicolas.dichtel@...nd.com>, davem@...emloft.net,
        netdev@...r.kernel.org, Matteo Croce <mcroce@...hat.com>,
        Erik Kline <ek@...gle.com>
Subject: Re: [PATCH net v2] ipv6: set all.accept_dad to 0 by default

On Tue, 14 Nov 2017 10:30:33 -0800
Girish Moodalbail <girish.moodalbail@...cle.com> wrote:

> On 11/14/17 5:21 AM, Nicolas Dichtel wrote:
> > With commits 35e015e1f577 and a2d3f3e33853, the global 'accept_dad' flag
> > is also taken into account (default value is 1). If either global or
> > per-interface flag is non-zero, DAD will be enabled on a given interface.
> > 
> > This is not backward compatible: before those patches, the user could
> > disable DAD just by setting the per-interface flag to 0. Now, the
> > user instead needs to set both flags to 0 to actually disable DAD.
> > 
> > Restore the previous behaviour by setting the default for the global
> > 'accept_dad' flag to 0. This way, DAD is still enabled by default,
> > as per-interface flags are set to 1 on device creation, but setting
> > them to 0 is enough to disable DAD on a given interface.
> > 
> > - Before 35e015e1f57a7 and a2d3f3e33853:
> >            global    per-interface    DAD enabled
> > [default]   1             1              yes
> >              X             0              no
> >              X             1              yes
> > 
> > - After 35e015e1f577 and a2d3f3e33853:
> >            global    per-interface    DAD enabled
> > [default]   1             1              yes
> >              0             0              no
> >              0             1              yes
> >              1             0              yes
> > 
> > - After this fix:
> >            global    per-interface    DAD enabled
> >              1             1              yes
> >              0             0              no
> > [default]   0             1              yes
> >              1             0              yes  
> 
> Above table can be summarized to..
> 
> - After this fix:
>            global    per-interface    DAD enabled
>              1             X              yes
>              0             0              no
> [default]   0             1              yes
> 
> So, if global is set to '1', then irrespective of what the per-interface value 
> is DAD will be enabled. Is it not confusing. Shouldn't the more specific value 
> override the general value?

Might be a bit confusing, yes, but in order to implement an overriding
mechanism you would need to implement a tristate option as Eric K.
proposed. That is, by default you would have -1 (meaning "don't care")
on per-interface flags, and if this value is changed then the
per-interface value wins over the global one.

Sensible, but I think it's outside of the scope of this patch, which is
just intended to restore a specific pre-existing userspace expectation.

> On the other hand, if the global is set to '0', then per-interface value will be 
> honored (overrides global). So, the meaning of global varies based on its value. 
> Isn't that confusing as well.

I don't find this confusing though. Setting the global flag always has
the meaning of "force enabling DAD on all interfaces".

You would have the same problem if you chose a logical AND between
global and per-interface flag. There, setting the global flag would mean
"force disabling DAD on all interfaces".

So the only indisputable improvement I see here would be to implement a
"don't care" value (either for global or for per-interface flags). But
I'd rather agree with Nicolas that we should fix a potentially broken
userspace assumption first.

-- 
Stefano

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