netdev - Re: [PATCH] bridge: extend BR_ISOLATE to full split-horizon

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Date:   Fri, 10 Dec 2021 17:49:28 +0100
From:   David Lamparter <equinox@...c24.net>
To:     Nikolay Aleksandrov <nikolay@...dia.com>,
        Alexandra Winter <wintera@...ux.ibm.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        David Lamparter <equinox@...c24.net>, netdev@...r.kernel.org
Subject: Re: [PATCH] bridge: extend BR_ISOLATE to full split-horizon

Thanks all for the feedback!
(rolled up several mails below)

On Thu, Dec 09, 2021 at 07:42:04AM -0800, Jakub Kicinski wrote:
> Does not apply to net-next, you'll need to repost even if the code is
> good. Please put [PATCH net-next] in the subject.

Apologies, I think I skipped a step in my rebase somewhere.

On Thu, Dec 09, 2021 at 06:08:03PM +0200, Nikolay Aleksandrov wrote:
> - please drop the sysfs part, we're not extending sysfs anymore

ACK (will remove the "horizon_group" file, change to "isolated" remains)

> - split the bridge change from the driver

ACK - I wasn't sure if it's OK if the intermediate doesn't compile due
to deleted BR_ISOLATED?  Or should I make 3 patches with only the 3rd
removing the flag definition?

> - drop the /* BR_ISOLATED - previously BIT(16) */ comment

Should I move up the other bits below it or just leave a hole at 16?

> - [IFLA_BRPORT_HORIZON_GROUP] = NLA_POLICY_MIN(NLA_S32, 0), why not just { .type = NLA_U32 } ?
>
> Why the limitation (UAPI limited to positive signed int. (recommended ifindex namespace)) ?
> You have the full unsigned space available, user-space can use it as it sees fit.
> You can just remove the comment about recommended ifindex.

No problem, I guess I'm overthinking this a bit.  Using the ifindex is
just a "good way" of avoiding confusion if multiple things in userspace
try to configure this.  Kernel really doesn't care.

> Also please extend the port isolation self-test with a test for a different horizon group.

ACK

> - just forbid having both set (tb[IFLA_BRPORT_ISOLATED] && tb[IFLA_BRPORT_HORIZON_GROUP])
>   user-space should use just one of the two, if isolated is set then it overwrites any older
>   IFLA_BRPORT_HORIZON_GROUP settings, that should simplify things considerably

So a bit of a hidden question here is which way "BR_ISOLATED" maps -
both "ISOLATED :=: group >= 1" and "ISOLATED :=: group == 1" are
possible.  But regardless of which way it's defined, there's to some
degree a risk of "old" tools accidentally wrecking the horizon group
setting.  That's why I ended up making BR_ISOLATE=1 not change the
horizon group if it's nonzero already...

On Thu, Dec 09, 2021 at 05:23:35PM +0100, Alexandra Winter wrote:
> Yes, please keep it compatible with userspace setting IFLA_BRPORT_ISOLATED only.

On Thu, Dec 09, 2021 at 06:23:25PM +0200, Nikolay Aleksandrov wrote:
> Actually they'll have to be exported together always... that's unfortunate. I get
> why you need the extra netlink logic, I think we'll have to keep it.

Yeah - we can't remove BR_ISOLATED from netlink GETs to keep compat, so
we need to accept it in SETs too in order to not break userspace handing
it right back in 1:1 for a get(-modify)-set...

On Thu, Dec 09, 2021 at 06:57:23PM +0200, Nikolay Aleksandrov wrote:
> So one relatively simple way to drop most of the logic is to have BRPORT_HORIZON_GROUP's
> attribute get set after ISOLATED so it always overwrites it, which is similar to the current
> situation but if you set only ISOLATED later it will function as intended (i.e. drop the logic
> for horizon_group when using the old attr). Still will have to disallow ISOLATED = 0/HORIZON_GROUP >0
> and ISOLATED=1/HORIZON_GROUP=0 though as these don't make sense.
>
> e.g.:
> if (tb[IFLA_BRPORT_ISOLATED])
> 	p->horizon_group = !!nla_get_u8(tb[IFLA_BRPORT_ISOLATED]);
> if (tb[IFLA_BRPORT_HORIZON_GROUP])
> 	p->horizon_group = nla_get_u32(tb[IFLA_BRPORT_HORIZON_GROUP]);
>
> This will be also in line with how they're exported (ISOLATED = 1 and HORIZON_GROUP >0).

This boils down to the same logic as is currently in the patch, except
it's written the other way around and with an else, i.e.

if (tb[IFLA_BRPORT_HORIZON_GROUP])
	p->horizon_group = ...
else if (tb[IFLA_BRPORT_ISOLATED])
	p->horizon_group = ...

With the else it seems more obvious to me, to avoid someone in the
future missing the fact that the 2 settings interact - so I would
preferably keep it like this.

Cheers,

-David