netdev - Re: [PATCH net-next v2] block/drbd: use nla_put_u64

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Message-ID: <20160504090529.GJ16459@soda.linbit>
Date:	Wed, 4 May 2016 11:05:30 +0200
From:	Lars Ellenberg <lars.ellenberg@...bit.com>
To:	David Miller <davem@...emloft.net>
Cc:	nicolas.dichtel@...nd.com, netdev@...r.kernel.org,
	philipp.reisner@...bit.com, drbd-dev@...ts.linbit.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v2] block/drbd: use nla_put_u64_64bit()

On Tue, May 03, 2016 at 12:05:56PM -0400, David Miller wrote:
> From: Lars Ellenberg <lars.ellenberg@...bit.com>
> Date: Tue, 3 May 2016 12:06:44 +0200
> 
> > Please just NOT use an additional "field",
> > but always use 0 to pad.
> 
> You can't, it doesn't work.

I did, and it *did* work.
At least, it appeared to.

I'm not talking about every user of netlink out there.
That I don't know. But specifically for DRBD netlink,
from what my experiments tell me, it works just fine.

> We are adding a new field to every netlink protocol family that has
> this alignment problem.

We don't have an "alignment problem" there, btw.
Last time I checked, we did work fine without this alignment magic,
we already take care of that, yes, even on affected architectures.

On Tue, May 03, 2016 at 12:06:52PM -0400, David Miller wrote:
> From: Lars Ellenberg <lars.ellenberg@...bit.com>
> Date: Tue, 3 May 2016 12:06:44 +0200
> 
> > Whereas using some arbitrary value will be wrong,
> > and will needlessly break userland.
> 
> It cannot break userland.

It can, if those tags have been used already.
There is DRBD out-of-tree as well,
it usually is ahead of in-tree DRBD.

But yes, I could obviously check and assign and reserve some
not-yet-used tag to all of them.

I don't see why, though, given that 0 (appearently) works fine.

Can you elaborate why and how that does not work?

	Lars