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

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Message-ID: <1462372026.5535.344.camel@edumazet-glaptop3.roam.corp.google.com>
Date:	Wed, 04 May 2016 07:27:06 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	nicolas.dichtel@...nd.com
Cc:	David Miller <davem@...emloft.net>, 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 Wed, 2016-05-04 at 14:49 +0200, Nicolas Dichtel wrote:
> Le 04/05/2016 11:05, Lars Ellenberg a écrit :
> [snip]
> > 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.
> The code adds several consecutive u64 attributes. The nl attribute header is 4
> bytes, thus the full attribute length is 12 bytes. If the first u64 is aligned
> on 8 (nla_data()), the next one is not aligned on 8: it starts 12 bytes (8 (u64)
> + 4 (nl attr hdr)) after the previous u64.

As I mentioned earlier ( https://lkml.org/lkml/2016/4/22/706 ), if both
kernel and user land do not blindly use *(u64 *)ptr to put/get these
values, there was no issue.

kernel was fine, and most user land apps were fine as well.

Only for compound struct  like tcp_info this was nice to get alignment
for performance reason (it removed one memcpy(), or the use of
put_unaligned() helpers)