Re: [PATCH net-next v2] block/drbd: use nla_put_u64_64bit()

From: Eric Dumazet
Date: Wed May 04 2016 - 10:27:16 EST

On Wed, 2016-05-04 at 14:49 +0200, Nicolas Dichtel wrote:
> Le 04/05/2016 11:05, Lars Ellenberg a Ãcrit :
> [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 ( ), 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)