RE: [PATCH net-next 0/7] Allow to monitor multicast cache event via rtnetlink

On 12/06/12 at 09:43am, Nicolas Dichtel wrote:

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Le 05/12/2012 18:54, David Miller a écrit :

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From: "David Laight" <redacted>
Date: Wed, 5 Dec 2012 11:41:33 -0000

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Probably worth commenting that the 64bit items might only be 32bit aligned.
Just to stop anyone trying to read/write them with pointer casts.

Rather, let's not create this situation at all.

It's totally inappropriate to have special code to handle every single
time we want to put 64-bit values into netlink messages.

We need a real solution to this issue.

The easiest way is to update *_ALIGNTO values (maybe we can keep
NLMSG_ALIGNTO to 4). But I think that many userland apps have these
values hardcoded and, the most important thing, this may increase
size of many netlink messages. Hence we need probably to find
something better.

We can't do this, as you say, ALIGNTO is compiled into all the
binaries.

A simple backwards compatible workaround would be to include an
unknown, empty padding attribute if needed. That would be 4 bytes
in size and could be used to include padding as needed.

What are you going to align the data with respect to?
I doubt you can assume that the start of the netlink
message itself is 8 byte aligned - so any attempt
to 8 byte align an item is probably doomed to failure.

	David