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

Le 06/12/2012 18:49, Thomas Graf a écrit :

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

quoted

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

quoted

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

quoted

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.

We could use nla_type = 0 as it is a reserved value that should
be available in all protocols. All readers (kernel and user space)
must ignore such an attribute just like any other unknown
attribute they encounter.

We could easily extend nla_put_u64() and variants to automatically
include such a padding attribute as needed.

In fact, it seems not so easy because most users of nlmsg_new() calculate
the exact needed length, thus if we add an unpredicted attribute, the message 
will be too small.