netdev - Re: [PATCH] IPROUTE: correct nla nested message generated by netem_parse

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Date:	Wed, 27 Aug 2008 16:41:22 +0200
From:	Thomas Graf <tgraf@...g.ch>
To:	Alexander Duyck <alexander.duyck@...il.com>
Cc:	Stephen Hemminger <shemminger@...tta.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	davem@...emloft.net, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	Alexander Duyck <alexander.h.duyck@...el.com>
Subject: Re: [PATCH] IPROUTE: correct nla nested message generated by netem_parse_opt

* Alexander Duyck <alexander.duyck@...il.com> 2008-08-21 19:37
> The problem is the current kernel ABI was changed in commit
> b9a2f2e450b0f770bb4347ae8d48eb2dea701e24 "netlink: Fix
> nla_parse_nested_compat() to call nla_parse() directly" to support a
> format that I have only seen generated in netem_parse_opt.  The kernel
> and prio_parse_opt qdisc both generate the other format which includes
> the extra attribute header to contain the nested attributes.  This
> patch fixes tc so that netem will use the correct nested attribute
> order for kernels prior to this commit and once this commit has been
> reverted.

How was it ever supposed to work? The code looked like the following
prior to commit b03f4672007e533c8dbf0965f995182586216bf1 which made
it used nla_parse_nested_compat()

-       /* Handle nested options after initial queue options.
-        * Should have put all options in nested format but too late now.
-        */
-       if (nla_len(opt) > sizeof(*qopt)) {
-               struct nlattr *tb[TCA_NETEM_MAX + 1];
-               if (nla_parse(tb, TCA_NETEM_MAX,
-                             nla_data(opt) + sizeof(*qopt),
-                             nla_len(opt) - sizeof(*qopt), NULL))

nla_parse_nested_compat() now does exactly what the above code does. So
in what way has the kernel ABI changed?

There is two ways of sending a fixed struct + attributes inside an
attribute:

a) (old and outdated method)
  attr foo
    fixed struct
    [nested attr 1]
    [nested attr 2]

This format can be parsed with nla_parse_nested_compat(attr foo)

b) (new method)
  attr foo
    fixed struct
    attr container
      [nested attr 1]
      [nested attr 2]

This format is parsed with nla_parse_nested(attr container)
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