netdev - genetlink misinterprets NEW as GET

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Date:	Tue, 4 Jan 2011 03:14:05 +0100 (CET)
From:	Jan Engelhardt <jengelh@...ozas.de>
To:	Netfilter Developer Mailing List <netfilter-devel@...r.kernel.org>
cc:	Linux Networking Developer Mailing List <netdev@...r.kernel.org>
Subject: genetlink misinterprets NEW as GET

Hey there,


I can't really say whether it's genetlink or netlink to blame,
but I noticed that a request with

	nlmsg_flags = NLM_F_CREATE | NLM_F_EXCL

to a genl-registered component can return -EOPNOTSUPP because it does 
not have a dumpit function defined in struct genl_ops. Make sense?
Not at first sight at least.
include/linux/netlink.h has this nice anecdote:

	/* Modifiers to GET request */
	#define NLM_F_ROOT      0x100
	#define NLM_F_MATCH     0x200
	#define NLM_F_ATOMIC    0x400
	#define NLM_F_DUMP      (NLM_F_ROOT|NLM_F_MATCH)
	
	/* Modifiers to NEW request */
	#define NLM_F_REPLACE   0x100
	#define NLM_F_EXCL      0x200
	#define NLM_F_CREATE    0x400
	#define NLM_F_APPEND    0x800

Except there is nothing that declares a particular Netlink message
as "GET" or "NEW". Subsequently, genetlink chokes:

	if (nlh->nlmsg_flags & NLM_F_DUMP)
		if (ops->dumpit == NULL)
			return -EOPNOTSUPP;

Because NLM_F_CREATE | NLM_F_EXCL == NLM_F_DUMP.
That, of course, is absolutely bogus.

[N.B.: I am also wondering whether
	(nlh->nlmsg_flags & NLM_F_DUMP) == NLM_F_DUMP
may have been desired, because NLM_F_DUMP is composed of two bits.]
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