netdev - Re: [PATCH] genetlink: fix usage of NLM_F_EXCL or NLM_F

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Date:	Tue, 30 Jul 2013 16:44:23 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	pablo@...filter.org
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH] genetlink: fix usage of NLM_F_EXCL or NLM_F_REPLACE

From: Pablo Neira Ayuso <pablo@...filter.org>
Date: Mon, 29 Jul 2013 12:30:04 +0200

> Currently, it is not possible to use neither NLM_F_EXCL nor
> NLM_F_REPLACE from genetlink. This is due to this checking in
> genl_family_rcv_msg:
> 
> 	if (nlh->nlmsg_flags & NLM_F_DUMP)
> 
> NLM_F_DUMP is NLM_F_MATCH|NLM_F_ROOT. Thus, if NLM_F_EXCL or
> NLM_F_REPLACE flag is set, genetlink believes that you're
> requesting a dump and it calls the .dumpit callback.
> 
> The solution that I propose is to refine this checking to
> make it stricter:
> 
> 	if ((nlh->nlmsg_flags & NLM_F_DUMP) == NLM_F_DUMP)
> 
> And given the combination NLM_F_REPLACE and NLM_F_EXCL does
> not make sense to me, it removes the ambiguity.
> 
> There was a patch that tried to fix this some time ago (0ab03c2
> netlink: test for all flags of the NLM_F_DUMP composite) but it
> tried to resolve this ambiguity in *all* existing netlink subsystems,
> not only genetlink. That patch was reverted since it broke iproute2,
> which is using NLM_F_ROOT to request the dump of the routing cache.
> 
> Signed-off-by: Pablo Neira Ayuso <pablo@...filter.org>

Yes, I remember that old attempt to fix this.

Ok, let's see what happens when we limit the scope of this change
to just genetlink users.

I honestly can't believe that NLM_F_EXCL and NLM_F_REPLACE are
completely unusable in normal rtnetlink interfaces.
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