From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Miller <davem@davemloft.net>
Subject: Re: [PATCH] include/net/genetlink.h: Allow genlmsg_cancel to
 accept a NULL argument
Date: Tue, 01 Feb 2011 14:54:10 -0800 (PST)
Message-ID: <20110201.145410.115936566.davem@davemloft.net>
References: <Pine.LNX.4.64.1101281642080.8546@pc-004.diku.dk>
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Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
	paul.moore@hp.com, kernel-janitors@vger.kernel.org
To: julia@diku.dk
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From: Julia Lawall <julia@diku.dk>
Date: Fri, 28 Jan 2011 16:43:40 +0100 (CET)

> nlmsg_cancel can accept NULL as its second argument, so for similarity,
> this patch extends genlmsg_cancel to be able to accept a NULL second
> argument as well.
> 
> Signed-off-by: Julia Lawall <julia@diku.dk>

I did a scan of all of the cases where this interface is used, and
I cannot find a situation where this capability would even be useful.

The use pattern is always:

	hdr = genlmsg_put(skb, ...);
	if (!hdr)
		goto out;

	NLA_PUT_*();
	NLA_PUT_*();
	....

	return genlmsg_end(skb, hdr);

nla_put_failure:
	genlmsg_cancel(skb, hdr);
out:
	return -EWHATEVER;

Always, hdr will be non-NULL.

We have to allocate the header first, then put the netlink
attributes.

Looking over users of nlmsg_cancel(), the situation seems to
match identically.

Therefore, it seems to me that it makes more sense to remove
the NULL check from nlmsg_cancel() than to add the NULL check
to genlmsg_cancel().

Thanks.