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SubjectRe: [PATCH] include/net/genetlink.h: Allow genlmsg_cancel to accept a NULL argument
On Tue, 1 Feb 2011, David Miller wrote:

> 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;

This pattern occurred in eg:

net/netlabel/netlabel_unlabeled.c

in the function netlbl_unlabel_staticlist_gen and in other netlabel code,
as well as in net/wireless/nl80211.c, but with the function nl80211hdr_put
instead of genlmsg_put. I submitted patches for all of these cases, so
that is perhaps why you don't see them. But someone suggested to change
genlmsg_cancel as well, to be as permissive as nlmsg_cancel.

For nlmsg_cancel, there are two occurrences in
net/netfilter/nf_conntrack_netlink.c where nlmsg_cancel is reachable with
the second argument NULL.

For nlmsg_cancel the ability to accept NULL as a second argument comes
from the fact that it only calls nlmsg_trim, which does nothing if NULL is
the second argument. nlmsg_trim is also called by nla_nest_cancel. There
are many calls to nla_nest_cancel with NULL as the second argument in the
directory net/sched, for example in the function gred_dump in
net/sched/sch_gred.c. net/sched also contains a call to nlmsg_trim with
NULL as the second argument, in the function flow_dump, in
net/sched/cls_flow.c.

The whole thing seems somewhat sloppy. I'm sure that all of the
above-cited occurrences could be rewritten as outlined above to skip over
the cancel/trim function.

julia


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