netdev - Re: [Linux-zigbee-devel] [PATCH net-next 4/5] ieee802154: don't ignore "to" argument in unbound dgram sendmsg

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Date:	Tue, 13 May 2014 17:13:40 -0300
From:	Werner Almesberger <werner@...esberger.net>
To:	Phoebe Buckheister <phoebe.buckheister@...m.fraunhofer.de>
Cc:	netdev@...r.kernel.org, davem@...emloft.net,
	linux-zigbee-devel@...ts.sourceforge.net
Subject: Re: [Linux-zigbee-devel] [PATCH net-next 4/5] ieee802154: don't
 ignore "to" argument in unbound dgram sendmsg

Phoebe Buckheister wrote:
> For some reason, unconnected 802.15.4 dgram sockets ignore the destination
> argument of sendmsg(), while bound sockets use it. Instead, send packets
> to the destination given by the user, and default to the connected
> destination only if no explicit target is given.

We discussed this a bit on IRC. Here's what I think happened and what
happens with your patch. Before:

conn'ed	msg_nam	action
-------	-------	------------------------------------------
no	no	send to initialization value (= broadcast)
no	yes	idem
yes	no	send to connected address
yes	yes	idem

After your patch:

conn'ed	msg_nam	action
-------	-------	------------------------------------------
no	no	send to initialization value (= broadcast)
no	yes	send to msg_name
yes	no	send to connected address
yes	yes	send to msg_name

This is certainly more correct. Blurting out a broadcast in the
no/no case isn't nice, though. It would be better to return either
ENOTCONN (POSIX [1]) or (even better, according to GNU libc [2])
EDESTADDRREQ.

POSIX [1] says we may get EISCONN in the yes/yes case and the Linux
man page [3] says we either get EISCONN or the msg_name argument is
ignored. So that also doesn't look quite right yet.

- Werner

[1] http://pubs.opengroup.org/onlinepubs/007904975/
[2] http://www.gnu.org/software/libc/manual/html_node/Error-Codes.html
[3] man sendmsg
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