linux-kernel - Re: [PATCH 0/10] af_unix: add multicast and filtering features to AF

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Message-ID: <4F4CF1BF.7080503@collabora.co.uk>
Date:	Tue, 28 Feb 2012 16:24:47 +0100
From:	Javier Martinez Canillas <javier.martinez@...labora.co.uk>
To:	David Lamparter <equinox@...c24.net>
CC:	Rodrigo Moya <rodrigo.moya@...labora.co.uk>,
	David Miller <davem@...emloft.net>, javier@...labora.co.uk,
	eric.dumazet@...il.com, lennart@...ttering.net,
	kay.sievers@...y.org, alban.crequy@...labora.co.uk,
	bart.cerneels@...labora.co.uk, sjoerd.simons@...labora.co.uk,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/10] af_unix: add multicast and filtering features to
 AF_UNIX

On 02/28/2012 03:28 PM, David Lamparter wrote:
> On Tue, Feb 28, 2012 at 11:47:39AM +0100, Rodrigo Moya wrote:
>> > - slow readers: dropping packets vs blocking the sender. Although
>> >   datagrams are not reliable on IP, datagrams on Unix sockets are
>> never
>> >   lost. So if one receiver has its buffer full the sender is blocked
>> > instead of dropping packets. That way we guarantee a reliable
>> > communication channel.
> 
> This sounds like a terribly nice way to f*ck the entire D-Bus system by
> having one broken (or malicious) desktop application. What's the
> intended way of coping with users that block the socket by not reading?
> 
> 
> -David L.

The problem is that D-bus expects a reliable transport method (TCP or
SOCK_STREAM Unix socks) but this is not the case with multicast Unix
sockets. Since our implementation is for SOCK_SEQPACKET and SOCK_DGRAM
socket types.

So, you have to either add another layer to the D-bus protocol to make
it reliable (acks, retransmissions, flow control, etc) or avoid losing
D-bus messages (by blocking the sender if one of the receivers has its
buffer full).

Regards,
Javier
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