Subject: Re: [PATCH v2 net-next] af_unix: dont send SCM_CREDENTIALS by default

From: Michal Schmidt <[email-masked]>
Date: Mon, 28 Nov 2011 14:23:05 +0100

On 09/20/2011 06:16 AM, Eric Dumazet wrote:
> Note : The man page does states :
>
> "To receive a struct ucred message the SO_PASSCRED option  must  be
> enabled  on  the socket."
>
> But it doesnt say if the SO_PASSCRED option must be enabled before the
> sender sends its message, or before receiver attempts to read it.
>
> Once a message is queued on an unix socket, flipping SO_PASSCRED cant
> change its content (adding or removing credentials), since sender might
> already have disappeared.
>
> So current code includes credentials in all sent messages, just in case
> receiver actually fetch credentials.
>
> There are probably programs that assume they can set SO_PASSCRED right
> before calling recvmsg(). Are we taking risk to break them, or are we
> gentle and provide a sysctl option to ease the transition, I dont
> know...

Such a case has just appeared:
https://bugzilla.redhat.com/show_bug.cgi?id=757628

systemd allows on-demand socket activation of services. It creates a 
listening socket without the SO_PASSCRED flag. When the first message 
arrives to the socket, systemd spawns the service and passes the 
socket's fd to it. The service sets SO_PASSCRED before actually 
receiving the message.

I can fix that in systemd, but there may be more cases like this.

Michal