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

[tim <tim.c.chen@intel.com>]

On Tue, 2011-09-20 at 06:16 +0200, Eric Dumazet wrote:
> Le lundi 19 septembre 2011 à 22:10 -0400, Valdis.Kletnieks@vt.edu a
> écrit :
> > On Mon, 19 Sep 2011 14:39:58 PDT, Tim Chen said:
> > > Do we have to worry about the case where peer socket changes its flag
> > > to SOCK_PASSCRED while packets are in flight?  If there isn't such
> > > pathological use case, the patch looks fine to me.
> > 
> > I wouldn't think so - if you're sending a packet, and retroactively trying to
> > change the flag and expect it to work, your program is too ugly to live.  After
> > all, if the scheduler had cut off your timeslice and scheduledthe receiving
> > process before you set the flag, that packet would be delivered and done with
> > anyhow, and no amount of wishing will set that flag on an already-delivered
> > packet.
> > 
> > What *is* worth checking is that we DTRT if a process/thread is doing a send on
> > one CPU, and another process/thread with a shared file descriptor for that
> > socket is diddling the flag.  But if we just define it as "atomic op to change
> > the flag and other observers get whatever value their CPU sees at that
> > instant", I'm OK with that too.. ;)
> > 
> 
> 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...
> 

Should we reconsider the original approach of reducing the
pid/credential references, with your fixes to correct its flaws in the
streaming msg case?

Tim