netdev - Why RTM_NEWADDR is sent before FIB update?

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Date:	Wed, 20 Oct 2010 13:23:10 +0300
From:	Timo Teräs <timo.teras@....fi>
To:	netdev@...r.kernel.org
Subject: Why RTM_NEWADDR is sent before FIB update?

Hi,

I'm wonder why does devinet.c:__inet_insert_ifa() (and other places)
send first the RTM_NEWADDR notification and only after that calls
inetaddr_chain notifiers?

The reason I'm asking that this gives a race condition to user land:
 1. process A changes IP address
 2. kernel sends RTM_NEWADDR
 3. process B gets notification
 4. process B tries to bind() to new IP but that fails with
EADDRNOTAVAIL because FIB is not yet updated and inet_addr_type() in
inet_bind() does not recognize the IP as local
 5. kernel calls inetaddr_chain notifiers which updates FIB

My understanding was that RTM_NEWADDR was notification about new address
being usable. But it's currently a notification about "new address will
be usable soon".

So should we:
 a) call first notifiers and after that send RTM_NEWADDR?
 b) synchronise inet_bind() with address changes somehow?
 c) live with the bad semantics of the notification in userland
 d) ??

- Timo
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