Subject: Re: [PATCH] ipv4: synchronize bind() with RTM_NEWADDR notifications

From: Timo Teräs <[email-masked]>
Date: Thu, 21 Oct 2010 13:41:37 +0300

On 10/21/2010 01:25 PM, Eric Dumazet wrote:
> Le jeudi 21 octobre 2010 à 13:12 +0300, Timo Teräs a écrit :
>> Otherwise we have 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
>>
>> IPv6 side seems to handle the notifications properly: bind()
>> immediately after RTM_NEWADDR succeeds as expected. This is because
>> ipv6_chk_addr() uses inet6_addr_lst which is updated before address
>> notification.
>>
>> Signed-off-by: Timo Teräs <[email-masked]>
>> ---
>>  net/ipv4/af_inet.c  |    9 +++++++++
>>  net/ipv6/af_inet6.c |    4 +++-
>>  2 files changed, 12 insertions(+), 1 deletions(-)
>>
>> diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
>> index 6a1100c..21200e4 100644
>> --- a/net/ipv4/af_inet.c
>> +++ b/net/ipv4/af_inet.c
>> @@ -466,6 +466,15 @@ int inet_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
>>  	if (addr_len < sizeof(struct sockaddr_in))
>>  		goto out;
>>  
>> +	/* Acquire rtnl_lock to synchronize with possible simultaneous
>> +	 * IP-address changes. This is needed because when RTM_NEWADDR
>> +	 * is sent the new IP is not yet in FIB, but alas inet_addr_type
>> +	 * checks the address type using FIB. Acquiring rtnl lock once
>> +	 * makse sure that any address for which RTM_NEWADDR was sent
>> +	 * earlier exists also in FIB. */
>> +	rtnl_lock();
>> +	rtnl_unlock();
> 
> You must be kidding ?
> 
> Really, this is a hot path...

Is inet_bind() called from non-userland context? If yes, then this is a
bad idea. Otherwise I don't think it's that hot path...

The other idea of doing notifier calls before RTM_NEWADDR sending is
worse because it changes ordering of userland visible netlink notifications.

This looked like the easiest way out. If this is unacceptable, I guess
we are left with changing inet_addr_type() to not use FIB.

Or is there better ideas?