Subject: Re: [BUG] in inet6_create

From: Pavel Emelyanov <[email-masked]>
Date: Fri, 02 Nov 2007 15:54:12 +0300

Pavel Emelyanov wrote:
> Roel Kluin wrote:
>> I got this bug recently, I am not sure whether this is related to any previously 
>> reported ones. It was a recently pulled git kernel. Also I have been hacking my
>> kernel a bit lately, but I think that I haven't got any changes in the currently
>> running kernel.
>>
>> FYI: my network card was not running (module not loaded, and I just started 
>> thunderbird)
>>
>> Roel
>>
>> More information needed?
> 
> I've tried to objdump my ipv6.ko, and found (at the different offset,
> but) the same codeline. It showed that the buggy place was in:
> 
>         list_for_each_rcu(p, &inetsw6[sock->type]) {
> 
> some list_head pointer was NULL.
> 
> I looked at the inet6_init (which seems to run at the moment of the
> oops according to the calltrace) and found that the ipv6 protocol 
> is first registered and only after this the inetsw6 lists are
> properly initialized.

Hm... A deeper look at the code showed that the proto_register() is
OK to be called before the list initialization.

Nevertheless, the faulty place is found correctly (providing that
Roel's objdump looks similar to mine, but it should - this codeline
is unique in the ipv6.ko).

Roel, are you sure, that the kernel you're running is not hacked
by some of you patches :)

> I suspect that this is a race: we create the socket right after
> the new protocol is registered, but before the list heads are 
> ready. The ->init call is called without the stopmachine, so
> other process run in parallel with it.
> 
> This patch should help, but I don't think that such a situation
> is easily reproducible.
> 
> Signed-off-by: Pavel Emelyanov <[email-masked]>
> 
> ---
> 
> diff --git a/net/ipv6/af_inet6.c b/net/ipv6/af_inet6.c
> index ecbd388..f9bd26f 100644
> --- a/net/ipv6/af_inet6.c
> +++ b/net/ipv6/af_inet6.c
> @@ -769,6 +769,10 @@ static int __init inet6_init(void)
>  #endif
>  #endif
>  
> +	/* Register the socket-side information for inet6_create.  */
> +	for(r = &inetsw6[0]; r < &inetsw6[SOCK_MAX]; ++r)
> +		INIT_LIST_HEAD(r);
> +
>  	err = proto_register(&tcpv6_prot, 1);
>  	if (err)
>  		goto out;
> @@ -786,10 +790,6 @@ static int __init inet6_init(void)
>  		goto out_unregister_udplite_proto;
>  
>  
> -	/* Register the socket-side information for inet6_create.  */
> -	for(r = &inetsw6[0]; r < &inetsw6[SOCK_MAX]; ++r)
> -		INIT_LIST_HEAD(r);
> -
>  	/* We MUST register RAW sockets before we create the ICMP6,
>  	 * IGMP6, or NDISC control sockets.
>  	 */
>