netdev - Re: Repeatable inet6_dump_fib crash in stock 4.12.0-rc4+

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Date:   Tue, 13 Jun 2017 14:28:22 -0600
From:   David Ahern <dsahern@...il.com>
To:     Ben Greear <greearb@...delatech.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Cc:     Cong Wang <xiyou.wangcong@...il.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: Repeatable inet6_dump_fib crash in stock 4.12.0-rc4+

On 6/13/17 2:16 PM, Ben Greear wrote:
> On 06/09/2017 02:25 PM, Eric Dumazet wrote:
>> On Fri, 2017-06-09 at 07:27 -0600, David Ahern wrote:
>>> On 6/8/17 11:55 PM, Cong Wang wrote:
>>>> On Thu, Jun 8, 2017 at 2:27 PM, Ben Greear <greearb@...delatech.com>
>>>> wrote:
>>>>>
>>>>> As far as I can tell, the patch did not help, or at least we still
>>>>> reproduce
>>>>> the
>>>>> crash easily.
>>>>
>>>> netlink dump is serialized by nlk->cb_mutex so I don't think that
>>>> patch makes any sense w.r.t race condition.
>>>
>>> From what I can see fn_sernum should be accessed under table lock, so
>>> when saving and checking it during a walk make sure it the lock is held.
>>> That has nothing to do with the netlink dump, but the table changing
>>> during a walk.
>>
>>
>> Yes, your patch makes total sense, of course.
> 
> I guess someone should go ahead and make an official patch and
> submit it, even if it doesn't fix my problem.

I can do that; was hoping to root cause the problem first.


> 
>>>>> (gdb) l *(fib6_walk_continue+0x76)
>>>>> 0x188c6 is in fib6_walk_continue
>>>>> (/home/greearb/git/linux-2.6/net/ipv6/ip6_fib.c:1593).
>>>>> 1588                            if (fn == w->root)
>>>>> 1589                                    return 0;
>>>>> 1590                            pn = fn->parent;
>>>>> 1591                            w->node = pn;
>>>>> 1592    #ifdef CONFIG_IPV6_SUBTREES
>>>>> 1593                            if (FIB6_SUBTREE(pn) == fn) {
>>>>
>>>> Apparently fn->parent is NULL here for some reason, but
>>>> I don't know if that is expected or not. If a simple NULL check
>>>> is not enough here, we have to trace why it is NULL.
>>>
>>> From my understanding, parent should not be null hence the attempts to
>>> fix access to table nodes under a lock. ie., figuring out why it is null
>>> here.
> 
> If someone has more suggestions, I'll be happy to test.

I have looked at the code again and nothing is jumping out. Will look
again later today.