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

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Date:   Tue, 13 Jun 2017 13:16:33 -0700
From:   Ben Greear <greearb@...delatech.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        David Ahern <dsahern@...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 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.

>>>> (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.

Thanks,
Ben


-- 
Ben Greear <greearb@...delatech.com>
Candela Technologies Inc  http://www.candelatech.com