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

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Date:   Fri, 9 Jun 2017 07:27:25 -0600
From:   David Ahern <dsahern@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>,
        Ben Greear <greearb@...delatech.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: Repeatable inet6_dump_fib crash in stock 4.12.0-rc4+

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.



>> (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.