netdev - Re: Refcount mismatch when unregistering netdevice from kernel

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Date:   Tue, 5 Jan 2021 11:09:43 -0800
From:   Wei Wang <weiwan@...gle.com>
To:     David Ahern <dsahern@...il.com>
Cc:     stranche@...eaurora.org, Eric Dumazet <eric.dumazet@...il.com>,
        Martin KaFai Lau <kafai@...com>,
        Mahesh Bandewar <maheshb@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Subash Abhinov Kasiviswanathan <subashab@...eaurora.org>
Subject: Re: Refcount mismatch when unregistering netdevice from kernel

On Mon, Jan 4, 2021 at 8:58 PM David Ahern <dsahern@...il.com> wrote:
>
> On 1/4/21 8:05 PM, stranche@...eaurora.org wrote:
> >
> > We're able to reproduce the refcount mismatch after some experimentation
> > as well.
> > Essentially, it consists of
> > 1) adding a default route (ip -6 route add dev XXX default)
> > 2) forcing the creation of an exception route via manually injecting an
> > ICMPv6
> > Packet Too Big into the device.
> > 3) Replace the default route (ip -6 route change dev XXX default)
> > 4) Delete the device. (ip link del XXX)
> >
> > After adding a call to flush out the exception cache for the route, the
> > mismatch
> > is no longer seen:
> > diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
> > index 7a0c877..95e4310 100644
> > --- a/net/ipv6/ip6_fib.c
> > +++ b/net/ipv6/ip6_fib.c
> > @@ -1215,6 +1215,7 @@ static int fib6_add_rt2node(struct fib6_node *fn,
> > struct fib6_info *rt,
> >                 }
> >                 nsiblings = iter->fib6_nsiblings;
> >                 iter->fib6_node = NULL;
> > +               rt6_flush_exceptions(iter);
> >                 fib6_purge_rt(iter, fn, info->nl_net);
> >                 if (rcu_access_pointer(fn->rr_ptr) == iter)
> >                         fn->rr_ptr = NULL;
>
> Ah, I see now. rt6_flush_exceptions is called by fib6_del_route, but
> that won't handle replace.
>
> If you look at fib6_purge_rt it already has a call to remove pcpu
> entries. This call to flush exceptions should go there and the existing
> one in fib6_del_route can be removed.
>
Thanks for catching this!
Agree with this proposed fix.


> Also, can you add the reproducer as another test case to
> tools/testing/selftests/net/pmtu.sh? We definitely need one for this
> sequence (route, exceptions, replace route).