netdev - Re: [PATCH net] ipv6: make ip6_create_rt_rcu return ip6_null

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Message-ID: <fbd50c1b-3a64-87f5-b662-56f12075c3c9@cumulusnetworks.com>
Date:   Tue, 19 Mar 2019 15:04:27 +0100
From:   David Ahern <dsa@...ulusnetworks.com>
To:     Paolo Abeni <pabeni@...hat.com>, Wei Wang <weiwan@...gle.com>
Cc:     Xin Long <lucien.xin@...il.com>,
        network dev <netdev@...r.kernel.org>,
        "David S . Miller" <davem@...emloft.net>
Subject: Re: [PATCH net] ipv6: make ip6_create_rt_rcu return ip6_null_entry
 instead of NULL

On 3/19/19 9:19 AM, Paolo Abeni wrote:
> Hi,
> 
> On Mon, 2019-03-18 at 14:30 -0700, Wei Wang wrote:
>> On Mon, Mar 18, 2019 at 12:48 PM David Ahern <dsa@...ulusnetworks.com> wrote:
>>> On 3/18/19 12:36 PM, Xin Long wrote:
>>>> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
>>>> index 4ef4bbd..754777d 100644
>>>> --- a/net/ipv6/route.c
>>>> +++ b/net/ipv6/route.c
>>>> @@ -1040,13 +1040,17 @@ static struct rt6_info *ip6_create_rt_rcu(struct fib6_info *rt)
>>>>       struct rt6_info *nrt;
>>>>
>>>>       if (!fib6_info_hold_safe(rt))
>>>> -             return NULL;
>>>> +             goto fallback;
>>>>
>>>>       nrt = ip6_dst_alloc(dev_net(dev), dev, flags);
>>>> -     if (nrt)
>>>> +     if (nrt) {
>>>>               ip6_rt_copy_init(nrt, rt);
>>>> -     else
>>>> +     } else {
>>>>               fib6_info_release(rt);
>>>> +fallback:
>>>> +             nrt = dev_net(dev)->ipv6.ip6_null_entry;
>>>> +             dst_hold(&nrt->dst);
>>>> +     }
>>>>
>>>>       return nrt;
>>>>  }
>>>
>>> It seems appropriate for ip6_create_rt_rcu to always return an rt6_info
>>> given its use and v6 design. Since ip6_dst_alloc can also fail
>>> (e.g.,
>>> exceed gc_thresh) the fallback case should apply to both.
>>
>> Agree. We should take care of ip6_dst_alloc() as well.
> 
> AFAICS, if ip6_dst_alloc() fails/returns a NULL ptr, with this patch we
> do enter the fallback path (and return a valid rt6_info). I think your
> concerns are already addressed, what I'm missing ?!?
> 

you are correct. It should be made clearer from a readability perspective.