netdev - Re: [PATCH] net-fq: Add WARN

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Message-ID: <CAM_iQpUdBst8xyzceq2OdVB2onF1qMR063zrkBVDBonZZNuQdA@mail.gmail.com>
Date:   Thu, 7 Jun 2018 14:52:37 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Ben Greear <greearb@...delatech.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH] net-fq: Add WARN_ON check for null flow.

On Thu, Jun 7, 2018 at 2:41 PM, Ben Greear <greearb@...delatech.com> wrote:
> On 06/07/2018 02:29 PM, Cong Wang wrote:
>>
>> On Thu, Jun 7, 2018 at 9:06 AM,  <greearb@...delatech.com> wrote:
>>>
>>> --- a/include/net/fq_impl.h
>>> +++ b/include/net/fq_impl.h
>>> @@ -80,6 +80,9 @@ static struct sk_buff *fq_tin_dequeue(struct fq *fq,
>>>
>>>         flow = list_first_entry(head, struct fq_flow, flowchain);
>>>
>>> +       if (WARN_ON_ONCE(!flow))
>>> +               return NULL;
>>> +
>>
>>
>> How could even possibly list_first_entry() returns NULL?
>> You need list_first_entry_or_null().
>>
>
> I don't know for certain flow as null, but something was NULL in this method
> near that line and it looked like a likely culprit.
>
> I guess possibly tin or fq was passed in as NULL?

A NULL pointer is not always 0. You can trigger a NULL-ptr-def with 0x3c
too, but you are checking against 0 in your patch, that is the problem and
that is why list_first_entry_or_null() exists.