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

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Message-ID: <7f4d0c7c-8617-346d-8073-42680662eab6@candelatech.com>
Date:   Thu, 7 Jun 2018 15:08:39 -0700
From:   Ben Greear <greearb@...delatech.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH] net-fq: Add WARN_ON check for null flow.

On 06/07/2018 02:52 PM, Cong Wang wrote:
> 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.
>

Ahh, I see what you mean, and that is my mistake.  In my case, it did seem to
be a mostly-null deref, not a 0x0 deref.

Thanks,
Ben

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