netdev - Re: [Patch net] net: make pskb_trim_rcsum

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Date:   Mon, 29 Oct 2018 20:08:44 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [Patch net] net: make pskb_trim_rcsum_slow() robust



On 10/29/2018 07:41 PM, Cong Wang wrote:
> On Mon, Oct 29, 2018 at 7:25 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
>>
>>
>>
>> On 10/29/2018 07:21 PM, Cong Wang wrote:
>>> On Mon, Oct 29, 2018 at 7:14 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
>>>>
>>>> Would not it be simpler to set ip_summed to CHECKSUM_NONE (no need to save old_csum) ?
>>>
>>> For !CHECKSUM_COMPLETE, ip_summed should be untouched, right?
>>>
>>> If you mean only setting to CHECKSUM_NONE for CHECKSUM_COMPLETE case,
>>> the end result may not be simpler.
>>
>> I meant to reinstate what was there before my patch in this error case
>>
>>        if (skb->ip_summed == CHECKSUM_COMPLETE)
>>                skb->ip_summed = CHECKSUM_NONE;
>>
>> That would only be run in error (quite unlikely) path, instead of saving old_csum in all cases.
> 
> I know your point, however, I am not sure that is a desired behavior.
> 
> On failure, I think the whole skb should be restored to its previous state
> before entering this function, changing it to CHECKSUM_NONE on failure
> is inconsistent with success case.
> 

Before my patch, we were changing skb->ip_summed to CHECKSUM_NONE, 
so why suddenly we need to be consistent ?

In any case, ip_check_defrag() should really drop this skb, as for other allocation
failures (like skb_share_check()), if really we want consistency.