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

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Date:   Mon, 29 Oct 2018 19:41:09 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     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 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.