netdev - Re: [PATCH net-next] net: always dump full packets with skb

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Date:   Tue, 6 Oct 2020 07:30:13 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Jacob Keller <jacob.e.keller@...el.com>
Cc:     Vladimir Oltean <vladimir.oltean@....com>,
        Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>
Subject: Re: [PATCH net-next] net: always dump full packets with skb_dump

On Mon, Oct 5, 2020 at 8:25 PM Jacob Keller <jacob.e.keller@...el.com> wrote:
>
>
>
> On 10/5/2020 7:48 AM, Vladimir Oltean wrote:
> > Currently skb_dump has a restriction to only dump full packet for the
> > first 5 socket buffers, then only headers will be printed. Remove this
> > arbitrary and confusing restriction, which is only documented vaguely
> > ("up to") in the comments above the prototype.
> >
>
> So, this limitation appeared very clearly in the original commit,
> 6413139dfc64 ("skbuff: increase verbosity when dumping skb data")..
>
> Searching the netdev list, that patch links back to this one as the
> original idea:
>
> https://patchwork.ozlabs.org/project/netdev/patch/20181121021309.6595-2-xiyou.wangcong@gmail.com/
>
> I can't find any further justification on that limit. I suppose the
> primary reasoning being if you somehow call this function in a loop this
> would avoid dumping the entire packet over and over?

Not in a loop per se, but indeed to avoid unbounded writing to the kernel log.

skb_dump is called from skb_warn_bad_offload and netdev_rx_csum_fault.
Previously when these were triggered, a few example bad packets were
sufficient to debug the issue.

A full dump can add a lot of data to the kernel log, so I limited to
what is strictly needed.