netdev - Re: [Patch net-next 2/2] net: dump whole skb data in netdev_rx_csum

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Date:   Thu, 22 Nov 2018 17:43:36 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        David Miller <davem@...emloft.net>
Subject: Re: [Patch net-next 2/2] net: dump whole skb data in netdev_rx_csum_fault()

On Wed, Nov 21, 2018 at 10:26 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Wed, Nov 21, 2018 at 10:17 AM Cong Wang <xiyou.wangcong@...il.com> wrote:
> >
> > On Wed, Nov 21, 2018 at 5:05 AM Eric Dumazet <eric.dumazet@...il.com> wrote:
> > >
> > >
> > >
> > > On 11/20/2018 06:13 PM, Cong Wang wrote:
> > > > Currently, we only dump a few selected skb fields in
> > > > netdev_rx_csum_fault(). It is not suffient for debugging checksum
> > > > fault. This patch introduces skb_dump() which dumps skb mac header,
> > > > network header and its whole skb->data too.
> > > >
> > > > Cc: Herbert Xu <herbert@...dor.apana.org.au>
> > > > Cc: Eric Dumazet <edumazet@...gle.com>
> > > > Cc: David Miller <davem@...emloft.net>
> > > > Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
> > > > ---
> > >
> > >
> > > > +     print_hex_dump(level, "skb data: ", DUMP_PREFIX_OFFSET, 16, 1,
> > > > +                    skb->data, skb->len, false);
> > >
> > > As I mentioned to David, we want all the bytes that were maybe already pulled
> > >
> > > (skb->head starting point, not skb->data)
> >
> > Hmm, with mac header and network header, it is effectively from skb->head, no?
> > Is there anything between skb->head and mac header?
>
> Oh, I guess we wanted a single hex dump, or we need some user program
> to be able to
> rebuild from different memory zones the original CHECKSUM_COMPLETE value.


Yeah, I can remove the prefix and dump the complete packet as
one single block. This means I also need to check where
skb->data points to.

>
> >
> > >
> > > Also we will miss the trimmed bytes if there were padding data.
> > > And it seems the various bugs we have are all tied to the pulled or trimmed bytes.
> > >
> >
> > Unless I miss something, the tailing padding data should be in range
> > [iphdr->tot_len, skb->len]. No?
>
>
> Not after we did the pskb_trim_rcsum() call, since it has effectively
> reduced skb->len by the number of padding bytes.

Sure, this patch can't change where netdev_rx_csum_fault() gets
called. We either need to move the checksum validation earlier,
or move the trimming later, none of them belongs to this patch.

Thanks.