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

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Message-ID: <5dcba6d2a6680938c69f42bffbaf1c58df43746b.camel@mellanox.com>
Date:   Wed, 21 Nov 2018 19:33:44 +0000
From:   Saeed Mahameed <saeedm@...lanox.com>
To:     "xiyou.wangcong@...il.com" <xiyou.wangcong@...il.com>,
        "edumazet@...gle.com" <edumazet@...gle.com>
CC:     "eric.dumazet@...il.com" <eric.dumazet@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>
Subject: Re: [Patch net-next 2/2] net: dump whole skb data in
 netdev_rx_csum_fault()

On Wed, 2018-11-21 at 10:26 -0800, Eric Dumazet 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.
> 

Normally the driver keeps some headroom @skb->head, so the actual mac
header starts @ skb->head + driver_specific_headroom

for example in mlx5 we do:
va = page_addr + offset
build_skb(va)
/* skb->data = va + headroom */
skb_reserve(mlx5_headroom)


> > > 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.