Re: [Patch net] mlx5: fixup checksum for ethernet padding

[Cong Wang <xiyou.wangcong@gmail.com>]

On Tue, Nov 27, 2018 at 3:48 PM Eric Dumazet <eric.dumazet@gmail.com> wrote:
>
>
>
> On 11/27/2018 03:21 PM, Cong Wang wrote:
> > When an ethernet frame is padded to meet the minimum ethernet frame
> > size, the padding octets are not covered by the hardware checksum.
> > Fortunately the padding octets are ususally zero's, which don't affect
> > checksum. However, we have a switch which pads non-zero octets, this
> > causes kernel hardware checksum fault repeatedly.
> >
> > Prior to commit 88078d98d1bb ("net: pskb_trim_rcsum() and CHECKSUM_COMPLETE are friends"),
> > skb checksum is forced to be CHECKSUM_NONE when padding is detected.
> > After it, we have to keep skb->csum updated, like what we do for FCS.
> >
> > The logic is a bit complicated when dealing with both FCS and padding,
> > so I wrap it up in a helper function mlx5e_csum_padding().
> >
> > I tested this patch with RXFCS on and off, it works fine without any
> > warning in both cases.
> >
> > Fixes: 88078d98d1bb ("net: pskb_trim_rcsum() and CHECKSUM_COMPLETE are friends"),
> > Cc: Saeed Mahameed <saeedm@mellanox.com>
> > Cc: Eric Dumazet <edumazet@google.com>
> > Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
> > ---
> >  .../net/ethernet/mellanox/mlx5/core/en_rx.c   | 36 +++++++++++++++++++
> >  1 file changed, 36 insertions(+)
> >
> > diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> > index 16985ca3248d..93c18647ca74 100644
> > --- a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> > +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> > @@ -732,6 +732,35 @@ static u8 get_ip_proto(struct sk_buff *skb, __be16 proto)
> >                                           ((struct ipv6hdr *)ip_p)->nexthdr;
> >  }
> >
> > +static void mlx5e_csum_padding(struct sk_buff *skb, int network_depth,
> > +                            __be16 proto, bool has_fcs)
> > +{
> > +     u32 frame_len = has_fcs ? skb->len - ETH_FCS_LEN : skb->len;
> > +     void *ip_p = skb->data + network_depth;
> > +     u32 pad_offset, pad_len;
> > +     void *pad;
> > +
> > +     if (likely(frame_len > ETH_ZLEN))
> > +             return;
> > +
>
>
> But the padding might be added on normal packets (say 1000 bytes + 3 bytes of padding) ?

I never see other padding cases than ETH_ZLEN. Does ethernet standard
require padding for other cases? I only read the section "3.2.8 Pad field" in
the standard.

>
> The bug here is that mlx5 csum only includes the data in IP frame.
>
> I would simply force skb->ip_summed to CHECKSUM_NONE if any padding is detected.
>
> Otherwise, your patch needs more work when multiple frags are used (ie num_frags > 1 )

You mean using skb_header_pointer()? Yeah if we need to handle cases
larger than ETH_ZLEN.

Thanks.