Re: [Qemu-devel] [PATCH] net: Transmit zero UDP checksum as 0xFFFF

[Jason Wang]

On 2017年11月15日 07:25, Ed Swierk wrote:
> The checksum algorithm used by IPv4, TCP and UDP allows a zero value
> to be represented by either 0x0000 and 0xFFFF. But per RFC 768, a zero
> UDP checksum must be transmitted as 0xFFFF, as 0x0000 is a special
> value meaning no checksum.
>
> Substitute 0xFFFF whenever a checksum is computed as zero on a UDP
> datagram. Doing this on IPv4 packets and TCP segments is unnecessary
> but legal.
>
> (While it is tempting to make the substitution in
> net_checksum_finish(), that function is also used by receivers to
> verify checksums, and in that case the expected value is always
> 0x0000.)

Then looks like you'd better have an wrapper for net_checksum_finish() 
and do things there.

> Signed-off-by: Ed Swierk <address@hidden>
> ---
>   hw/net/e1000.c      | 5 +++--
>   hw/net/net_rx_pkt.c | 3 +++
>   hw/net/net_tx_pkt.c | 6 ++++++
>   hw/net/vmxnet3.c    | 7 +++++--
>   4 files changed, 17 insertions(+), 4 deletions(-)
>
> diff --git a/hw/net/e1000.c b/hw/net/e1000.c
> index d642314..97242a1 100644
> --- a/hw/net/e1000.c
> +++ b/hw/net/e1000.c
> @@ -505,8 +505,9 @@ putsum(uint8_t *data, uint32_t n, uint32_t sloc, uint32_t 
> css, uint32_t cse)
>       if (cse && cse < n)
>           n = cse + 1;
>       if (sloc < n-1) {
> -        sum = net_checksum_add(n-css, data+css);
> -        stw_be_p(data + sloc, net_checksum_finish(sum));
> +        sum = net_raw_checksum(data + css, n - css);
> +        /* For UDP, zero checksum must be sent as 0xFFFF */
> +        stw_be_p(data + sloc, sum ? sum : 0xffff);
>       }
>   }
>   
> diff --git a/hw/net/net_rx_pkt.c b/hw/net/net_rx_pkt.c
> index 1019b50..e820132 100644
> --- a/hw/net/net_rx_pkt.c
> +++ b/hw/net/net_rx_pkt.c
> @@ -588,6 +588,9 @@ bool net_rx_pkt_fix_l4_csum(struct NetRxPkt *pkt)
>   
>       /* Calculate L4 checksum */
>       csum = cpu_to_be16(_net_rx_pkt_calc_l4_csum(pkt));
> +    if (!csum) {
> +        csum = 0xFFFF; /* For UDP, zero checksum must be sent as 0xFFFF */
> +    }

I thought we should only do this for tx?

Thanks