Re: [PATCH 04/14] Add csum_udp4() helper for calculating UDP over IPv4 checksums

[Stefano Brivio <sbrivio@redhat.com>]

On Mon, 17 Oct 2022 19:57:57 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> At least two places in passt fill in UDP over IPv4 checksums, although
> since UDP checksums are optional with IPv4 that just amounts to storing
> a 0 (in tap_ip_send()) or leaving a 0 from an earlier initialization (in
> dhcp()).  For consistency, add a helper for this "calculation".
> 
> Just for the heck of it, add the option (compile time disabled for now) to
> calculate real UDP checksums.
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  checksum.c | 33 +++++++++++++++++++++++++++++++++
>  checksum.h |  3 +++
>  dhcp.c     |  2 +-
>  tap.c      |  2 +-
>  4 files changed, 38 insertions(+), 2 deletions(-)
> 
> diff --git a/checksum.c b/checksum.c
> index 0849fb1..72f1cfb 100644
> --- a/checksum.c
> +++ b/checksum.c
> @@ -56,6 +56,11 @@
>  #include <linux/icmp.h>
>  #include <linux/icmpv6.h>
>  
> +/* Checksums are optional for UDP over IPv4, so we usually just set
> + * them to 0.  Change this 1 to calculate real UDP over IPv4 checksums

to 1

> + */
> +#define UDP4_REAL_CHECKSUMS	0
> +
>  /**
>   * sum_16b() - Calculate sum of 16-bit words
>   * @buf:	Input buffer
> @@ -109,6 +114,34 @@ uint16_t csum_unaligned(const void *buf, size_t len, uint32_t init)
>  	return (uint16_t)~csum_fold(sum_16b(buf, len) + init);
>  }
>  
> +/**
> + * csum_udp4() - Calculate checksum for a UDP over IPv4 packet

and set

> + * @udp4hr:	UDP header, initialized apart from checksum
> + * @saddr:	IPv4 source address
> + * @daddr:	IPv4 destination address
> + * @payload:	ICMPv4 packet payload
> + * @len:	Length of @payload (not including UDP)
> + */
> +void csum_udp4(struct udphdr *udp4hr,
> +	       in_addr_t saddr, in_addr_t daddr,
> +	       const void *payload, size_t len)
> +{
> +	/* UDP checksums are optional, so don't bother */
> +	udp4hr->check = 0;
> +
> +	if (UDP4_REAL_CHECKSUMS) {
> +		/* UNTESTED: if we did want real UDPv4 checksums, this
> +		 * is roughly what we'd need */
> +		uint32_t psum = csum_fold(htonl(saddr))
> +			+ csum_fold(htonl(daddr))
> +			+ htons(len + sizeof(*udp4hr))
> +			+ htons(IPPROTO_UDP);
> +		/* Add in partial checksum for the UDP header alone */
> +		psum += sum_16b(udp4hr, sizeof(*udp4hr));
> +		udp4hr->check = csum_unaligned(payload, len, psum);
> +	}
> +}
> +
>  /**
>   * csum_icmp4() - Calculate checksum for an ICMPv4 packet
>   * @icmp4hr:	ICMPv4 header, initialized apart from checksum
> diff --git a/checksum.h b/checksum.h
> index 1b9f48e..a9502b9 100644
> --- a/checksum.h
> +++ b/checksum.h
> @@ -13,6 +13,9 @@ struct icmp6hdr;
>  uint32_t sum_16b(const void *buf, size_t len);
>  uint16_t csum_fold(uint32_t sum);
>  uint16_t csum_unaligned(const void *buf, size_t len, uint32_t init);
> +void csum_udp4(struct udphdr *udp4hr,
> +	       in_addr_t saddr, in_addr_t daddr,
> +	       const void *payload, size_t len);

Horizontal space.

-- 
Stefano