Re: [PATCH 01/14] Add csum_icmp6() helper for calculating ICMPv6 checksums

[Stefano Brivio <sbrivio@redhat.com>]

On Mon, 17 Oct 2022 19:57:54 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> At least two places in passt calculate ICMPv6 checksums, ndp() and
> tap_ip_send().  Add a helper to handle this calculation in both places.
> For future flexibility, the new helper takes parameters for the fields in
> the IPv6 pseudo-header, so an IPv6 header or pseudo-header doesn't need to
> be explicitly constructed.  It also allows the ICMPv6 header and payload to
> be in separate buffers, although we don't use this yet.
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  checksum.c | 27 +++++++++++++++++++++++++++
>  checksum.h |  7 +++++++
>  ndp.c      |  5 +----
>  tap.c      |  6 ++----
>  4 files changed, 37 insertions(+), 8 deletions(-)
> 
> diff --git a/checksum.c b/checksum.c
> index 56ad01e..0e207c8 100644
> --- a/checksum.c
> +++ b/checksum.c
> @@ -52,6 +52,8 @@
>  #include <stddef.h>
>  #include <stdint.h>
>  
> +#include <linux/icmpv6.h>
> +
>  /**
>   * sum_16b() - Calculate sum of 16-bit words
>   * @buf:	Input buffer
> @@ -105,6 +107,31 @@ uint16_t csum_unaligned(const void *buf, size_t len, uint32_t init)
>  	return (uint16_t)~csum_fold(sum_16b(buf, len) + init);
>  }
>  
> +/**
> + * csum_icmp6() - Calculate checksum for an ICMPv6 packet

"Calculate and set" ...?

> + * @icmp6hr:	ICMPv6 header, initialized apart from checksum
> + * @saddr:	IPv6 source address
> + * @daddr:	IPv6 destination address
> + * @payload:	ICMP packet payload
> + * @len:	Length of @payload (not including ICMPv6 header)
> + */
> +void csum_icmp6(struct icmp6hdr *icmp6hr,
> +		const struct in6_addr *saddr,
> +		const struct in6_addr *daddr,

I think:
		const struct in6_addr *saddr, const struct in6_addr *daddr,

would be easier on eyes.

> +		const void *payload,
> +		size_t len)
> +{
> +	/* Partial checksum for the pseudo-IPv6 header */
> +	uint32_t psum = sum_16b(saddr, sizeof(*saddr)) +
> +		sum_16b(daddr, sizeof(*daddr)) +
> +		htons(len + sizeof(*icmp6hr)) + htons(IPPROTO_ICMPV6);

Maybe:

	uint32_t psum = sum_16b(saddr, sizeof(*saddr)) +
			sum_16b(daddr, sizeof(*daddr)) +
			htons(len + sizeof(*icmp6hr))  + htons(IPPROTO_ICMPV6);

for me, it turns things from "sum a bunch of things" to "addresses and
something else".

> +
> +	icmp6hr->icmp6_cksum = 0;
> +	/* Add in partial checksum for the ICMPv6 header alone */
> +	psum += sum_16b(icmp6hr, sizeof(*icmp6hr));
> +	icmp6hr->icmp6_cksum = csum_unaligned(payload, len, psum);
> +}
> +
>  /**
>   * csum_tcp4() - Calculate TCP checksum for IPv4 and set in place
>   * @iph:	Packet buffer, IP header
> diff --git a/checksum.h b/checksum.h
> index 5418406..2c72200 100644
> --- a/checksum.h
> +++ b/checksum.h
> @@ -6,9 +6,16 @@
>  #ifndef CHECKSUM_H
>  #define CHECKSUM_H
>  
> +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_icmp6(struct icmp6hdr *ih,
> +		const struct in6_addr *saddr,
> +		const struct in6_addr *daddr,
> +		const void *payload,
> +		size_t len);

It looks a bit like Haskell. ;) I would really use the horizontal space
we have.

>  void csum_tcp4(struct iphdr *iph);
>  uint16_t csum(const void *buf, size_t len, uint32_t init);
>  
> diff --git a/ndp.c b/ndp.c
> index dec36a9..03f1d06 100644
> --- a/ndp.c
> +++ b/ndp.c
> @@ -189,10 +189,7 @@ dns_done:
>  		ip6hr->saddr = c->ip6.addr_ll;
>  
>  	ip6hr->payload_len = htons(sizeof(*ihr) + len);
> -	ip6hr->hop_limit = IPPROTO_ICMPV6;
> -	ihr->icmp6_cksum = 0;
> -	ihr->icmp6_cksum = csum_unaligned(ip6hr, sizeof(*ip6hr) +
> -						 sizeof(*ihr) + len, 0);
> +	csum_icmp6(ihr, &ip6hr->saddr, &ip6hr->daddr, ihr + 1, len);

Nice to see this all going away!

>  	ip6hr->version = 6;
>  	ip6hr->nexthdr = IPPROTO_ICMPV6;
> diff --git a/tap.c b/tap.c
> index 8b6d9bc..aafc92b 100644
> --- a/tap.c
> +++ b/tap.c
> @@ -191,10 +191,8 @@ void tap_ip_send(const struct ctx *c, const struct in6_addr *src, uint8_t proto,
>  		} else if (proto == IPPROTO_ICMPV6) {
>  			struct icmp6hdr *ih = (struct icmp6hdr *)(ip6h + 1);
>  
> -			ih->icmp6_cksum = 0;
> -			ih->icmp6_cksum = csum_unaligned(ip6h,
> -							 len + sizeof(*ip6h),
> -							 0);
> +			csum_icmp6(ih, &ip6h->saddr, &ip6h->daddr,
> +				   ih + 1, len - sizeof(*ih));
>  		}
>  		ip6h->version = 6;
>  		ip6h->nexthdr = proto;

-- 
Stefano