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

On Tue, Oct 18, 2022 at 05:03:09AM +0200, Stefano Brivio wrote:
11;rgb:ffff/ffff/ffff> On Mon, 17 Oct 2022 19:57:57 +1100
...
  David Gibson <david(a)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(a)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 
Done.

...
 
  + */
 +#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 
Done.

...
 
  + *
@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. 
Done.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson