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 checksumsto 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 packetand 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