Re: [PATCH v3 6/9] checksum: use csum_ip4_header() in udp.c and tcp.c

[Stefano Brivio <sbrivio@redhat.com>]

On Sat, 17 Feb 2024 16:07:22 +0100
Laurent Vivier <lvivier@redhat.com> wrote:

> We can find the same function to compute the IPv4 header
> checksum in tcp.c, udp.c and tap.c
> 
> Use the function defined for tap.c, csum_ip4_header(), but
> with the code used in tcp.c and udp.c as it doesn't need a fully
> initialiazed IPv4 header, only protocol, tot_len, saddr and daddr.
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
> 
> Notes:
>     v3:
>       - function parameters provide tot_len, saddr, daddr and protocol
>        rather than an iphdr
>     
>     v2:
>       - use csum_ip4_header() from checksum.c
>       - use code from tcp.c and udp.c in csum_ip4_header()
>       - use "const struct iphfr *", check is not updated by the
>         function but by the caller.
> 
>  checksum.c | 17 +++++++++++++----
>  checksum.h |  3 ++-
>  tap.c      |  3 ++-
>  tcp.c      | 24 +++---------------------
>  udp.c      | 20 ++------------------
>  5 files changed, 22 insertions(+), 45 deletions(-)
> 
> diff --git a/checksum.c b/checksum.c
> index 74e3742bc6f6..511b296a9a80 100644
> --- a/checksum.c
> +++ b/checksum.c
> @@ -57,6 +57,7 @@
>  #include <linux/icmpv6.h>
>  
>  #include "util.h"
> +#include "ip.h"
>  #include "checksum.h"
>  
>  /* Checksums are optional for UDP over IPv4, so we usually just set
> @@ -116,13 +117,21 @@ uint16_t csum_fold(uint32_t sum)
>  uint16_t csum(const void *buf, size_t len, uint32_t init);
>  
>  /**
> - * csum_ip4_header() - Calculate and set IPv4 header checksum
> + * csum_ip4_header() - Calculate IPv4 header checksum
>   * @ip4h:	IPv4 header
>   */
> -void csum_ip4_header(struct iphdr *ip4h)
> +uint16_t csum_ip4_header(uint16_t tot_len, uint8_t protocol,
> +			 uint32_t saddr, uint32_t daddr)
>  {
> -	ip4h->check = 0;
> -	ip4h->check = csum(ip4h, (size_t)ip4h->ihl * 4, 0);
> +	uint32_t sum = L2_BUF_IP4_PSUM(protocol);

Now that we use this macro, Coverity Scan realises that it's broken:

#define L2_BUF_IP4_PSUM(proto) ((uint32_t)htons_constant(0x4500) +     \
				(uint32_t)htons_constant(0xff00 | (proto)))

...but proto is eight (lower) bits, so this actually ignores 'proto'.

-- 
Stefano