Re: [RFC 02/12] ip: Add ip4_default_prefix_len() helper function for class-based prefix

[David Gibson <david@gibson.dropbear.id.au>]

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On Sun, Dec 14, 2025 at 08:54:31PM -0500, Jon Maloy wrote:
> We add a helper function to calculate the default IPv4 prefix length
> based on address class. This is used to replace the current inline
> calculation in conf_ip4(), and is also a preparation for more uses
> of this functionality in the coming commits.
> 
> Signed-off-by: Jon Maloy <jmaloy@redhat.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

Not a complaint about this patch, since it's just code motion, but the
whole idea of network classes is obsolete these days.  I wonder if we
should deprecate automatic picking of prefix_len based on class.

> ---
>  conf.c | 15 +++------------
>  ip.c   | 21 +++++++++++++++++++++
>  ip.h   |  2 ++
>  3 files changed, 26 insertions(+), 12 deletions(-)
> 
> diff --git a/conf.c b/conf.c
> index 0e96f36..31acc20 100644
> --- a/conf.c
> +++ b/conf.c
> @@ -733,18 +733,9 @@ static unsigned int conf_ip4(unsigned int ifi, struct ip4_ctx *ip4,
>  	}
>  
>  	/* Apply default prefix_len to first address if not set */
> -	if (!ip4->addrs[0].prefix_len) {
> -		in_addr_t a = ntohl(ip4->addrs[0].addr.s_addr);
> -
> -		if (IN_CLASSA(a))
> -			ip4->addrs[0].prefix_len = 8;
> -		else if (IN_CLASSB(a))
> -			ip4->addrs[0].prefix_len = 16;
> -		else if (IN_CLASSC(a))
> -			ip4->addrs[0].prefix_len = 24;
> -		else
> -			ip4->addrs[0].prefix_len = 32;
> -	}
> +	if (!ip4->addrs[0].prefix_len)
> +		ip4->addrs[0].prefix_len =
> +			ip4_default_prefix_len(&ip4->addrs[0].addr);
>  
>  	ip4->addr_seen = ip4->addrs[0].addr;
>  
> diff --git a/ip.c b/ip.c
> index 9a7f4c5..2519c71 100644
> --- a/ip.c
> +++ b/ip.c
> @@ -13,6 +13,8 @@
>   */
>  
>  #include <stddef.h>
> +#include <netinet/in.h>
> +
>  #include "util.h"
>  #include "ip.h"
>  
> @@ -67,3 +69,22 @@ found:
>  	*proto = nh;
>  	return true;
>  }
> +
> +/**
> + * ip4_default_prefix_len() - Get default prefix length for IPv4 address
> + * @addr:	IPv4 address
> + *
> + * Return: prefix length based on address class (8/16/24), or 32 for other
> + */
> +int ip4_default_prefix_len(const struct in_addr *addr)
> +{
> +	in_addr_t a = ntohl(addr->s_addr);
> +
> +	if (IN_CLASSA(a))
> +		return 8;
> +	if (IN_CLASSB(a))
> +		return 16;
> +	if (IN_CLASSC(a))
> +		return 24;
> +	return 32;
> +}
> diff --git a/ip.h b/ip.h
> index 748cb1f..065b78b 100644
> --- a/ip.h
> +++ b/ip.h
> @@ -139,6 +139,8 @@ static const struct in_addr in4addr_broadcast = { 0xffffffff };
>  #define IP4_MAX_ADDRS		8
>  #define IP6_MAX_ADDRS		16
>  
> +int ip4_default_prefix_len(const struct in_addr *addr);
> +
>  /**
>   * struct ip4_addr_entry - IPv4 address with prefix length
>   * @addr:		IPv4 address
> -- 
> 2.51.1
> 

-- 
David Gibson (he or they)	| 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

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