Re: [PATCH v3 02/11] ip: Add IN4_MASK() macro for IPv4 netmask calculation

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

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On Fri, Jan 30, 2026 at 04:44:38PM -0500, Jon Maloy wrote:
> Add a convenience macro to compute an IPv4 netmask from a prefix length.
> This simplifies netmask calculations throughout the codebase.
> 
> Signed-off-by: Jon Maloy <jmaloy@redhat.com>

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

> ---
>  conf.c | 2 +-
>  dhcp.c | 2 +-
>  ip.h   | 2 ++
>  3 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/conf.c b/conf.c
> index 98d5d17..5188c02 100644
> --- a/conf.c
> +++ b/conf.c
> @@ -1147,7 +1147,7 @@ static void conf_print(const struct ctx *c)
>  		if (!c->no_dhcp) {
>  			uint32_t mask;
>  
> -			mask = htonl(0xffffffff << (32 - c->ip4.prefix_len));
> +			mask = IN4_MASK(c->ip4.prefix_len);
>  
>  			info("DHCP:");
>  			info("    assign: %s",
> diff --git a/dhcp.c b/dhcp.c
> index 6b9c2e3..c552f01 100644
> --- a/dhcp.c
> +++ b/dhcp.c
> @@ -404,7 +404,7 @@ int dhcp(const struct ctx *c, struct iov_tail *data)
>  
>  	info("    from %s", eth_ntop(m->chaddr, macstr, sizeof(macstr)));
>  
> -	mask.s_addr = htonl(0xffffffff << (32 - c->ip4.prefix_len));
> +	mask.s_addr = IN4_MASK(c->ip4.prefix_len);
>  	memcpy(opts[1].s,  &mask,                sizeof(mask));
>  	memcpy(opts[3].s,  &c->ip4.guest_gw,     sizeof(c->ip4.guest_gw));
>  	memcpy(opts[54].s, &c->ip4.our_tap_addr, sizeof(c->ip4.our_tap_addr));
> diff --git a/ip.h b/ip.h
> index bd28640..c829d84 100644
> --- a/ip.h
> +++ b/ip.h
> @@ -17,6 +17,8 @@
>  	(ntohl(((struct in_addr *)(a))->s_addr) >> IN_CLASSA_NSHIFT == IN_LOOPBACKNET)
>  #define IN4_IS_ADDR_MULTICAST(a) \
>  	(IN_MULTICAST(ntohl(((struct in_addr *)(a))->s_addr)))
> +#define IN4_MASK(prefix_len) \
> +	(htonl(0xffffffff << (128 - (prefix_len))))
>  #define IN4_ARE_ADDR_EQUAL(a, b) \
>  	(((struct in_addr *)(a))->s_addr == ((struct in_addr *)b)->s_addr)
>  #define IN4ADDR_LOOPBACK_INIT \
> -- 
> 2.52.0
> 

-- 
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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