[PATCH] dhcp: Fix netmask calculation for option 1 from prefix length

[PATCH] dhcp: Fix netmask calculation for option 1 from prefix length

[Stefano Brivio]

Similar to the conf_print() fix from commit 4129764ecaeb ("conf: Fix
mask calculation from prefix_len in conf_print()"): to calculate an
IPv4 netmask from the prefix length, we need to left shift 32 all-one
bits by 32 minus the prefix length -- not by the prefix length
itself.

Reported-by: Yalan Zhang <yalzhang@redhat.com>
Fixes: dd09cceaee21 ("Minor improvements to IPv4 netmask handling")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
---
 dhcp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/dhcp.c b/dhcp.c
index 6088886..edab780 100644
--- a/dhcp.c
+++ b/dhcp.c
@@ -335,7 +335,7 @@ int dhcp(const struct ctx *c, const struct pool *p)
 	     m->chaddr[3], m->chaddr[4], m->chaddr[5]);
 
 	m->yiaddr = c->ip4.addr;
-	mask.s_addr = htonl(0xffffffff << c->ip4.prefix_len);
+	mask.s_addr = htonl(0xffffffff << (32 - c->ip4.prefix_len));
 	memcpy(opts[1].s,  &mask,        sizeof(mask));
 	memcpy(opts[3].s,  &c->ip4.gw,   sizeof(c->ip4.gw));
 	memcpy(opts[54].s, &c->ip4.gw,   sizeof(c->ip4.gw));
-- 
@@ -335,7 +335,7 @@ int dhcp(const struct ctx *c, const struct pool *p)
 	     m->chaddr[3], m->chaddr[4], m->chaddr[5]);
 
 	m->yiaddr = c->ip4.addr;
-	mask.s_addr = htonl(0xffffffff << c->ip4.prefix_len);
+	mask.s_addr = htonl(0xffffffff << (32 - c->ip4.prefix_len));
 	memcpy(opts[1].s,  &mask,        sizeof(mask));
 	memcpy(opts[3].s,  &c->ip4.gw,   sizeof(c->ip4.gw));
 	memcpy(opts[54].s, &c->ip4.gw,   sizeof(c->ip4.gw));
-- 
2.35.1

Re: [PATCH] dhcp: Fix netmask calculation for option 1 from prefix length

[David Gibson]

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On Thu, Feb 16, 2023 at 02:07:52AM +0100, Stefano Brivio wrote:
> Similar to the conf_print() fix from commit 4129764ecaeb ("conf: Fix
> mask calculation from prefix_len in conf_print()"): to calculate an
> IPv4 netmask from the prefix length, we need to left shift 32 all-one
> bits by 32 minus the prefix length -- not by the prefix length
> itself.
> 
> Reported-by: Yalan Zhang <yalzhang@redhat.com>
> Fixes: dd09cceaee21 ("Minor improvements to IPv4 netmask handling")
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

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

> ---
>  dhcp.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/dhcp.c b/dhcp.c
> index 6088886..edab780 100644
> --- a/dhcp.c
> +++ b/dhcp.c
> @@ -335,7 +335,7 @@ int dhcp(const struct ctx *c, const struct pool *p)
>  	     m->chaddr[3], m->chaddr[4], m->chaddr[5]);
>  
>  	m->yiaddr = c->ip4.addr;
> -	mask.s_addr = htonl(0xffffffff << c->ip4.prefix_len);
> +	mask.s_addr = htonl(0xffffffff << (32 - c->ip4.prefix_len));
>  	memcpy(opts[1].s,  &mask,        sizeof(mask));
>  	memcpy(opts[3].s,  &c->ip4.gw,   sizeof(c->ip4.gw));
>  	memcpy(opts[54].s, &c->ip4.gw,   sizeof(c->ip4.gw));

-- 
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

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