Re: [PATCH 1/3] dhcp: Use -1 as "missing option" length instead of 0

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

[-- Attachment #1: Type: text/plain, Size: 3468 bytes --]

On Mon, Nov 25, 2024 at 01:04:21AM +0100, Stefano Brivio wrote:
> We want to add support for option 80 (Rapid Commit, RFC 4039), whose
> length is 0.
> 
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
> ---
>  dhcp.c | 27 ++++++++++++++++++++-------
>  1 file changed, 20 insertions(+), 7 deletions(-)
> 
> diff --git a/dhcp.c b/dhcp.c
> index a06f143..2fe4a4d 100644
> --- a/dhcp.c
> +++ b/dhcp.c
> @@ -36,9 +36,9 @@
>  /**
>   * struct opt - DHCP option
>   * @sent:	Convenience flag, set while filling replies
> - * @slen:	Length of option defined for server
> + * @slen:	Length of option defined for server, -1 if not going to be sent
>   * @s:		Option payload from server
> - * @clen:	Length of option received from client
> + * @clen:	Length of option received from client, -1 if not received
>   * @c:		Option payload from client
>   */
>  struct opt {
> @@ -154,17 +154,17 @@ static int fill(struct msg *m)
>  	 * option 53 at the beginning of the list.
>  	 * Put it there explicitly, unless requested via option 55.
>  	 */
> -	if (!memchr(opts[55].c, 53, opts[55].clen))
> +	if (opts[55].clen > 0 && !memchr(opts[55].c, 53, opts[55].clen))
>  		fill_one(m, 53, &offset);
>  
>  	for (i = 0; i < opts[55].clen; i++) {
>  		o = opts[55].c[i];
> -		if (opts[o].slen)
> +		if (opts[o].slen != -1)
>  			fill_one(m, o, &offset);
>  	}
>  
>  	for (o = 0; o < 255; o++) {
> -		if (opts[o].slen && !opts[o].sent)
> +		if (opts[o].slen != -1 && !opts[o].sent)
>  			fill_one(m, o, &offset);
>  	}
>  
> @@ -264,6 +264,9 @@ static void opt_set_dns_search(const struct ctx *c, size_t max_len)
>  						 ".\xc0");
>  		}
>  	}
> +
> +	if (!opts[119].slen)
> +		opts[119].slen = -1;
>  }
>  
>  /**
> @@ -313,6 +316,13 @@ int dhcp(const struct ctx *c, const struct pool *p)
>  
>  	offset += offsetof(struct msg, o);
>  
> +	for (i = 0; i < ARRAY_SIZE(opts); i++) {
> +		if (!opts[i].slen)
> +			opts[i].slen = -1;
> +
> +		opts[i].clen = -1;
> +	}

Could this move to dhcp_init()?  I think there you wouldn't need test
and could unconditionally initialize all the lengths to -1 before
initializing the options we actually use.

>  	while (opt_off + 2 < opt_len) {
>  		const uint8_t *olen, *val;
>  		uint8_t *type;
> @@ -334,8 +344,9 @@ int dhcp(const struct ctx *c, const struct pool *p)
>  	if (opts[53].c[0] == DHCPDISCOVER) {
>  		info("DHCP: offer to discover");
>  		opts[53].s[0] = DHCPOFFER;
> -	} else if (opts[53].c[0] == DHCPREQUEST || !opts[53].clen) {
> -		info("%s: ack to request", opts[53].clen ? "DHCP" : "BOOTP");
> +	} else if (opts[53].c[0] == DHCPREQUEST || opts[53].clen <= 0) {
> +		info("%s: ack to request",
> +		     (opts[53].clen <= 0) ? "DHCP" : "BOOTP");

Should this be <= 0, or < 0?  i.e. Wouldn't even an empty option 53
indicate we're dealing with DHCP rather than BOOTP?

>  		opts[53].s[0] = DHCPACK;
>  	} else {
>  		return -1;
> @@ -374,6 +385,8 @@ int dhcp(const struct ctx *c, const struct pool *p)
>  		((struct in_addr *)opts[6].s)[i] = c->ip4.dns[i];
>  		opts[6].slen += sizeof(uint32_t);
>  	}
> +	if (!opts[6].slen)
> +		opts[6].slen = -1;
>  
>  	if (!c->no_dhcp_dns_search)
>  		opt_set_dns_search(c, sizeof(m->o));

-- 
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

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 833 bytes --]