Re: [PATCH v3 5/6] dhcp: Add option overload

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

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On Mon, Jun 01, 2026 at 01:07:55PM +0530, Anshu Kumari wrote:
> A user can enter lots of options in command-line which may not fit in
> existing buffer, So when the options field is full, overflow remaining
> DHCP options into the file and sname fields per RFC 2132 option 52.
> 
> Also, when the file field is not used for overload, copy the boot
> file URL there directly for legacy PXE clients.
> 
> Link: https://bugs.passt.top/show_bug.cgi?id=192
> Signed-off-by: Anshu Kumari <anskuma@redhat.com>

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

For the code proper.  The commit message is perhaps buries the lede in
that in that this is also the patch that actually starts putting the
custom_opts into DHCP replies.  Mind you, that would change if we
parsed directly into opts[], which might be nicer anyway.


> ---
> v3:
>   - Added RFC 2132 Section 9.3 reference comment on overload
>     constants.
>   - Use ARRAY_SIZE(opts) instead of raw 255 in fill_overflow().
>   - Swapped overflow order: try sname (64 bytes) first, then file
>     (128 bytes) — better packing and keeps file field available for
>     boot file name.
>   - Removed '&' from &reply.file.
>   - Removed '+1' from memcpy — reply.file already zeroed.
>   - opt_set_dns_search() max_len: OPT_MAX - 3 instead of
>     sizeof(m->o).
> 
> v2:
>   - Added #define DHCP_OVERLOAD_FILE and #define DHCP_OVERLOAD_SNAME constants
>   - Added comment documenting space reservation: /* Reserve 3 bytes for option 52 */
>   - Fixed DNS search length: sizeof(m->o) only, not combined with file+sname
>   - Removed dhcp_boot references — reply.file copy now reads from opts[67]
>   - Used DHCP_OVERLOAD_FILE constant in reply.file guard
> ---
>  dhcp.c | 91 +++++++++++++++++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 81 insertions(+), 10 deletions(-)
> 
> diff --git a/dhcp.c b/dhcp.c
> index 5c6a492..a9d56fd 100644
> --- a/dhcp.c
> +++ b/dhcp.c
> @@ -372,14 +372,64 @@ static bool fill_one(uint8_t *buf, size_t size, int o, int *offset)
>  	return false;
>  }
>  
> +/* RFC 2132, Section 9.3 - Option Overload*/
> +#define DHCP_OVERLOAD_FILE	1
> +#define DHCP_OVERLOAD_SNAME	2
> +
>  /**
> - * fill() - Fill options in message
> + * fill_overflow() - Fill remaining options into sname and file fields
> + * @m:		Message whose sname/file fields may be used for overflow
> + *
> + * Try the smaller sname field first: small options go there, leaving
> + * the larger file field available for big options and for the boot
> + * file name (option 67) if set.
> + *
> + * Return: option 52 overload value: 0 if no overflow,
> + *         DHCP_OVERLOAD_FILE for file, DHCP_OVERLOAD_SNAME for sname,
> + *         or both OR'd together
> + */
> +static int fill_overflow(struct msg *m)
> +{
> +	int sname_off = 0, file_off = 0, overload = 0;
> +	int o;
> +
> +	for (o = 0; (size_t)o < ARRAY_SIZE(opts); o++) {
> +		if (opts[o].slen == -1 || opts[o].sent)
> +			continue;
> +		fill_one(m->sname, sizeof(m->sname) - 1, o, &sname_off);
> +	}
> +
> +	for (o = 0; (size_t)o < ARRAY_SIZE(opts); o++) {
> +		if (opts[o].slen == -1 || opts[o].sent)
> +			continue;
> +		if (fill_one(m->file, sizeof(m->file) - 1, o, &file_off))
> +			debug("DHCP: skipping option %i (overload full)", o);
> +	}
> +
> +	if (sname_off) {
> +		m->sname[sname_off] = 255;
> +		overload |= DHCP_OVERLOAD_SNAME;
> +	}
> +
> +	if (file_off) {
> +		m->file[file_off] = 255;
> +		overload |= DHCP_OVERLOAD_FILE;
> +	}
> +
> +	return overload;
> +}
> +
> +/**
> + * fill() - Fill options in message, with overload into file/sname if needed
>   * @m:		Message to fill
> + * @overload:	Set to option 52 value (0 if none, 1/2/3 per RFC 2132)
>   *
>   * Return: current size of options field
>   */
> -static int fill(struct msg *m)
> +static int fill(struct msg *m, int *overload)
>  {
> +	/* Reserve 3 bytes for option 52 (overload) if needed */
> +	size_t size = OPT_MAX - 3;
>  	int i, o, offset = 0;
>  
>  	for (o = 0; o < 255; o++)
> @@ -390,20 +440,25 @@ static int fill(struct msg *m)
>  	 * Put it there explicitly, unless requested via option 55.
>  	 */
>  	if (opts[55].clen > 0 && !memchr(opts[55].c, 53, opts[55].clen))
> -		if (fill_one(m->o, OPT_MAX, 53, &offset))
> -			 debug("DHCP: skipping option 53");
> +		fill_one(m->o, size, 53, &offset);
>  
>  	for (i = 0; i < opts[55].clen; i++) {
>  		o = opts[55].c[i];
>  		if (opts[o].slen != -1)
> -			if (fill_one(m->o, OPT_MAX, o, &offset))
> -				debug("DHCP: skipping option %i", o);
> +			fill_one(m->o, size, o, &offset);
>  	}
>  
>  	for (o = 0; o < 255; o++) {
>  		if (opts[o].slen != -1 && !opts[o].sent)
> -			if (fill_one(m->o, OPT_MAX, o, &offset))
> -				debug("DHCP: skipping option %i", o);
> +			fill_one(m->o, size, o, &offset);
> +	}
> +
> +	*overload = fill_overflow(m);
> +
> +	if (*overload) {
> +		m->o[offset++] = 52;
> +		m->o[offset++] = 1;
> +		m->o[offset++] = *overload;
>  	}
>  
>  	m->o[offset++] = 255;
> @@ -528,6 +583,7 @@ int dhcp(const struct ctx *c, struct iov_tail *data)
>  	struct msg const *m;
>  	struct msg reply;
>  	unsigned int i;
> +	int overload;
>  
>  	eh = IOV_REMOVE_HEADER(data, eh_storage);
>  	iph = IOV_PEEK_HEADER(data, iph_storage);
> @@ -677,9 +733,24 @@ int dhcp(const struct ctx *c, struct iov_tail *data)
>  	}
>  
>  	if (!c->no_dhcp_dns_search)
> -		opt_set_dns_search(c, sizeof(m->o));
> +		opt_set_dns_search(c, OPT_MAX - 3);
> +
> +	for (i = 0; i < (unsigned int)c->custom_opts_count; i++) {
> +		uint8_t code = c->custom_opts[i].code;
> +
> +		opts[code].slen = c->custom_opts[i].len;
> +		memcpy(opts[code].s, c->custom_opts[i].val,
> +		       c->custom_opts[i].len);
> +	}
> +
> +	dlen = offsetof(struct msg, o) + fill(&reply, &overload);
>  
> -	dlen = offsetof(struct msg, o) + fill(&reply);
> +	/* Copy boot file name into the file field for legacy PXE clients,
> +	 * unless the file field is already used for option overload.
> +	 */
> +	if (!(overload & DHCP_OVERLOAD_FILE) &&
> +	    opts[67].slen > 0 && (size_t)opts[67].slen < sizeof(reply.file))
> +		memcpy(reply.file, opts[67].s, opts[67].slen);
>  
>  	if (m->flags & FLAG_BROADCAST)
>  		dst = in4addr_broadcast;
> -- 
> 2.54.0
> 

-- 
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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