Re: [PATCH v7 6/7] dhcp: Add option overload

[Stefano Brivio <sbrivio@redhat.com>]

On Thu,  1 Oct 2026 18:45:59 +0530
Anshu Kumari <anskuma@redhat.com> wrote:

> When the options field is full, overflow remaining DHCP options into
> the sname and file fields per RFC 2132 option 52.
> 
> Per RFC 2132, Section 9.5, the boot file name is always placed in the
> 'file' header field.  When a boot file is set, the file field is
> reserved from overload and overflow uses only the sname field.
> 
> Link: https://bugs.passt.top/show_bug.cgi?id=192
> Signed-off-by: Anshu Kumari <anskuma@redhat.com>
> ---
> 
> v7:
>   - Removed overload function parameter
>   - Added foreach_opt() macro to iterate option table
> 
> Till v6 both "patch 5/7" and "patch 6/7" were one.
> ---
>  dhcp.c | 78 ++++++++++++++++++++++++++++++++++++++++++++++++++--------
>  1 file changed, 68 insertions(+), 10 deletions(-)
> 
> diff --git a/dhcp.c b/dhcp.c
> index 6f69fd62..63ac8e3d 100644
> --- a/dhcp.c
> +++ b/dhcp.c
> @@ -67,6 +67,8 @@ struct opt {
>  
>  static struct opt opts[256];
>  
> +#define foreach_opt(o)	for ((o) = 0; (size_t)(o) < ARRAY_SIZE(opts); (o)++)
> +
>  #define DHCPDISCOVER	1
>  #define DHCPOFFER	2
>  #define DHCPREQUEST	3
> @@ -440,13 +442,30 @@ static void fill_one(uint8_t *buf, size_t size, int o, int *offset)
>  }
>  
>  /**
> - * fill() - Fill options in message
> - * @m:		Message to fill
> + * enum dhcp_overload - DHCP option overload values (RFC 2132, Section 9.3)
> + * @DHCP_OVERLOAD_NONE:		No overload
> + * @DHCP_OVERLOAD_FILE:		file field carries options
> + * @DHCP_OVERLOAD_SNAME:	sname field carries options
> + */
> +enum dhcp_overload {
> +	DHCP_OVERLOAD_NONE  = 0,
> +	DHCP_OVERLOAD_FILE  = 1,
> +	DHCP_OVERLOAD_SNAME = 2,
> +};
> +
> +/**
> + * fill() - Fill options in message, with overload into file/sname if needed
> + * @m:			Message to fill
> + * @has_bootfile:	Reserve file field for boot file name
>   *
>   * Return: current size of options field
>   */
> -static int fill(struct msg *m)
> +static int fill(struct msg *m, bool has_bootfile)
>  {
> +	enum dhcp_overload overload = DHCP_OVERLOAD_NONE;
> +	int sname_off = 0, file_off = 0;
> +	/* 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++)
> @@ -457,17 +476,54 @@ 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))
> -		fill_one(m->o, OPT_MAX, 53, &offset);
> +		fill_one(m->o, size, 53, &offset);
>  
>  	for (i = 0; i < opts[55].clen; i++) {
>  		o = opts[55].c[i];
>  		if (opts[o].conf != OPT_UNSET)
> -			fill_one(m->o, OPT_MAX, o, &offset);
> +			fill_one(m->o, size, o, &offset);
>  	}
>  
>  	for (o = 0; o < 255; o++) {
>  		if (opts[o].conf != OPT_UNSET && !opts[o].sent)
> -			fill_one(m->o, OPT_MAX, o, &offset);
> +			fill_one(m->o, size, o, &offset);
> +	}
> +
> +	/* Overflow unsent options into sname, then file */
> +	foreach_opt(o) {

This comes from my suggestion on v6 but those were really subsequent
steps (building a more specialised iterator on top of foreach_opt()),
not alternatives. Look at flow_foreach() and flow_foreach_of_type()
as examples. That is, here you could use:

	foreach_unsent_opt(o)
		fill_one(m->sname, sizeof(m->sname) - 1, o, &sname_off);

with:

#define foreach_unset_opt(o)						     \
	foreach_opt((o))						     \
		/* NOLINTNEXTLINE(readability-inconsistent-ifelse-braces) */ \
		if (opts[(o)].conf != OPT_UNSET && !opts[(o)].sent)

...the option with the reverse condition and "continue; else" could also
work, I'm not sure what's the most practical here. The direct option
looks more... direct, to me.

The two loops below could use this iterator, and perhaps the one above
as well (I haven't tried).

Note that in 7/7 you could probably switch to this other iterator as
well, without the explicit need for the base foreach_opt(o) iterator,
but I would suggest to keep them as two different macros anyway, it's
clearer and more reusable.

> +		if (opts[o].conf == OPT_UNSET || opts[o].sent)
> +			continue;
> +		fill_one(m->sname, sizeof(m->sname) - 1, o, &sname_off);
> +	}
> +
> +	if (!has_bootfile) {
> +		foreach_opt(o) {
> +			if (opts[o].conf == OPT_UNSET || opts[o].sent)
> +				continue;
> +			fill_one(m->file, sizeof(m->file) - 1, o, &file_off);
> +		}
> +	}
> +
> +	/* Report any options that could not be sent */
> +	foreach_opt(o) {
> +		if (opts[o].conf != OPT_UNSET && !opts[o].sent)
> +			debug("DHCP: skipping option %i", 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;
> +	}
> +
> +	if (overload) {
> +		m->o[offset++] = 52;
> +		m->o[offset++] = 1;
> +		m->o[offset++] = overload;
>  	}
>  
>  	m->o[offset++] = 255;
> @@ -761,16 +817,18 @@ int dhcp(const struct ctx *c, struct iov_tail *data)
>  		}
>  	}
>  
> -	if (!c->no_dhcp_dns_search)
> -		opt_set_dns_search(c, sizeof(m->o));
> +	if (!c->no_dhcp_dns_search) {
> +		/* 3 bytes reserved for option 52 (code, length, value) */
> +		opt_set_dns_search(c, OPT_MAX - 3);
> +	}
>  
>  	/* RFC 2132, Section 9.5: put boot file name in the 'file' header
> -	 * field.
> +	 * field.  Reserve the file field from overload.
>  	 */
>  	has_bootfile = opts[67].slen > 0 &&
>  		       (size_t)opts[67].slen < sizeof(reply.file);
>  
> -	dlen = offsetof(struct msg, o) + fill(&reply);
> +	dlen = offsetof(struct msg, o) + fill(&reply, has_bootfile);
>  
>  	if (has_bootfile)
>  		memcpy(reply.file, opts[67].s, opts[67].slen);

-- 
Stefano