Re: [PATCH v2 3/6] dhcp: Add option type table and value parser

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

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On Tue, May 26, 2026 at 06:01:10PM +0530, Anshu Kumari wrote:
> Add an RFC 2132 type lookup table mapping DHCP option codes to their
> expected value formats, and a dhcp_opt_parse() function that converts
> CLI string values into their binary wire representation.
> 
> Wire dhcp_opt_parse() into the --dhcp-opt handler so that values are
> validated and encoded at configuration time.
> 
> Link: https://bugs.passt.top/show_bug.cgi?id=192
> Signed-off-by: Anshu Kumari <anskuma@redhat.com>

Laurent's comments seconded, a few other ones here.

> ---
> v2:
>   - Replaced struct lookup table + dhcp_opt_type_lookup() function with flat dhcp_opt_types[256] array indexed by code.
>   - Consolidated DHCP_OPT_UINT8/UINT16/UINT32 into single DHCP_OPT_INTEGER with dhcp_opt_int_width[256] table.
>   - Dropped DHCP_OPT_ROUTES / option 121 entirely.
>   - Added kerneldoc for enum dhcp_opt_type values.
>   - Removed curly braces from switch cases, declarations before switch.
>   - Added newlines before return statements.
>   - Changed IP list delimiter from space to comma (--dhcp-opt 6,1.1.1.1,8.8.8.8).
>   - Defined DHCP_OPT_PARSE_BUF constant for bare 1024.
>   - Added len and val[255] fields to struct here (moved from patch 1).
>   - Added kerneldoc for @custom_opts.len and @custom_opts.val.
>   - Wired dhcp_opt_parse() into case 32 (--dhcp-boot) to populate val/len.
> ---
>  conf.c  |  16 ++++++
>  dhcp.c  | 148 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  dhcp.h  |  17 +++++++
>  passt.h |   4 ++
>  4 files changed, 185 insertions(+)
> 
> diff --git a/conf.c b/conf.c
> index ae8ee26..3a5f45d 100644
> --- a/conf.c
> +++ b/conf.c
> @@ -1266,6 +1266,7 @@ void conf(struct ctx *c, int argc, char **argv)
>  	char *end;
>  	uid_t uid;
>  	gid_t gid;
> +	int len;
>  	
>  
>  	if (c->mode == MODE_PASTA)
> @@ -1482,7 +1483,14 @@ void conf(struct ctx *c, int argc, char **argv)
>  				die("Too many DHCP options (max %d)",
>  				    MAX_CUSTOM_DHCP_OPTS);
>  
> +			ret = dhcp_opt_parse(67, optarg,
> +					     c->custom_opts[c->custom_opts_count].val,
> +					     sizeof(c->custom_opts[0].val));
> +			if (ret < 0)
> +				die("Invalid boot file value: %s", optarg);
> +
>  			c->custom_opts[c->custom_opts_count].code = 67;
> +			c->custom_opts[c->custom_opts_count].len = ret;
>  			if (snprintf_check(c->custom_opts[c->custom_opts_count].str,
>  					   sizeof(c->custom_opts[0].str),
>  					   "%s", optarg))
> @@ -1503,7 +1511,15 @@ void conf(struct ctx *c, int argc, char **argv)
>  				die("Too many --dhcp-opt entries (max %d)",
>  				    MAX_CUSTOM_DHCP_OPTS);
>  
> +			len = dhcp_opt_parse(optcode, comma + 1,
> +					     c->custom_opts[c->custom_opts_count].val,
> +					     sizeof(c->custom_opts[0].val));
> +			if (len < 0)
> +				die("Invalid value for DHCP option %lu: %s",
> +				    optcode, comma + 1);
> +
>  			c->custom_opts[c->custom_opts_count].code = optcode;
> +			c->custom_opts[c->custom_opts_count].len = len;
>  			if (snprintf_check(c->custom_opts[c->custom_opts_count].str,
>  					   sizeof(c->custom_opts[0].str),
>  					   "%s", comma + 1))
> diff --git a/dhcp.c b/dhcp.c
> index 1ff8cba..e1c95ad 100644
> --- a/dhcp.c
> +++ b/dhcp.c
> @@ -33,6 +33,154 @@
>  #include "log.h"
>  #include "dhcp.h"
>  
> +/**
> + * dhcp_opt_types - Maps option code to RFC 2132 value type, indexed by code
> + */
> +static const enum dhcp_opt_type dhcp_opt_types[256] = {
> +	[1]  = DHCP_OPT_IPV4,		/* Subnet Mask */
> +	[2]  = DHCP_OPT_INTEGER,	/* Time Offset */
> +	[3]  = DHCP_OPT_IPV4_LIST,	/* Router */
> +	[4]  = DHCP_OPT_IPV4_LIST,	/* Time Server */
> +	[5]  = DHCP_OPT_IPV4_LIST,	/* Name Server */
> +	[6]  = DHCP_OPT_IPV4_LIST,	/* Domain Name Server */
> +	[7]  = DHCP_OPT_IPV4_LIST,	/* Log Server */
> +	[8]  = DHCP_OPT_IPV4_LIST,	/* Cookie Server */
> +	[9]  = DHCP_OPT_IPV4_LIST,	/* LPR Server */
> +	[10] = DHCP_OPT_IPV4_LIST,	/* Impress Server */
> +	[11] = DHCP_OPT_IPV4_LIST,	/* Resource Location Server */
> +	[12] = DHCP_OPT_STR,		/* Host Name */
> +	[13] = DHCP_OPT_INTEGER,	/* Boot File Size */
> +	[15] = DHCP_OPT_STR,		/* Domain Name */
> +	[16] = DHCP_OPT_IPV4,		/* Swap Server */
> +	[17] = DHCP_OPT_STR,		/* Root Path */
> +	[19] = DHCP_OPT_INTEGER,	/* IP Forwarding */
> +	[23] = DHCP_OPT_INTEGER,	/* Default IP TTL */
> +	[26] = DHCP_OPT_INTEGER,	/* Interface MTU */
> +	[28] = DHCP_OPT_IPV4,		/* Broadcast Address */
> +	[33] = DHCP_OPT_IPV4_LIST,	/* Static Routes */
> +	[37] = DHCP_OPT_INTEGER,	/* TCP Default TTL */
> +	[38] = DHCP_OPT_INTEGER,	/* TCP Keepalive Interval */
> +	[40] = DHCP_OPT_STR,		/* NIS Domain Name */
> +	[41] = DHCP_OPT_IPV4_LIST,	/* NIS Servers */
> +	[42] = DHCP_OPT_IPV4_LIST,	/* NTP Servers */
> +	[44] = DHCP_OPT_IPV4_LIST,	/* NetBIOS Name Server */
> +	[50] = DHCP_OPT_IPV4,		/* Requested IP Address */
> +	[51] = DHCP_OPT_INTEGER,	/* IP Address Lease Time */
> +	[53] = DHCP_OPT_INTEGER,	/* DHCP Message Type */
> +	[54] = DHCP_OPT_IPV4,		/* Server Identifier */
> +	[55] = DHCP_OPT_STR,		/* Parameter Request List */
> +	[57] = DHCP_OPT_INTEGER,	/* Max DHCP Message Size */
> +	[58] = DHCP_OPT_INTEGER,	/* Renewal (T1) Time */
> +	[59] = DHCP_OPT_INTEGER,	/* Rebinding (T2) Time */
> +	[60] = DHCP_OPT_STR,		/* Vendor Class Identifier */
> +	[61] = DHCP_OPT_STR,		/* Client Identifier */
> +	[66] = DHCP_OPT_STR,		/* TFTP Server Name */
> +	[67] = DHCP_OPT_STR,		/* Bootfile Name */
> +	[119] = DHCP_OPT_STR,		/* Domain Search List */
> +	[252] = DHCP_OPT_STR,		/* WPAD URL */
> +};
> +
> +/**
> + * dhcp_opt_int_width - Integer width in bytes for INTEGER-typed options
> + */
> +static const uint8_t dhcp_opt_int_width[256] = {

I know I suggested this approach, but now seeing a whole array[256] it
seems unpleasantly bulky.  I'd suggest instead encoding the width into
some bits of the DHCP_OPT_* constants.

> +	[2]  = 4,	/* Time Offset */
> +	[13] = 2,	/* Boot File Size */
> +	[19] = 1,	/* IP Forwarding */
> +	[23] = 1,	/* Default IP TTL */
> +	[26] = 2,	/* Interface MTU */
> +	[37] = 1,	/* TCP Default TTL */
> +	[38] = 4,	/* TCP Keepalive Interval */
> +	[51] = 4,	/* IP Address Lease Time */
> +	[53] = 1,	/* DHCP Message Type */
> +	[57] = 2,	/* Max DHCP Message Size */
> +	[58] = 4,	/* Renewal (T1) Time */
> +	[59] = 4,	/* Rebinding (T2) Time */
> +};
> +
> +#define DHCP_OPT_PARSE_BUF	1024
> +
> +/**
> + * dhcp_opt_parse() - Parse a DHCP option value
> + * @code:	DHCP option code
> + * @str:	DHCP Value string from command line
> + * @buf:	Output buffer
> + * @buf_len:	Size of output buffer
> + *
> + * Return: number of bytes written to @buf, or -1 on error
> + */
> +int dhcp_opt_parse(uint8_t code, const char *str, uint8_t *buf, size_t buf_len)
> +{
> +	enum dhcp_opt_type type = dhcp_opt_types[code];
> +	char tmp[DHCP_OPT_PARSE_BUF];
> +	char *tok, *saveptr, *end;
> +	struct in_addr addr;
> +	unsigned long val;
> +	unsigned int i;
> +	uint8_t width;
> +	size_t slen;
> +	int len;
> +
> +	switch (type) {
> +	case DHCP_OPT_NONE:
> +		die("Unsupported DHCP option: %u,"
> +		    " see passt(1) for supported codes", code);
> +	case DHCP_OPT_IPV4:
> +		if (inet_pton(AF_INET, str, &addr) != 1)
> +			return -1;
> +
> +		if (buf_len < sizeof(addr))
> +			return -1;
> +
> +		memcpy(buf, &addr, sizeof(addr));
> +
> +		return sizeof(addr);
> +	case DHCP_OPT_IPV4_LIST:

I think it should be possible to share more logic between the single
IPv4 and IPv4 list options (parse them both as a list, but error if
there's >1 in the single case).

> +		len = 0;
> +
> +		if (snprintf_check(tmp, sizeof(tmp), "%s", str))
> +			return -1;
> +
> +		for (tok = strtok_r(tmp, ",", &saveptr); tok;
> +		     tok = strtok_r(NULL, ",", &saveptr)) {
> +			if (inet_pton(AF_INET, tok, &addr) != 1)
> +				return -1;
> +
> +			if (len + (int)sizeof(addr) > (int)buf_len)
> +				return -1;
> +
> +			memcpy(buf + len, &addr, sizeof(addr));
> +			len += sizeof(addr);
> +		}
> +		return len;
> +	case DHCP_OPT_INTEGER:
> +		width = dhcp_opt_int_width[code];
> +		val = strtoul(str, &end, 0);

Most strtoul() errors can be discerned by checking end, but not all.
You also need to explicitly set errno to 0 beforehand and check it
again afterwards (clunky, but that's the interface we have).

> +
> +		if (*end || buf_len < width)
> +			return -1;
> +
> +		if (width < 4 && val >= (1UL << (width * 8)))

strtoul() will parse 64-bit values on most current platforms, so you
need this range enforcement for width==4 as well.

> +			return -1;
> +
> +		for (i = 0; i < width; i++)
> +			buf[i] = (val >> ((width - 1 - i) * 8)) & 0xff;

Might be easier to follow as:
	for (...) {
		buf[i] = val & 0xff;
		val >>= 8;
	}

> +
> +		return width;
> +	case DHCP_OPT_STR:
> +		slen = strlen(str);
> +
> +		if (!slen || slen >= buf_len)
> +			return -1;
> +
> +		strncpy((char *)buf, str, buf_len);

You've already know the length so you can make this a memcpy().

> +
> +		return slen;
> +	}
> +
> +	return -1;
> +}
> +
>  /**
>   * struct opt - DHCP option
>   * @sent:	Convenience flag, set while filling replies
> diff --git a/dhcp.h b/dhcp.h
> index cd50c99..3da571c 100644
> --- a/dhcp.h
> +++ b/dhcp.h
> @@ -6,7 +6,24 @@
>  #ifndef DHCP_H
>  #define DHCP_H
>  
> +/**
> + * enum dhcp_opt_type - DHCP option value types per RFC 2132
> + * @DHCP_OPT_NONE:	Unsupported or unknown option
> + * @DHCP_OPT_STR:	Variable-length string
> + * @DHCP_OPT_IPV4:	Single IPv4 address
> + * @DHCP_OPT_IPV4_LIST:Multiple IPv4 addresses, comma-separated
> + * @DHCP_OPT_INTEGER:	Unsigned integer (1, 2, or 4 bytes)
> + */
> +enum dhcp_opt_type {
> +	DHCP_OPT_NONE,
> +	DHCP_OPT_STR,
> +	DHCP_OPT_IPV4,
> +	DHCP_OPT_IPV4_LIST,
> +	DHCP_OPT_INTEGER,
> +};

This can move to dhcp.c, since it's not used in any other files.

> +
>  int dhcp(const struct ctx *c, struct iov_tail *data);
>  void dhcp_init(void);
> +int dhcp_opt_parse(uint8_t code, const char *str, uint8_t *buf, size_t buf_len);
>  
>  #endif /* DHCP_H */
> diff --git a/passt.h b/passt.h
> index 3a0816f..751fee3 100644
> --- a/passt.h
> +++ b/passt.h
> @@ -184,6 +184,8 @@ struct ip6_ctx {
>   * @fqdn:		Guest FQDN
>   * @custom_opts:	User-specified DHCP options from --dhcp-opt
>   * @custom_opts.code:	DHCP option code
> + * @custom_opts.len:	Length of binary value in @val
> + * @custom_opts.val:	Binary-encoded option value
>   * @custom_opts.str:	Original string value from command line
>   * @custom_opts_count:	Number of entries in @custom_opts
>   * @ifi6:		Template interface for IPv6, -1: none, 0: IPv6 disabled
> @@ -271,6 +273,8 @@ struct ctx {
>  
>  	struct {
>  		uint8_t code;
> +		uint8_t len;
> +		uint8_t val[255];
>  		char str[256];
>  	} custom_opts[MAX_CUSTOM_DHCP_OPTS];
>  	int custom_opts_count;
> -- 
> 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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