Re: [PATCH v3 3/6] dhcp: Add option type table and value parser

On Mon, Jun 01, 2026 at 01:07:53PM +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 dhcp_add_option() 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 --- v3: - Replaced DHCP_OPT_INTEGER with separate DHCP_OPT_INT8/INT16/INT32 enums, removed dhcp_opt_int_width[] array. - Shared logic between DHCP_OPT_IPV4 and DHCP_OPT_IPV4_LIST — parse both as list, error if >1 in single case. - Added errno = 0 before strtoul() and check after. - Fixed range check: 1ULL << (width * 8) for all widths including width==4. - strncpy → memcpy for DHCP_OPT_STR. - Moved enum to dhcp.c since not used in other files. - Removed options 55, 61 (client-only), 119 (DNS compression, use --dhcp-search instead), 33 (IP pairs not supported). - DHCP_OPT_PARSE_BUF 1024 → char tmp[256]. - Upgraded dhcp_add_option() to call dhcp_opt_parse() and populate val[]/len. - Aligned array entries for readability. - Added tab after @DHCP_OPT_IPV4_LIST: in kerneldoc. - Reject empty value strings before parsing - Reject leading/trailing/consecutive commas in IP list values.

Thanks for the detailed changelogs, by the way. I know these are a bunch of work to maintain, but they really help when reviewing.

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. --- dhcp.c | 180 +++++++++++++++++++++++++++++++++++++++++++++++++++++++- passt.h | 4 ++ 2 files changed, 181 insertions(+), 3 deletions(-)

diff --git a/dhcp.c b/dhcp.c index c5fbf37..07a42b9 100644 --- a/dhcp.c +++ b/dhcp.c @@ -23,6 +23,7 @@ #include #include #include +#include

#include "util.h" #include "ip.h" @@ -33,6 +34,170 @@ #include "log.h" #include "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_INT8: Unsigned 8-bit integer + * @DHCP_OPT_INT16: Unsigned 16-bit integer + * @DHCP_OPT_INT32: Unsigned 32-bit integer + */ +enum dhcp_opt_type { + DHCP_OPT_NONE, + DHCP_OPT_STR, + DHCP_OPT_IPV4, + DHCP_OPT_IPV4_LIST, + DHCP_OPT_INT8, + DHCP_OPT_INT16, + DHCP_OPT_INT32, +}; + +/** + * 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_INT32, /* Time Offset */ + [3] = DHCP_OPT_IPV4_LIST, /* Router */

I'm still a bit unsure if we want to allow user modification of the options, like this one, which we already manage ourselves.

+ [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_INT16, /* Boot File Size */ + [15] = DHCP_OPT_STR, /* Domain Name */ + [16] = DHCP_OPT_IPV4, /* Swap Server */ + [17] = DHCP_OPT_STR, /* Root Path */ + [19] = DHCP_OPT_INT8, /* IP Forwarding */ + [23] = DHCP_OPT_INT8, /* Default IP TTL */ + [26] = DHCP_OPT_INT16, /* Interface MTU */ + [28] = DHCP_OPT_IPV4, /* Broadcast Address */ + [37] = DHCP_OPT_INT8, /* TCP Default TTL */ + [38] = DHCP_OPT_INT32, /* 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_INT32, /* IP Address Lease Time */ + [53] = DHCP_OPT_INT8, /* DHCP Message Type */ + [54] = DHCP_OPT_IPV4, /* Server Identifier */ + [57] = DHCP_OPT_INT16, /* Max DHCP Message Size */ + [58] = DHCP_OPT_INT32, /* Renewal (T1) Time */ + [59] = DHCP_OPT_INT32, /* Rebinding (T2) Time */ + [60] = DHCP_OPT_STR, /* Vendor Class Identifier */ + [66] = DHCP_OPT_STR, /* TFTP Server Name */ + [67] = DHCP_OPT_STR, /* Bootfile Name */ + [252] = DHCP_OPT_STR, /* WPAD URL */ +}; + +/** + * dhcp_opt_parse() - Parse a DHCP option value + * @code: DHCP option code + * @str: Value string from command line + * @buf: Output buffer for binary value + * @buf_len: Size of output buffer + * + * Return: number of bytes written to @buf, or -1 on error + */ +static 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 *tok, *saveptr, *end; + struct in_addr addr; + unsigned long val; + unsigned int i; + uint8_t width; + char tmp[256]; + size_t slen; + int len; + + if (!*str) + die("Empty value for DHCP option %u", code); + + switch (type) { + case DHCP_OPT_NONE: + die("Unsupported DHCP option: %u," + " see passt(1) for supported codes", code); + case DHCP_OPT_IPV4: + case DHCP_OPT_IPV4_LIST: + len = 0; + + /* Reject empty, leading/trailing, or consecutive commas */ + if (!*str || *str == ',' || str[strlen(str) - 1] == ',' || + strstr(str, ",,")) + return -1; + + if (snprintf_check(tmp, sizeof(tmp), "%s", str)) + return -1;

The arbitrary 256 byte buffer limit here isn't great. The string encoding of an IPv4 address can be nearly 4 times as long as the binary encoding, so we could potentially hit this with a longish, but not ridiculous address list.

+ for (tok = strtok_r(tmp, ",", &saveptr); tok; + tok = strtok_r(NULL, ",", &saveptr)) {

One way to avoid that would be to avoid using strtok_r() which relies in in-place modifying the input. Instead you'd need to repeatedly find the length of the next chunk with strchr() or strcpsn(), then extract each one into a tmp of length INET_ADDRSTRLEN to call inet_pton(). On the plus side, that should naturally deal with the case of extraneous commas (it would show up as an empth entry), rather than requiring an explicit check at the top.

+ if (inet_pton(AF_INET, tok, &addr) != 1) + return -1; + + if (len + (int)sizeof(addr) > (int)buf_len) + return -1;

You could make this check before the inet_pton(), then do the conversion directly into buf, avoiding the addr temporary.

+ + memcpy(buf + len, &addr, sizeof(addr)); + len += sizeof(addr); + + if (type == DHCP_OPT_IPV4) + break; + } + + if (type == DHCP_OPT_IPV4 && strtok_r(NULL, ",", &saveptr)) + return -1; + + return len; + case DHCP_OPT_INT8: + case DHCP_OPT_INT16: + case DHCP_OPT_INT32: + if (type == DHCP_OPT_INT8) + width = 1; + else if (type == DHCP_OPT_INT16) + width = 2; + else + width = 4; + + errno = 0; + val = strtoul(str, &end, 0); + + if (*end || errno) + return -1; + + if (buf_len < width) + return -1; + + if (val >= (1ULL << (width * 8))) + return -1; + + for (i = width; i > 0; i--) { + buf[i - 1] = val & 0xff; + val >>= 8; + } + + return width; + case DHCP_OPT_STR: + slen = strlen(str); + + if (!slen || slen >= buf_len) + return -1; + + memcpy(buf, str, slen);

Do you need to include the terminating \0 here? If so you'll need slen + 1.

+ + return slen; + } + + return -1; +}

/** * dhcp_add_option() - Add or update a custom DHCP option @@ -40,14 +205,15 @@ * @code: DHCP option code * @val_str: Value string from command line * - * If @code was already added, the previous value is overwritten. - * Calls die() on any error. + * Parses @val_str according to the type registered for @code in + * dhcp_opt_types[]. If @code was already added, the previous value + * is overwritten. Calls die() on any error. * * Return: 0 on success */ int dhcp_add_option(struct ctx *c, uint8_t code, const char *val_str) { - int idx; + int idx, ret;

for (idx = 0; idx < c->custom_opts_count; idx++) { if (c->custom_opts[idx].code == code) @@ -61,7 +227,15 @@ int dhcp_add_option(struct ctx *c, uint8_t code, const char *val_str) c->custom_opts_count++; }

+ ret = dhcp_opt_parse(code, val_str, + c->custom_opts[idx].val, + sizeof(c->custom_opts[0].val));

Now that this parsing and adding code is all in dhcp.c, could we parse the options directly into the existing opts[] global, rather than requiring both the string and parsed forms in c->custom_opts?

+ if (ret < 0) + die("Invalid value for DHCP option %u: %s", + code, val_str); + c->custom_opts[idx].code = code; + c->custom_opts[idx].len = ret;

if (snprintf_check(c->custom_opts[idx].str, sizeof(c->custom_opts[0].str), 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