[PATCH v6 6/6] dhcp: Add RFC 3396 option splitting for concatenation-requiring options

[Anshu Kumari <anskuma@redhat.com>]

Implement option splitting per RFC 3396 for options that may exceed
255 bytes.  A new concat_req[] lookup table marks options requiring
concatenation (currently option 81, Client FQDN per RFC 4702).

The opts[].s buffer is resized from 255 to 497 bytes
(OPT_CONCAT_MAX) to hold the maximum data that can be split
across the options field, file field, and sname field.

When a concatenation-requiring option does not fit as a single
option in any field, fill() calls fill_split() to split it across
fields in RFC 3396 order: options field first, then file, then
sname.

Link: https://bugs.passt.top/show_bug.cgi?id=192
Signed-off-by: Anshu Kumari <anskuma@redhat.com>
---
v6:
- Merged v5 patches 6/7 and 7/7 into a single patch.
- Replaced DHCP_OPT_STR_CONCAT enum value and is_concat_opt()
  helper with a concat_req[] boolean lookup table.
- Used MIN() macro instead of ternary for chunk size.
- Fixed space calculation to account for 2-byte code+length
  overhead per chunk.

v5:
  - New patch: implement option splitting per RFC 3396 for options exceeding 255 bytes
  - Add DHCP_OPT_STR_CONCAT type, is_concat_opt(), fill_split() helpers
  - Resize opts[].s from 255 to OPT_CONCAT_MAX (497) bytes
  - Add /* fallthrough */ between DHCP_OPT_STR and DHCP_OPT_STR_CONCAT case
---
 dhcp.c | 99 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 98 insertions(+), 1 deletion(-)

diff --git a/dhcp.c b/dhcp.c
index 43ce133..eda04d1 100644
--- a/dhcp.c
+++ b/dhcp.c
@@ -35,6 +35,11 @@
 #include "dhcp.h"
 #include "parse.h"
 
+/* RFC 3396: maximum option data that can be split across options field,
+ * file field, and sname field (minus code+length overhead per portion).
+ */
+#define OPT_CONCAT_MAX	497
+
 /**
  * enum opt_state - DHCP option state
  * @OPT_UNSET:		Option not configured
@@ -59,7 +64,7 @@ enum opt_state {
 struct opt {
 	int sent;
 	int slen;
-	uint8_t s[255];
+	uint8_t s[OPT_CONCAT_MAX];
 	int clen;
 	uint8_t c[255];
 	enum opt_state state;
@@ -215,6 +220,13 @@ static const enum dhcp_opt_type dhcp_opt_types[] = {
 	[252] = DHCP_OPT_STR,		/* WPAD URL */
 };
 
+/**
+ * concat_req - Options requiring RFC 3396 concatenation, indexed by code
+ */
+static const bool concat_req[256] = {
+	[81] = true,	/* Client FQDN (RFC 4702, Section 2) */
+};
+
 /**
  * dhcp_opt_parse() - Parse a DHCP option value
  * @code:	DHCP option code
@@ -309,6 +321,9 @@ static int dhcp_opt_parse(uint8_t code, const char *str,
 	case DHCP_OPT_STR:
 		slen = strlen(str);
 
+		if (!concat_req[code] && slen > 255)
+			return -1;
+
 		if (slen >= buf_len)
 			return -1;
 
@@ -442,6 +457,40 @@ enum dhcp_overload {
 	DHCP_OVERLOAD_SNAME = 2,
 };
 
+/**
+ * fill_split() - Write a split portion of an option into a buffer
+ * @buf:	Buffer to write into
+ * @size:	Usable size of @buf
+ * @o:		Option number (code)
+ * @offset:	Current offset within @buf, updated on write
+ * @data:	Pointer to remaining option data to write
+ * @remaining:	Bytes of option data still to write
+ *
+ * Return: number of data bytes written (excluding code+length header)
+ */
+static size_t fill_split(uint8_t *buf, size_t size, int o, int *offset,
+			 const uint8_t *data, size_t remaining)
+{
+	size_t avail, chunk;
+
+	if (*offset + 2 >= (int)size)
+		return 0;
+
+	avail = size - *offset - 2;
+	chunk = MIN(remaining, avail);
+	if (!chunk)
+		return 0;
+
+	buf[*offset] = o;
+	buf[*offset + 1] = chunk;
+	*offset += 2;
+
+	memcpy(buf + *offset, data, chunk);
+	*offset += chunk;
+
+	return chunk;
+}
+
 /**
  * fill() - Fill options in message, with overload into file/sname if needed
  * @m:			Message to fill
@@ -495,6 +544,54 @@ static int fill(struct msg *m, enum dhcp_overload *overload, bool has_bootfile)
 		}
 	}
 
+	/* RFC 3396: split concatenation-requiring options that didn't fit
+	 * as a single option.  Split order: options, file, sname.
+	 */
+	for (o = 0; (size_t)o < ARRAY_SIZE(opts); o++) {
+		size_t file_cap, sname_cap, total, written;
+
+		if (opts[o].state == OPT_UNSET || opts[o].sent ||
+		    !concat_req[o])
+			continue;
+
+		sname_cap = sizeof(m->sname) - 1 > (size_t)sname_off ?
+			    sizeof(m->sname) - 1 - sname_off : 0;
+
+		if (has_bootfile || sizeof(m->file) - 1 <= (size_t)file_off)
+			file_cap = 0;
+		else
+			file_cap = sizeof(m->file) - 1 - file_off;
+
+		total = (size > (size_t)offset ? size - offset - 2 : 0)
+			+ (file_cap > 2 ? file_cap - 2 : 0)
+			+ (sname_cap > 2 ? sname_cap - 2 : 0);
+
+		if (total < (size_t)opts[o].slen) {
+			debug("DHCP: skipping option %i (no space to split)",
+			      o);
+			continue;
+		}
+
+		written = 0;
+		written += fill_split(m->o, size, o, &offset,
+				      opts[o].s, opts[o].slen);
+		if (written < (size_t)opts[o].slen && !has_bootfile)
+			written += fill_split(m->file,
+					      sizeof(m->file) - 1, o,
+					      &file_off,
+					      opts[o].s + written,
+					      opts[o].slen - written);
+		if (written < (size_t)opts[o].slen)
+			written += fill_split(m->sname,
+					      sizeof(m->sname) - 1, o,
+					      &sname_off,
+					      opts[o].s + written,
+					      opts[o].slen - written);
+
+		if (written >= (size_t)opts[o].slen)
+			opts[o].sent = 1;
+	}
+
 	/* Report any options that could not be sent */
 	for (o = 0; (size_t)o < ARRAY_SIZE(opts); o++) {
 		if (opts[o].state != OPT_UNSET && !opts[o].sent)
-- 
2.55.0