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

[Anshu Kumari <anskuma@redhat.com>]

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>
---
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