[PATCH v8 5/6] dhcp: Add option overload

[Anshu Kumari <anskuma@redhat.com>]

Change fill_one() to return void because now we use a single loop
to report all unsent options.

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>
---
v8:
  - commit message rewritten - explain about why fill_one() now 
    returns void.
  - foreach_unsent_opt() macro added.
---
 dhcp.c | 93 +++++++++++++++++++++++++++++++++++++++++++---------------
 1 file changed, 69 insertions(+), 24 deletions(-)

diff --git a/dhcp.c b/dhcp.c
index 73aa1a13..04a6d389 100644
--- a/dhcp.c
+++ b/dhcp.c
@@ -67,6 +67,12 @@ struct opt {
 
 static struct opt opts[256];
 
+#define foreach_opt(o)	for ((o) = 0; (size_t)(o) < ARRAY_SIZE(opts); (o)++)
+
+#define foreach_unsent_opt(o)						\
+	foreach_opt((o))						\
+		if (opts[(o)].conf != OPT_UNSET && !opts[(o)].sent)
+
 #define DHCPDISCOVER	1
 #define DHCPOFFER	2
 #define DHCPREQUEST	3
@@ -419,16 +425,14 @@ const char *dhcp_opt_to_str(uint8_t code, char *buf, size_t buf_len)
  * @size:	Usable size of @buf (excluding end marker)
  * @o:		Option number
  * @offset:	Current offset within @buf, updated on insertion
- *
- * Return: false if @buf has space to write the option, true otherwise
  */
-static bool fill_one(uint8_t *buf, size_t size, int o, int *offset)
+static void fill_one(uint8_t *buf, size_t size, int o, int *offset)
 {
 	size_t slen = opts[o].slen;
 
 	/* If we don't have space to write the option, then just skip */
 	if (*offset + 2 /* code and length of option */ + slen > size)
-		return true;
+		return;
 
 	buf[*offset] = o;
 	buf[*offset + 1] = slen;
@@ -440,17 +444,33 @@ static bool fill_one(uint8_t *buf, size_t size, int o, int *offset)
 
 	opts[o].sent = 1;
 	*offset += slen;
-	return false;
 }
 
 /**
- * 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++)
@@ -460,24 +480,47 @@ static int fill(struct msg *m)
 	 * option 53 at the beginning of the list.
 	 * 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");
-	}
+	if (opts[55].clen > 0 && !memchr(opts[55].c, 53, opts[55].clen))
+		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) {
-			if (fill_one(m->o, OPT_MAX, o, &offset))
-				debug("DHCP: skipping option %i", o);
-		}
+		if (opts[o].conf != OPT_UNSET)
+			fill_one(m->o, size, o, &offset);
 	}
 
 	for (o = 0; o < 255; o++) {
-		if (opts[o].conf != OPT_UNSET && !opts[o].sent) {
-			if (fill_one(m->o, OPT_MAX, o, &offset))
-				debug("DHCP: skipping option %i", o);
-		}
+		if (opts[o].conf != OPT_UNSET && !opts[o].sent)
+			fill_one(m->o, size, o, &offset);
+	}
+
+	/* Overflow unsent options into sname, then file */
+	foreach_unsent_opt(o)
+		fill_one(m->sname, sizeof(m->sname) - 1, o, &sname_off);
+
+	if (!has_bootfile) {
+		foreach_unsent_opt(o)
+			fill_one(m->file, sizeof(m->file) - 1, o, &file_off);
+	}
+
+	/* Report any options that could not be sent */
+	foreach_unsent_opt(o)
+		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;
@@ -770,16 +813,18 @@ 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);
-- 
2.55.0