[PATCH v3 2/2] dhcpv6: Inject custom options into DHCPv6 replies

[Anshu Kumari <anskuma@redhat.com>]

Append user-specified options from --dhcpv6-opt to DHCPv6 reply
messages.  Options are read from the pre-parsed static storage
populated by dhcpv6_set_opt() at startup, and skipped with a
debug message if they exceed the available space.

Link: https://bugs.passt.top/show_bug.cgi?id=192
Signed-off-by: Anshu Kumari <anskuma@redhat.com>
---
v3:
  - rename dhcpv6_custom_opts_fill() to dhcpv6_user_opts_fill().
  - Iterate directly over the pre-parsed binary dhcpv6_user_opts[]
    array instead of re-parsing string values at reply time.

v2:
  - Updated dhcpv6_custom_opts_fill() to parse str at reply time
    using dhcpv6_opt_parse() instead of copying cached val/len.
---
 dhcpv6.c | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/dhcpv6.c b/dhcpv6.c
index 3964dd9..041b589 100644
--- a/dhcpv6.c
+++ b/dhcpv6.c
@@ -865,6 +865,43 @@ static size_t dhcpv6_client_fqdn_fill(const struct iov_tail *data,
 	return offset + sizeof(struct opt_hdr) + opt_len;
 }
 
+/**
+ * dhcpv6_user_opts_fill() - Append user-specified options to reply
+ * @buf:	Response message buffer
+ * @offset:	Current offset in buffer
+ *
+ * Return: updated offset after appending user options
+ */
+static size_t dhcpv6_user_opts_fill(char *buf, size_t offset)
+{
+	unsigned int i;
+
+	for (i = 0; i < ARRAY_SIZE(dhcpv6_user_opts); i++) {
+		int vlen = dhcpv6_user_opts[i].len;
+		struct opt_hdr *hdr;
+
+		if (!vlen)
+			continue;
+
+		if (offset + sizeof(struct opt_hdr) + vlen >
+		    OPT_MAX_SIZE) {
+			debug("DHCPv6: option %u doesn't fit,"
+			      " skipping", i);
+			continue;
+		}
+
+		hdr = (struct opt_hdr *)(buf + offset);
+		hdr->t = htons(i);
+		hdr->l = htons(vlen);
+		offset += sizeof(struct opt_hdr);
+
+		memcpy(buf + offset, dhcpv6_user_opts[i].val, vlen);
+		offset += vlen;
+	}
+
+	return offset;
+}
+
 /**
  * dhcpv6() - Check if this is a DHCPv6 message, reply as needed
  * @c:		Execution context
@@ -1003,6 +1040,8 @@ int dhcpv6(struct ctx *c, struct iov_tail *data,
 	    sizeof(struct opt_hdr) + ntohs(client_id->l);
 	n = dhcpv6_dns_fill(c, (char *)&resp, n);
 	n = dhcpv6_client_fqdn_fill(data, c, (char *)&resp, n);
+	n = dhcpv6_user_opts_fill((char *)&resp, n);
+
 	resp.hdr.xid = mh->xid;
 
 	tap_udp6_send(c, src, 547, saddr, 546, mh->xid, &resp, n);
-- 
2.55.0