[PATCH libnetfilter_queue v4] src: Add nfq_nlmsg_put2() - user specifies header flags

[Duncan Roe <duncan_roe@optusnet.com.au>]

Enable mnl programs to check whether a config request was accepted.
(nfnl programs do this already).

v4: other requested changes

v3: force on NLM_F_REQUEST

v2: take flags as an arg (Pablo request)
Signed-off-by: Duncan Roe <duncan_roe@optusnet.com.au>
---
 .../libnetfilter_queue/libnetfilter_queue.h   |  1 +
 src/nlmsg.c                                   | 54 ++++++++++++++++++-
 2 files changed, 54 insertions(+), 1 deletion(-)

diff --git a/include/libnetfilter_queue/libnetfilter_queue.h b/include/libnetfilter_queue/libnetfilter_queue.h
index 3d8e444..f254984 100644
--- a/include/libnetfilter_queue/libnetfilter_queue.h
+++ b/include/libnetfilter_queue/libnetfilter_queue.h
@@ -151,6 +151,7 @@ void nfq_nlmsg_verdict_put_pkt(struct nlmsghdr *nlh, const void *pkt, uint32_t p
 
 int nfq_nlmsg_parse(const struct nlmsghdr *nlh, struct nlattr **attr);
 struct nlmsghdr *nfq_nlmsg_put(char *buf, int type, uint32_t queue_num);
+struct nlmsghdr *nfq_nlmsg_put2(char *buf, int type, uint32_t queue_num, uint16_t flags);
 
 #ifdef __cplusplus
 } /* extern "C" */
diff --git a/src/nlmsg.c b/src/nlmsg.c
index 5400dd7..0c6229e 100644
--- a/src/nlmsg.c
+++ b/src/nlmsg.c
@@ -309,10 +309,62 @@ int nfq_nlmsg_parse(const struct nlmsghdr *nlh, struct nlattr **attr)
  */
 EXPORT_SYMBOL
 struct nlmsghdr *nfq_nlmsg_put(char *buf, int type, uint32_t queue_num)
+{
+	return nfq_nlmsg_put2(buf, type, queue_num, 0);
+}
+
+/**
+ * nfq_nlmsg_put2 - Set up a netlink header with user-specified flags
+ *                  in a memory buffer
+ * \param *buf Pointer to memory buffer
+ * \param type Either NFQNL_MSG_CONFIG or NFQNL_MSG_VERDICT
+ * \param queue_num Queue number
+ * \param flags additional flags to put in message header, commonly NLM_F_ACK
+ * \returns Pointer to netlink header
+ *
+ * Use NLM_F_ACK before performing an action that might fail, e.g.
+ * attempt to configure NFQA_CFG_F_SECCTX on a system not runnine SELinux.
+ * \n
+ * The kernel always sends a message in response to a failed command.
+ * NLM_F_ACK instructs the kernel to also send a message in response
+ * to a successful command.
+ * \n
+ * This code snippet demonstrates reading these responses:
+ * \verbatim
+	nlh = nfq_nlmsg_put2(nltxbuf, NFQNL_MSG_CONFIG, queue_num, NLM_F_ACK);
+	mnl_attr_put_u32(nlh, NFQA_CFG_FLAGS, NFQA_CFG_F_SECCTX);
+	mnl_attr_put_u32(nlh, NFQA_CFG_MASK, NFQA_CFG_F_SECCTX);
+
+	if (mnl_socket_sendto(nl, nlh, nlh->nlmsg_len) < 0) {
+		perror("mnl_socket_send");
+		exit(EXIT_FAILURE);
+	}
+
+	ret = mnl_socket_recvfrom(nl, nlrxbuf, sizeof nlrxbuf);
+	if (ret == -1) {
+		perror("mnl_socket_recvfrom");
+		exit(EXIT_FAILURE);
+	}
+
+	ret = mnl_cb_run(nlrxbuf, ret, 0, portid, NULL, NULL);
+	if (ret == -1)
+		perror("configure NFQA_CFG_F_SECCTX");
+\endverbatim
+ *
+ * \note
+ * The program above can continue after the error because NFQA_CFG_F_SECCTX
+ * was the only item in the preceding **mnl_socket_sendto**.
+ * If there had been other items, they would not have been actioned and it would
+ * not now be safe to proceed.
+ */
+
+EXPORT_SYMBOL
+struct nlmsghdr *nfq_nlmsg_put2(char *buf, int type, uint32_t queue_num,
+				uint16_t flags)
 {
 	struct nlmsghdr *nlh = mnl_nlmsg_put_header(buf);
 	nlh->nlmsg_type = (NFNL_SUBSYS_QUEUE << 8) | type;
-	nlh->nlmsg_flags = NLM_F_REQUEST;
+	nlh->nlmsg_flags = NLM_F_REQUEST | flags;
 
 	struct nfgenmsg *nfg = mnl_nlmsg_put_extra_header(nlh, sizeof(*nfg));
 	nfg->nfgen_family = AF_UNSPEC;
-- 
2.35.8