udata: add TLV user data infrastructure

These functions allow to create a buffer (struct nftnl_udata_buf) of
user data attributes in TLV format (struct nftnl_udata). It is inspired
by libmnl/src/attr.c. It can be used to store several TLVs sequentially
into an object.

Example:

	struct nftnl_udata_buf *buf;
	struct nftnl_udata *attr;
	const char *str = "Hello World!";

	buf = nftnl_udata_buf_alloc(UDATA_SIZE);
	if (!buf) {
		perror("OOM");
		exit(EXIT_FAILURE);
	}

	if (!nftnl_udata_put_strz(buf, MY_TYPE, str)) {
		perror("Can't put attribute \"%s\"", str);
		exit(EXIT_FAILURE);
	}

	nftnl_udata_for_each(buf, attr)
		printf("%s\n", (char *)nftnl_udata_attr_value(attr));

	nftnl_udata_buf_free(buf);

Signed-off-by: Carlos Falgueras García <carlosfg@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 40 insertions, 0 deletions

diff --git a/include/udata.h b/include/udata.h
new file mode 100644
index 0000000..407a3b9
--- /dev/null
+++ b/include/udata.h
@@ -0,0 +1,40 @@
+#ifndef _LIBNFTNL_UDATA_INTERNAL_H_
+#define _LIBNFTNL_UDATA_INTERNAL_H_
+
+#include <stdint.h>
+#include <stddef.h>
+
+/*
+ * TLV structures:
+ * nftnl_udata
+ *  <-------- HEADER --------> <------ PAYLOAD ------>
+ * +------------+-------------+- - - - - - - - - - - -+
+ * |    type    |     len     |         value         |
+ * |  (1 byte)  |   (1 byte)  |                       |
+ * +--------------------------+- - - - - - - - - - - -+
+ *  <-- sizeof(nftnl_udata) -> <-- nftnl_udata->len -->
+ */
+struct nftnl_udata {
+	uint8_t		type;
+	uint8_t		len;
+	unsigned char	value[];
+} __attribute__((__packed__));
+
+/*
+ *              +---------------------------------++
+ *              | data[]                          ||
+ *              |   ||                            ||
+ *              |   \/                            \/
+ *  +-------+-------+-------+-------+ ... +-------+- - - - - - -+
+ *  | size  |  end  |  TLV  |  TLV  |     |  TLV  |    Empty    |
+ *  +-------+-------+-------+-------+ ... +-------+- - - - - - -+
+ *                  |<---- nftnl_udata_len() ---->|
+ *                  |<----------- nftnl_udata_size() ---------->|
+ */
+struct nftnl_udata_buf {
+	uint32_t	size;
+	char		*end;
+	char		data[];
+};
+
+#endif /* _LIBNFTNL_UDATA_INTERNAL_H_ */