extv: introduced a low-level extension parsing code

This will simplify the parsing and handling of extensions throughout
the TLS 1.3 message contents.

Signed-off-by: Nikos Mavrogiannopoulos <nmav@redhat.com>

1 files changed, 127 insertions, 0 deletions

diff --git a/lib/extv.c b/lib/extv.c
new file mode 100644
index 0000000000..d68f852769
--- /dev/null
+++ b/lib/extv.c
@@ -0,0 +1,127 @@
+/*
+ * Copyright (C) 2017 Red Hat, Inc.
+ *
+ * Author: Nikos Mavrogiannopoulos
+ *
+ * This file is part of GnuTLS.
+ *
+ * The GnuTLS is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public License
+ * as published by the Free Software Foundation; either version 2.1 of
+ * the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>
+ *
+ */
+
+#include "gnutls_int.h"
+#include "extensions.h"
+#include "errors.h"
+#include "extv.h"
+
+/* Iterates through all extensions found, and calls the cb()
+ * function with their data */
+int _gnutls_extv_parse(void *ctx,
+		       int (*cb)(void *ctx, uint16_t tls_id, const uint8_t *data, int data_size),
+		       const uint8_t * data, int data_size)
+{
+	int next, ret;
+	int pos = 0;
+	uint16_t tls_id;
+	const uint8_t *sdata;
+	uint16_t size;
+
+	if (data_size == 0)
+		return 0;
+
+	DECR_LENGTH_RET(data_size, 2, GNUTLS_E_UNEXPECTED_EXTENSIONS_LENGTH);
+	next = _gnutls_read_uint16(data);
+	pos += 2;
+
+	DECR_LENGTH_RET(data_size, next, GNUTLS_E_UNEXPECTED_EXTENSIONS_LENGTH);
+
+	if (next == 0 && data_size == 0) /* field is present, but has zero length? Ignore it. */
+		return 0;
+	else if (data_size > 0) /* forbid unaccounted data */
+		return gnutls_assert_val(GNUTLS_E_UNEXPECTED_EXTENSIONS_LENGTH);
+
+	do {
+		DECR_LENGTH_RET(next, 2, GNUTLS_E_UNEXPECTED_EXTENSIONS_LENGTH);
+		tls_id = _gnutls_read_uint16(&data[pos]);
+		pos += 2;
+
+		DECR_LENGTH_RET(next, 2, GNUTLS_E_UNEXPECTED_EXTENSIONS_LENGTH);
+		size = _gnutls_read_uint16(&data[pos]);
+		pos += 2;
+
+		DECR_LENGTH_RET(next, size, GNUTLS_E_UNEXPECTED_EXTENSIONS_LENGTH);
+		sdata = &data[pos];
+		pos += size;
+
+		ret = cb(ctx, tls_id, sdata, size);
+		if (ret < 0)
+			return gnutls_assert_val(ret);
+	}
+	while (next > 2);
+
+	/* forbid leftovers */
+	if (next > 0)
+		return gnutls_assert_val(GNUTLS_E_UNEXPECTED_EXTENSIONS_LENGTH);
+
+	return 0;
+
+}
+
+/* Returns:
+ *  * On success the number of bytes appended (always positive), or zero if not sent
+ *  * On failure, a negative error code.
+ */
+int _gnutls_extv_append(gnutls_buffer_st *buf,
+			uint16_t tls_id,
+		        void *ctx,
+		        int (*cb)(void *ctx, gnutls_buffer_st *buf))
+{
+	int size_pos, appended, ret;
+	size_t size_prev;
+
+	ret = _gnutls_buffer_append_prefix(buf, 16, tls_id);
+	if (ret < 0)
+		return gnutls_assert_val(ret);
+
+	size_pos = buf->length;
+	ret = _gnutls_buffer_append_prefix(buf, 16, 0);
+	if (ret < 0)
+		return gnutls_assert_val(ret);
+
+	size_prev = buf->length;
+	ret = cb(ctx, buf);
+	if (ret < 0 && ret != GNUTLS_E_INT_RET_0) {
+		return gnutls_assert_val(ret);
+	}
+
+	/* returning GNUTLS_E_INT_RET_0 means to send an empty
+	 * extension of this type.
+	 */
+	appended = buf->length - size_prev;
+
+	if (appended > 0 || ret == GNUTLS_E_INT_RET_0) {
+		if (ret == GNUTLS_E_INT_RET_0)
+			appended = 0;
+
+		/* write the real size */
+		_gnutls_write_uint16(appended,
+				     &buf->data[size_pos]);
+	} else if (appended == 0) {
+		buf->length -= 4;	/* reset type and size */
+		return 0;
+	}
+
+	return appended + 4;
+}
+