lib/dns/rdataslab.c

/*	$NetBSD: rdataslab.c,v 1.1.2.2 2024/02/24 13:07:01 martin Exp $	*/

/*
 * Copyright (C) Internet Systems Consortium, Inc. ("ISC")
 *
 * SPDX-License-Identifier: MPL-2.0
 *
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, you can obtain one at https://mozilla.org/MPL/2.0/.
 *
 * See the COPYRIGHT file distributed with this work for additional
 * information regarding copyright ownership.
 */

/*! \file */

#include <stdbool.h>
#include <stdlib.h>

#include <isc/mem.h>
#include <isc/region.h>
#include <isc/string.h> /* Required for HP/UX (and others?) */
#include <isc/util.h>

#include <dns/rdata.h>
#include <dns/rdataset.h>
#include <dns/rdataslab.h>
#include <dns/result.h>

/*
 * The rdataslab structure allows iteration to occur in both load order
 * and DNSSEC order.  The structure is as follows:
 *
 *	header		(reservelen bytes)
 *	record count	(2 bytes)
 *	offset table	(4 x record count bytes in load order)
 *	data records
 *		data length	(2 bytes)
 *		order		(2 bytes)
 *		meta data	(1 byte for RRSIG's)
 *		data		(data length bytes)
 *
 * If DNS_RDATASET_FIXED is defined to be zero (0) the format of a
 * rdataslab is as follows:
 *
 *	header		(reservelen bytes)
 *	record count	(2 bytes)
 *	data records
 *		data length	(2 bytes)
 *		meta data	(1 byte for RRSIG's)
 *		data		(data length bytes)
 *
 * Offsets are from the end of the header.
 *
 * Load order traversal is performed by walking the offset table to find
 * the start of the record (DNS_RDATASET_FIXED = 1).
 *
 * DNSSEC order traversal is performed by walking the data records.
 *
 * The order is stored with record to allow for efficient reconstruction
 * of the offset table following a merge or subtraction.
 *
 * The iterator methods in rbtdb support both load order and DNSSEC order
 * iteration.
 *
 * WARNING:
 *	rbtdb.c directly interacts with the slab's raw structures.  If the
 *	structure changes then rbtdb.c also needs to be updated to reflect
 *	the changes.  See the areas tagged with "RDATASLAB".
 */

struct xrdata {
	dns_rdata_t rdata;
	unsigned int order;
};

/*% Note: the "const void *" are just to make qsort happy.  */
static int
compare_rdata(const void *p1, const void *p2) {
	const struct xrdata *x1 = p1;
	const struct xrdata *x2 = p2;
	return (dns_rdata_compare(&x1->rdata, &x2->rdata));
}

#if DNS_RDATASET_FIXED
static void
fillin_offsets(unsigned char *offsetbase, unsigned int *offsettable,
	       unsigned length) {
	unsigned int i, j;
	unsigned char *raw;

	for (i = 0, j = 0; i < length; i++) {
		if (offsettable[i] == 0) {
			continue;
		}

		/*
		 * Fill in offset table.
		 */
		raw = &offsetbase[j * 4 + 2];
		*raw++ = (offsettable[i] & 0xff000000) >> 24;
		*raw++ = (offsettable[i] & 0xff0000) >> 16;
		*raw++ = (offsettable[i] & 0xff00) >> 8;
		*raw = offsettable[i] & 0xff;

		/*
		 * Fill in table index.
		 */
		raw = offsetbase + offsettable[i] + 2;
		*raw++ = (j & 0xff00) >> 8;
		*raw = j++ & 0xff;
	}
}
#endif /* if DNS_RDATASET_FIXED */

isc_result_t
dns_rdataslab_fromrdataset(dns_rdataset_t *rdataset, isc_mem_t *mctx,
			   isc_region_t *region, unsigned int reservelen) {
	/*
	 * Use &removed as a sentinel pointer for duplicate
	 * rdata as rdata.data == NULL is valid.
	 */
	static unsigned char removed;
	struct xrdata *x;
	unsigned char *rawbuf;
#if DNS_RDATASET_FIXED
	unsigned char *offsetbase;
#endif /* if DNS_RDATASET_FIXED */
	unsigned int buflen;
	isc_result_t result;
	unsigned int nitems;
	unsigned int nalloc;
	unsigned int i;
#if DNS_RDATASET_FIXED
	unsigned int *offsettable;
#endif /* if DNS_RDATASET_FIXED */
	unsigned int length;

	buflen = reservelen + 2;

	nitems = dns_rdataset_count(rdataset);

	/*
	 * If there are no rdata then we can just need to allocate a header
	 * with zero a record count.
	 */
	if (nitems == 0) {
		if (rdataset->type != 0) {
			return (ISC_R_FAILURE);
		}
		rawbuf = isc_mem_get(mctx, buflen);
		region->base = rawbuf;
		region->length = buflen;
		rawbuf += reservelen;
		*rawbuf++ = 0;
		*rawbuf = 0;
		return (ISC_R_SUCCESS);
	}

	if (nitems > 0xffff) {
		return (ISC_R_NOSPACE);
	}

	/*
	 * Remember the original number of items.
	 */
	nalloc = nitems;
	x = isc_mem_get(mctx, nalloc * sizeof(struct xrdata));

	/*
	 * Save all of the rdata members into an array.
	 */
	result = dns_rdataset_first(rdataset);
	if (result != ISC_R_SUCCESS && result != ISC_R_NOMORE) {
		goto free_rdatas;
	}
	for (i = 0; i < nalloc && result == ISC_R_SUCCESS; i++) {
		INSIST(result == ISC_R_SUCCESS);
		dns_rdata_init(&x[i].rdata);
		dns_rdataset_current(rdataset, &x[i].rdata);
		INSIST(x[i].rdata.data != &removed);
#if DNS_RDATASET_FIXED
		x[i].order = i;
#endif /* if DNS_RDATASET_FIXED */
		result = dns_rdataset_next(rdataset);
	}
	if (i != nalloc || result != ISC_R_NOMORE) {
		/*
		 * Somehow we iterated over fewer rdatas than
		 * dns_rdataset_count() said there were or there
		 * were more items than dns_rdataset_count said
		 * there were.
		 */
		result = ISC_R_FAILURE;
		goto free_rdatas;
	}

	/*
	 * Put into DNSSEC order.
	 */
	if (nalloc > 1U) {
		qsort(x, nalloc, sizeof(struct xrdata), compare_rdata);
	}

	/*
	 * Remove duplicates and compute the total storage required.
	 *
	 * If an rdata is not a duplicate, accumulate the storage size
	 * required for the rdata.  We do not store the class, type, etc,
	 * just the rdata, so our overhead is 2 bytes for the number of
	 * records, and 8 for each rdata, (length(2), offset(4) and order(2))
	 * and then the rdata itself.
	 */
	for (i = 1; i < nalloc; i++) {
		if (compare_rdata(&x[i - 1].rdata, &x[i].rdata) == 0) {
			x[i - 1].rdata.data = &removed;
#if DNS_RDATASET_FIXED
			/*
			 * Preserve the least order so A, B, A -> A, B
			 * after duplicate removal.
			 */
			if (x[i - 1].order < x[i].order) {
				x[i].order = x[i - 1].order;
			}
#endif /* if DNS_RDATASET_FIXED */
			nitems--;
		} else {
#if DNS_RDATASET_FIXED
			buflen += (8 + x[i - 1].rdata.length);
#else  /* if DNS_RDATASET_FIXED */
			buflen += (2 + x[i - 1].rdata.length);
#endif /* if DNS_RDATASET_FIXED */
			/*
			 * Provide space to store the per RR meta data.
			 */
			if (rdataset->type == dns_rdatatype_rrsig) {
				buflen++;
			}
		}
	}

	/*
	 * Don't forget the last item!
	 */
#if DNS_RDATASET_FIXED
	buflen += (8 + x[i - 1].rdata.length);
#else  /* if DNS_RDATASET_FIXED */
	buflen += (2 + x[i - 1].rdata.length);
#endif /* if DNS_RDATASET_FIXED */
	/*
	 * Provide space to store the per RR meta data.
	 */
	if (rdataset->type == dns_rdatatype_rrsig) {
		buflen++;
	}

	/*
	 * Ensure that singleton types are actually singletons.
	 */
	if (nitems > 1 && dns_rdatatype_issingleton(rdataset->type)) {
		/*
		 * We have a singleton type, but there's more than one
		 * RR in the rdataset.
		 */
		result = DNS_R_SINGLETON;
		goto free_rdatas;
	}

	/*
	 * Allocate the memory, set up a buffer, start copying in
	 * data.
	 */
	rawbuf = isc_mem_get(mctx, buflen);

#if DNS_RDATASET_FIXED
	/* Allocate temporary offset table. */
	offsettable = isc_mem_get(mctx, nalloc * sizeof(unsigned int));
	memset(offsettable, 0, nalloc * sizeof(unsigned int));
#endif /* if DNS_RDATASET_FIXED */

	region->base = rawbuf;
	region->length = buflen;

	memset(rawbuf, 0, buflen);
	rawbuf += reservelen;

#if DNS_RDATASET_FIXED
	offsetbase = rawbuf;
#endif /* if DNS_RDATASET_FIXED */

	*rawbuf++ = (nitems & 0xff00) >> 8;
	*rawbuf++ = (nitems & 0x00ff);

#if DNS_RDATASET_FIXED
	/* Skip load order table.  Filled in later. */
	rawbuf += nitems * 4;
#endif /* if DNS_RDATASET_FIXED */

	for (i = 0; i < nalloc; i++) {
		if (x[i].rdata.data == &removed) {
			continue;
		}
#if DNS_RDATASET_FIXED
		offsettable[x[i].order] = rawbuf - offsetbase;
#endif /* if DNS_RDATASET_FIXED */
		length = x[i].rdata.length;
		if (rdataset->type == dns_rdatatype_rrsig) {
			length++;
		}
		INSIST(length <= 0xffff);
		*rawbuf++ = (length & 0xff00) >> 8;
		*rawbuf++ = (length & 0x00ff);
#if DNS_RDATASET_FIXED
		rawbuf += 2; /* filled in later */
#endif			     /* if DNS_RDATASET_FIXED */
		/*
		 * Store the per RR meta data.
		 */
		if (rdataset->type == dns_rdatatype_rrsig) {
			*rawbuf++ = (x[i].rdata.flags & DNS_RDATA_OFFLINE)
					    ? DNS_RDATASLAB_OFFLINE
					    : 0;
		}
		memmove(rawbuf, x[i].rdata.data, x[i].rdata.length);
		rawbuf += x[i].rdata.length;
	}

#if DNS_RDATASET_FIXED
	fillin_offsets(offsetbase, offsettable, nalloc);
	isc_mem_put(mctx, offsettable, nalloc * sizeof(unsigned int));
#endif /* if DNS_RDATASET_FIXED */

	result = ISC_R_SUCCESS;

free_rdatas:
	isc_mem_put(mctx, x, nalloc * sizeof(struct xrdata));
	return (result);
}

unsigned int
dns_rdataslab_size(unsigned char *slab, unsigned int reservelen) {
	unsigned int count, length;
	unsigned char *current;

	REQUIRE(slab != NULL);

	current = slab + reservelen;
	count = *current++ * 256;
	count += *current++;
#if DNS_RDATASET_FIXED
	current += (4 * count);
#endif /* if DNS_RDATASET_FIXED */
	while (count > 0) {
		count--;
		length = *current++ * 256;
		length += *current++;
#if DNS_RDATASET_FIXED
		current += length + 2;
#else  /* if DNS_RDATASET_FIXED */
		current += length;
#endif /* if DNS_RDATASET_FIXED */
	}

	return ((unsigned int)(current - slab));
}

unsigned int
dns_rdataslab_rdatasize(unsigned char *slab, unsigned int reservelen) {
	unsigned int count, length, rdatalen = 0;
	unsigned char *current;

	REQUIRE(slab != NULL);

	current = slab + reservelen;
	count = *current++ * 256;
	count += *current++;
#if DNS_RDATASET_FIXED
	current += (4 * count);
#endif /* if DNS_RDATASET_FIXED */
	while (count > 0) {
		count--;
		length = *current++ * 256;
		length += *current++;
		rdatalen += length;
#if DNS_RDATASET_FIXED
		current += length + 2;
#else  /* if DNS_RDATASET_FIXED */
		current += length;
#endif /* if DNS_RDATASET_FIXED */
	}

	return (rdatalen);
}

unsigned int
dns_rdataslab_count(unsigned char *slab, unsigned int reservelen) {
	unsigned int count;
	unsigned char *current;

	REQUIRE(slab != NULL);

	current = slab + reservelen;
	count = *current++ * 256;
	count += *current++;
	return (count);
}

/*
 * Make the dns_rdata_t 'rdata' refer to the slab item
 * beginning at '*current', which is part of a slab of type
 * 'type' and class 'rdclass', and advance '*current' to
 * point to the next item in the slab.
 */
static void
rdata_from_slab(unsigned char **current, dns_rdataclass_t rdclass,
		dns_rdatatype_t type, dns_rdata_t *rdata) {
	unsigned char *tcurrent = *current;
	isc_region_t region;
	unsigned int length;
	bool offline = false;

	length = *tcurrent++ * 256;
	length += *tcurrent++;

	if (type == dns_rdatatype_rrsig) {
		if ((*tcurrent & DNS_RDATASLAB_OFFLINE) != 0) {
			offline = true;
		}
		length--;
		tcurrent++;
	}
	region.length = length;
#if DNS_RDATASET_FIXED
	tcurrent += 2;
#endif /* if DNS_RDATASET_FIXED */
	region.base = tcurrent;
	tcurrent += region.length;
	dns_rdata_fromregion(rdata, rdclass, type, &region);
	if (offline) {
		rdata->flags |= DNS_RDATA_OFFLINE;
	}
	*current = tcurrent;
}

/*
 * Return true iff 'slab' (slab data of type 'type' and class 'rdclass')
 * contains an rdata identical to 'rdata'.  This does case insensitive
 * comparisons per DNSSEC.
 */
static bool
rdata_in_slab(unsigned char *slab, unsigned int reservelen,
	      dns_rdataclass_t rdclass, dns_rdatatype_t type,
	      dns_rdata_t *rdata) {
	unsigned int count, i;
	unsigned char *current;
	dns_rdata_t trdata = DNS_RDATA_INIT;
	int n;

	current = slab + reservelen;
	count = *current++ * 256;
	count += *current++;

#if DNS_RDATASET_FIXED
	current += (4 * count);
#endif /* if DNS_RDATASET_FIXED */

	for (i = 0; i < count; i++) {
		rdata_from_slab(&current, rdclass, type, &trdata);

		n = dns_rdata_compare(&trdata, rdata);
		if (n == 0) {
			return (true);
		}
		if (n > 0) { /* In DNSSEC order. */
			break;
		}
		dns_rdata_reset(&trdata);
	}
	return (false);
}

isc_result_t
dns_rdataslab_merge(unsigned char *oslab, unsigned char *nslab,
		    unsigned int reservelen, isc_mem_t *mctx,
		    dns_rdataclass_t rdclass, dns_rdatatype_t type,
		    unsigned int flags, unsigned char **tslabp) {
	unsigned char *ocurrent, *ostart, *ncurrent, *tstart, *tcurrent, *data;
	unsigned int ocount, ncount, count, olength, tlength, tcount, length;
	dns_rdata_t ordata = DNS_RDATA_INIT;
	dns_rdata_t nrdata = DNS_RDATA_INIT;
	bool added_something = false;
	unsigned int oadded = 0;
	unsigned int nadded = 0;
	unsigned int nncount = 0;
#if DNS_RDATASET_FIXED
	unsigned int oncount;
	unsigned int norder = 0;
	unsigned int oorder = 0;
	unsigned char *offsetbase;
	unsigned int *offsettable;
#endif /* if DNS_RDATASET_FIXED */

	/*
	 * XXX  Need parameter to allow "delete rdatasets in nslab" merge,
	 * or perhaps another merge routine for this purpose.
	 */

	REQUIRE(tslabp != NULL && *tslabp == NULL);
	REQUIRE(oslab != NULL && nslab != NULL);

	ocurrent = oslab + reservelen;
	ocount = *ocurrent++ * 256;
	ocount += *ocurrent++;
#if DNS_RDATASET_FIXED
	ocurrent += (4 * ocount);
#endif /* if DNS_RDATASET_FIXED */
	ostart = ocurrent;
	ncurrent = nslab + reservelen;
	ncount = *ncurrent++ * 256;
	ncount += *ncurrent++;
#if DNS_RDATASET_FIXED
	ncurrent += (4 * ncount);
#endif /* if DNS_RDATASET_FIXED */
	INSIST(ocount > 0 && ncount > 0);

#if DNS_RDATASET_FIXED
	oncount = ncount;
#endif /* if DNS_RDATASET_FIXED */

	/*
	 * Yes, this is inefficient!
	 */

	/*
	 * Figure out the length of the old slab's data.
	 */
	olength = 0;
	for (count = 0; count < ocount; count++) {
		length = *ocurrent++ * 256;
		length += *ocurrent++;
#if DNS_RDATASET_FIXED
		olength += length + 8;
		ocurrent += length + 2;
#else  /* if DNS_RDATASET_FIXED */
		olength += length + 2;
		ocurrent += length;
#endif /* if DNS_RDATASET_FIXED */
	}

	/*
	 * Start figuring out the target length and count.
	 */
	tlength = reservelen + 2 + olength;
	tcount = ocount;

	/*
	 * Add in the length of rdata in the new slab that aren't in
	 * the old slab.
	 */
	do {
		dns_rdata_init(&nrdata);
		rdata_from_slab(&ncurrent, rdclass, type, &nrdata);
		if (!rdata_in_slab(oslab, reservelen, rdclass, type, &nrdata)) {
			/*
			 * This rdata isn't in the old slab.
			 */
#if DNS_RDATASET_FIXED
			tlength += nrdata.length + 8;
#else  /* if DNS_RDATASET_FIXED */
			tlength += nrdata.length + 2;
#endif /* if DNS_RDATASET_FIXED */
			if (type == dns_rdatatype_rrsig) {
				tlength++;
			}
			tcount++;
			nncount++;
			added_something = true;
		}
		ncount--;
	} while (ncount > 0);
	ncount = nncount;

	if (((flags & DNS_RDATASLAB_EXACT) != 0) && (tcount != ncount + ocount))
	{
		return (DNS_R_NOTEXACT);
	}

	if (!added_something && (flags & DNS_RDATASLAB_FORCE) == 0) {
		return (DNS_R_UNCHANGED);
	}

	/*
	 * Ensure that singleton types are actually singletons.
	 */
	if (tcount > 1 && dns_rdatatype_issingleton(type)) {
		/*
		 * We have a singleton type, but there's more than one
		 * RR in the rdataset.
		 */
		return (DNS_R_SINGLETON);
	}

	if (tcount > 0xffff) {
		return (ISC_R_NOSPACE);
	}

	/*
	 * Copy the reserved area from the new slab.
	 */
	tstart = isc_mem_get(mctx, tlength);
	memmove(tstart, nslab, reservelen);
	tcurrent = tstart + reservelen;
#if DNS_RDATASET_FIXED
	offsetbase = tcurrent;
#endif /* if DNS_RDATASET_FIXED */

	/*
	 * Write the new count.
	 */
	*tcurrent++ = (tcount & 0xff00) >> 8;
	*tcurrent++ = (tcount & 0x00ff);

#if DNS_RDATASET_FIXED
	/*
	 * Skip offset table.
	 */
	tcurrent += (tcount * 4);

	offsettable = isc_mem_get(mctx,
				  (ocount + oncount) * sizeof(unsigned int));
	memset(offsettable, 0, (ocount + oncount) * sizeof(unsigned int));
#endif /* if DNS_RDATASET_FIXED */

	/*
	 * Merge the two slabs.
	 */
	ocurrent = ostart;
	INSIST(ocount != 0);
#if DNS_RDATASET_FIXED
	oorder = ocurrent[2] * 256 + ocurrent[3];
	INSIST(oorder < ocount);
#endif /* if DNS_RDATASET_FIXED */
	rdata_from_slab(&ocurrent, rdclass, type, &ordata);

	ncurrent = nslab + reservelen + 2;
#if DNS_RDATASET_FIXED
	ncurrent += (4 * oncount);
#endif /* if DNS_RDATASET_FIXED */

	if (ncount > 0) {
		do {
			dns_rdata_reset(&nrdata);
#if DNS_RDATASET_FIXED
			norder = ncurrent[2] * 256 + ncurrent[3];

			INSIST(norder < oncount);
#endif /* if DNS_RDATASET_FIXED */
			rdata_from_slab(&ncurrent, rdclass, type, &nrdata);
		} while (rdata_in_slab(oslab, reservelen, rdclass, type,
				       &nrdata));
	}

	while (oadded < ocount || nadded < ncount) {
		bool fromold;
		if (oadded == ocount) {
			fromold = false;
		} else if (nadded == ncount) {
			fromold = true;
		} else {
			fromold = (dns_rdata_compare(&ordata, &nrdata) < 0);
		}
		if (fromold) {
#if DNS_RDATASET_FIXED
			offsettable[oorder] = tcurrent - offsetbase;
#endif /* if DNS_RDATASET_FIXED */
			length = ordata.length;
			data = ordata.data;
			if (type == dns_rdatatype_rrsig) {
				length++;
				data--;
			}
			*tcurrent++ = (length & 0xff00) >> 8;
			*tcurrent++ = (length & 0x00ff);
#if DNS_RDATASET_FIXED
			tcurrent += 2; /* fill in later */
#endif				       /* if DNS_RDATASET_FIXED */
			memmove(tcurrent, data, length);
			tcurrent += length;
			oadded++;
			if (oadded < ocount) {
				dns_rdata_reset(&ordata);
#if DNS_RDATASET_FIXED
				oorder = ocurrent[2] * 256 + ocurrent[3];
				INSIST(oorder < ocount);
#endif /* if DNS_RDATASET_FIXED */
				rdata_from_slab(&ocurrent, rdclass, type,
						&ordata);
			}
		} else {
#if DNS_RDATASET_FIXED
			offsettable[ocount + norder] = tcurrent - offsetbase;
#endif /* if DNS_RDATASET_FIXED */
			length = nrdata.length;
			data = nrdata.data;
			if (type == dns_rdatatype_rrsig) {
				length++;
				data--;
			}
			*tcurrent++ = (length & 0xff00) >> 8;
			*tcurrent++ = (length & 0x00ff);
#if DNS_RDATASET_FIXED
			tcurrent += 2; /* fill in later */
#endif				       /* if DNS_RDATASET_FIXED */
			memmove(tcurrent, data, length);
			tcurrent += length;
			nadded++;
			if (nadded < ncount) {
				do {
					dns_rdata_reset(&nrdata);
#if DNS_RDATASET_FIXED
					norder = ncurrent[2] * 256 +
						 ncurrent[3];
					INSIST(norder < oncount);
#endif /* if DNS_RDATASET_FIXED */
					rdata_from_slab(&ncurrent, rdclass,
							type, &nrdata);
				} while (rdata_in_slab(oslab, reservelen,
						       rdclass, type, &nrdata));
			}
		}
	}

#if DNS_RDATASET_FIXED
	fillin_offsets(offsetbase, offsettable, ocount + oncount);

	isc_mem_put(mctx, offsettable,
		    (ocount + oncount) * sizeof(unsigned int));
#endif /* if DNS_RDATASET_FIXED */

	INSIST(tcurrent == tstart + tlength);

	*tslabp = tstart;

	return (ISC_R_SUCCESS);
}

isc_result_t
dns_rdataslab_subtract(unsigned char *mslab, unsigned char *sslab,
		       unsigned int reservelen, isc_mem_t *mctx,
		       dns_rdataclass_t rdclass, dns_rdatatype_t type,
		       unsigned int flags, unsigned char **tslabp) {
	unsigned char *mcurrent, *sstart, *scurrent, *tstart, *tcurrent;
	unsigned int mcount, scount, rcount, count, tlength, tcount, i;
	dns_rdata_t srdata = DNS_RDATA_INIT;
	dns_rdata_t mrdata = DNS_RDATA_INIT;
#if DNS_RDATASET_FIXED
	unsigned char *offsetbase;
	unsigned int *offsettable;
	unsigned int order;
#endif /* if DNS_RDATASET_FIXED */

	REQUIRE(tslabp != NULL && *tslabp == NULL);
	REQUIRE(mslab != NULL && sslab != NULL);

	mcurrent = mslab + reservelen;
	mcount = *mcurrent++ * 256;
	mcount += *mcurrent++;
	scurrent = sslab + reservelen;
	scount = *scurrent++ * 256;
	scount += *scurrent++;
	INSIST(mcount > 0 && scount > 0);

	/*
	 * Yes, this is inefficient!
	 */

	/*
	 * Start figuring out the target length and count.
	 */
	tlength = reservelen + 2;
	tcount = 0;
	rcount = 0;

#if DNS_RDATASET_FIXED
	mcurrent += 4 * mcount;
	scurrent += 4 * scount;
#endif /* if DNS_RDATASET_FIXED */
	sstart = scurrent;

	/*
	 * Add in the length of rdata in the mslab that aren't in
	 * the sslab.
	 */
	for (i = 0; i < mcount; i++) {
		unsigned char *mrdatabegin = mcurrent;
		rdata_from_slab(&mcurrent, rdclass, type, &mrdata);
		scurrent = sstart;
		for (count = 0; count < scount; count++) {
			dns_rdata_reset(&srdata);
			rdata_from_slab(&scurrent, rdclass, type, &srdata);
			if (dns_rdata_compare(&mrdata, &srdata) == 0) {
				break;
			}
		}
		if (count == scount) {
			/*
			 * This rdata isn't in the sslab, and thus isn't
			 * being subtracted.
			 */
			tlength += (unsigned int)(mcurrent - mrdatabegin);
			tcount++;
		} else {
			rcount++;
		}
		dns_rdata_reset(&mrdata);
	}

#if DNS_RDATASET_FIXED
	tlength += (4 * tcount);
#endif /* if DNS_RDATASET_FIXED */

	/*
	 * Check that all the records originally existed.  The numeric
	 * check only works as rdataslabs do not contain duplicates.
	 */
	if (((flags & DNS_RDATASLAB_EXACT) != 0) && (rcount != scount)) {
		return (DNS_R_NOTEXACT);
	}

	/*
	 * Don't continue if the new rdataslab would be empty.
	 */
	if (tcount == 0) {
		return (DNS_R_NXRRSET);
	}

	/*
	 * If nothing is going to change, we can stop.
	 */
	if (rcount == 0) {
		return (DNS_R_UNCHANGED);
	}

	/*
	 * Copy the reserved area from the mslab.
	 */
	tstart = isc_mem_get(mctx, tlength);
	memmove(tstart, mslab, reservelen);
	tcurrent = tstart + reservelen;
#if DNS_RDATASET_FIXED
	offsetbase = tcurrent;

	offsettable = isc_mem_get(mctx, mcount * sizeof(unsigned int));
	memset(offsettable, 0, mcount * sizeof(unsigned int));
#endif /* if DNS_RDATASET_FIXED */

	/*
	 * Write the new count.
	 */
	*tcurrent++ = (tcount & 0xff00) >> 8;
	*tcurrent++ = (tcount & 0x00ff);

#if DNS_RDATASET_FIXED
	tcurrent += (4 * tcount);
#endif /* if DNS_RDATASET_FIXED */

	/*
	 * Copy the parts of mslab not in sslab.
	 */
	mcurrent = mslab + reservelen;
	mcount = *mcurrent++ * 256;
	mcount += *mcurrent++;
#if DNS_RDATASET_FIXED
	mcurrent += (4 * mcount);
#endif /* if DNS_RDATASET_FIXED */
	for (i = 0; i < mcount; i++) {
		unsigned char *mrdatabegin = mcurrent;
#if DNS_RDATASET_FIXED
		order = mcurrent[2] * 256 + mcurrent[3];
		INSIST(order < mcount);
#endif /* if DNS_RDATASET_FIXED */
		rdata_from_slab(&mcurrent, rdclass, type, &mrdata);
		scurrent = sstart;
		for (count = 0; count < scount; count++) {
			dns_rdata_reset(&srdata);
			rdata_from_slab(&scurrent, rdclass, type, &srdata);
			if (dns_rdata_compare(&mrdata, &srdata) == 0) {
				break;
			}
		}
		if (count == scount) {
			/*
			 * This rdata isn't in the sslab, and thus should be
			 * copied to the tslab.
			 */
			unsigned int length;
			length = (unsigned int)(mcurrent - mrdatabegin);
#if DNS_RDATASET_FIXED
			offsettable[order] = tcurrent - offsetbase;
#endif /* if DNS_RDATASET_FIXED */
			memmove(tcurrent, mrdatabegin, length);
			tcurrent += length;
		}
		dns_rdata_reset(&mrdata);
	}

#if DNS_RDATASET_FIXED
	fillin_offsets(offsetbase, offsettable, mcount);

	isc_mem_put(mctx, offsettable, mcount * sizeof(unsigned int));
#endif /* if DNS_RDATASET_FIXED */

	INSIST(tcurrent == tstart + tlength);

	*tslabp = tstart;

	return (ISC_R_SUCCESS);
}

bool
dns_rdataslab_equal(unsigned char *slab1, unsigned char *slab2,
		    unsigned int reservelen) {
	unsigned char *current1, *current2;
	unsigned int count1, count2;
	unsigned int length1, length2;

	current1 = slab1 + reservelen;
	count1 = *current1++ * 256;
	count1 += *current1++;

	current2 = slab2 + reservelen;
	count2 = *current2++ * 256;
	count2 += *current2++;

	if (count1 != count2) {
		return (false);
	}

#if DNS_RDATASET_FIXED
	current1 += (4 * count1);
	current2 += (4 * count2);
#endif /* if DNS_RDATASET_FIXED */

	while (count1 > 0) {
		length1 = *current1++ * 256;
		length1 += *current1++;

		length2 = *current2++ * 256;
		length2 += *current2++;

#if DNS_RDATASET_FIXED
		current1 += 2;
		current2 += 2;
#endif /* if DNS_RDATASET_FIXED */

		if (length1 != length2 ||
		    memcmp(current1, current2, length1) != 0)
		{
			return (false);
		}

		current1 += length1;
		current2 += length1;

		count1--;
	}
	return (true);
}

bool
dns_rdataslab_equalx(unsigned char *slab1, unsigned char *slab2,
		     unsigned int reservelen, dns_rdataclass_t rdclass,
		     dns_rdatatype_t type) {
	unsigned char *current1, *current2;
	unsigned int count1, count2;
	dns_rdata_t rdata1 = DNS_RDATA_INIT;
	dns_rdata_t rdata2 = DNS_RDATA_INIT;

	current1 = slab1 + reservelen;
	count1 = *current1++ * 256;
	count1 += *current1++;

	current2 = slab2 + reservelen;
	count2 = *current2++ * 256;
	count2 += *current2++;

	if (count1 != count2) {
		return (false);
	}

#if DNS_RDATASET_FIXED
	current1 += (4 * count1);
	current2 += (4 * count2);
#endif /* if DNS_RDATASET_FIXED */

	while (count1-- > 0) {
		rdata_from_slab(&current1, rdclass, type, &rdata1);
		rdata_from_slab(&current2, rdclass, type, &rdata2);
		if (dns_rdata_compare(&rdata1, &rdata2) != 0) {
			return (false);
		}
		dns_rdata_reset(&rdata1);
		dns_rdata_reset(&rdata2);
	}
	return (true);
}