rdataslab.c

Go to the documentation of this file.

/*
 * Copyright (C) 2004-2015  Internet Systems Consortium, Inc. ("ISC")
 * Copyright (C) 1999-2003  Internet Software Consortium.
 *
 * Permission to use, copy, modify, and/or distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES WITH
 * REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
 * AND FITNESS.  IN NO EVENT SHALL ISC BE LIABLE FOR ANY SPECIAL, DIRECT,
 * INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
 * LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE
 * OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
 * PERFORMANCE OF THIS SOFTWARE.
 */

/* $Id$ */

/*! \file */

#include <config.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/result.h>
#include <dns/rdata.h>
#include <dns/rdataset.h>
#include <dns/rdataslab.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 here currently only support DNSSEC order iteration.
 *
 * 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

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 sentinal 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
        unsigned int    buflen;
        isc_result_t    result;
        unsigned int    nitems;
        unsigned int    nalloc;
        unsigned int    i;
#if DNS_RDATASET_FIXED
        unsigned int   *offsettable;
#endif
        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);
                if (rawbuf == NULL)
                        return (ISC_R_NOMEMORY);
                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));
        if (x == NULL)
                return (ISC_R_NOMEMORY);

        /*
         * 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
                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
                        nitems--;
                } else {
#if DNS_RDATASET_FIXED
                        buflen += (8 + x[i-1].rdata.length);
#else
                        buflen += (2 + x[i-1].rdata.length);
#endif
                        /*
                         * 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
        buflen += (2 + x[i-1].rdata.length);
#endif
        /*
         * 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 (rawbuf == NULL) {
                result = ISC_R_NOMEMORY;
                goto free_rdatas;
        }

#if DNS_RDATASET_FIXED
        /* Allocate temporary offset table. */
        offsettable = isc_mem_get(mctx, nalloc * sizeof(unsigned int));
        if (offsettable == NULL) {
                isc_mem_put(mctx, rawbuf, buflen);
                result = ISC_R_NOMEMORY;
                goto free_rdatas;
        }
        memset(offsettable, 0, nalloc * sizeof(unsigned int));
#endif

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

        rawbuf += reservelen;
#if DNS_RDATASET_FIXED
        offsetbase = rawbuf;
#endif

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

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

        for (i = 0; i < nalloc; i++) {
                if (x[i].rdata.data == &removed)
                        continue;
#if DNS_RDATASET_FIXED
                offsettable[x[i].order] = rawbuf - offsetbase;
#endif
                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
                /*
                 * 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

        result = ISC_R_SUCCESS;

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

static void
rdataset_disassociate(dns_rdataset_t *rdataset) {
        UNUSED(rdataset);
}

static isc_result_t
rdataset_first(dns_rdataset_t *rdataset) {
        unsigned char *raw = rdataset->private3;
        unsigned int count;

        count = raw[0] * 256 + raw[1];
        if (count == 0) {
                rdataset->private5 = NULL;
                return (ISC_R_NOMORE);
        }
#if DNS_RDATASET_FIXED
        raw += 2 + (4 * count);
#else
        raw += 2;
#endif
        /*
         * The privateuint4 field is the number of rdata beyond the cursor
         * position, so we decrement the total count by one before storing
         * it.
         */
        count--;
        rdataset->privateuint4 = count;
        rdataset->private5 = raw;

        return (ISC_R_SUCCESS);
}

static isc_result_t
rdataset_next(dns_rdataset_t *rdataset) {
        unsigned int count;
        unsigned int length;
        unsigned char *raw;

        count = rdataset->privateuint4;
        if (count == 0)
                return (ISC_R_NOMORE);
        count--;
        rdataset->privateuint4 = count;
        raw = rdataset->private5;
        length = raw[0] * 256 + raw[1];
#if DNS_RDATASET_FIXED
        raw += length + 4;
#else
        raw += length + 2;
#endif
        rdataset->private5 = raw;

        return (ISC_R_SUCCESS);
}

static void
rdataset_current(dns_rdataset_t *rdataset, dns_rdata_t *rdata) {
        unsigned char *raw = rdataset->private5;
        isc_region_t r;
        unsigned int length;
        unsigned int flags = 0;

        REQUIRE(raw != NULL);

        length = raw[0] * 256 + raw[1];
#if DNS_RDATASET_FIXED
        raw += 4;
#else
        raw += 2;
#endif
        if (rdataset->type == dns_rdatatype_rrsig) {
                if (*raw & DNS_RDATASLAB_OFFLINE)
                        flags |= DNS_RDATA_OFFLINE;
                length--;
                raw++;
        }
        r.length = length;
        r.base = raw;
        dns_rdata_fromregion(rdata, rdataset->rdclass, rdataset->type, &r);
        rdata->flags |= flags;
}

static void
rdataset_clone(dns_rdataset_t *source, dns_rdataset_t *target) {
        *target = *source;

        /*
         * Reset iterator state.
         */
        target->privateuint4 = 0;
        target->private5 = NULL;
}

static unsigned int
rdataset_count(dns_rdataset_t *rdataset) {
        unsigned char *raw = rdataset->private3;
        unsigned int count;

        count = raw[0] * 256 + raw[1];

        return (count);
}

static dns_rdatasetmethods_t rdataset_methods = {
        rdataset_disassociate,
        rdataset_first,
        rdataset_next,
        rdataset_current,
        rdataset_clone,
        rdataset_count,
        NULL,
        NULL,
        NULL,
        NULL,
        NULL,
        NULL,
        NULL,
        NULL,
        NULL,
        NULL,
        NULL,
        NULL
};

void
dns_rdataslab_tordataset(unsigned char *slab, unsigned int reservelen,
                         dns_rdataclass_t rdclass, dns_rdatatype_t rdtype,
                         dns_rdatatype_t covers, dns_ttl_t ttl,
                         dns_rdataset_t *rdataset)
{
        REQUIRE(slab != NULL);
        REQUIRE(!dns_rdataset_isassociated(rdataset));

        rdataset->methods = &rdataset_methods;
        rdataset->rdclass = rdclass;
        rdataset->type = rdtype;
        rdataset->covers = covers;
        rdataset->ttl = ttl;
        rdataset->trust = 0;
        rdataset->private1 = NULL;
        rdataset->private2 = NULL;
        rdataset->private3 = slab + reservelen;

        /*
         * Reset iterator state.
         */
        rdataset->privateuint4 = 0;
        rdataset->private5 = NULL;
}

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

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

/*
 * 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 inline 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;
        isc_boolean_t offline = ISC_FALSE;

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

        if (type == dns_rdatatype_rrsig) {
                if ((*tcurrent & DNS_RDATASLAB_OFFLINE) != 0)
                        offline = ISC_TRUE;
                length--;
                tcurrent++;
        }
        region.length = length;
#if DNS_RDATASET_FIXED
        tcurrent += 2;
#endif
        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 inline isc_boolean_t
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

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

                n = dns_rdata_compare(&trdata, rdata);
                if (n == 0)
                        return (ISC_TRUE);
                if (n > 0)      /* In DNSSEC order. */
                        break;
                dns_rdata_reset(&trdata);
        }
        return (ISC_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;
        isc_boolean_t added_something = ISC_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

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

#if DNS_RDATASET_FIXED
        oncount = ncount;
#endif

        /*
         * 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
                olength += length + 2;
                ocurrent += length;
#endif
        }

        /*
         * 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
                        tlength += nrdata.length + 2;
#endif
                        if (type == dns_rdatatype_rrsig)
                                tlength++;
                        tcount++;
                        nncount++;
                        added_something = ISC_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);
        if (tstart == NULL)
                return (ISC_R_NOMEMORY);
        memmove(tstart, nslab, reservelen);
        tcurrent = tstart + reservelen;
#if DNS_RDATASET_FIXED
        offsetbase = tcurrent;
#endif

        /*
         * 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));
        if (offsettable == NULL) {
                isc_mem_put(mctx, tstart, tlength);
                return (ISC_R_NOMEMORY);
        }
        memset(offsettable, 0, (ocount + oncount) * sizeof(unsigned int));
#endif

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

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

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

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

        while (oadded < ocount || nadded < ncount) {
                isc_boolean_t fromold;
                if (oadded == ocount)
                        fromold = ISC_FALSE;
                else if (nadded == ncount)
                        fromold = ISC_TRUE;
                else
                        fromold = ISC_TF(compare_rdata(&ordata, &nrdata) < 0);
                if (fromold) {
#if DNS_RDATASET_FIXED
                        offsettable[oorder] = tcurrent - offsetbase;
#endif
                        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
                        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
                                rdata_from_slab(&ocurrent, rdclass, type,
                                                &ordata);
                        }
                } else {
#if DNS_RDATASET_FIXED
                        offsettable[ocount + norder] = tcurrent - offsetbase;
#endif
                        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
                        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
                                        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

        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

        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
        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

        /*
         * 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);
        if (tstart == NULL)
                return (ISC_R_NOMEMORY);
        memmove(tstart, mslab, reservelen);
        tcurrent = tstart + reservelen;
#if DNS_RDATASET_FIXED
        offsetbase = tcurrent;

        offsettable = isc_mem_get(mctx, mcount * sizeof(unsigned int));
        if (offsettable == NULL) {
                isc_mem_put(mctx, tstart, tlength);
                return (ISC_R_NOMEMORY);
        }
        memset(offsettable, 0, mcount * sizeof(unsigned int));
#endif

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

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

        /*
         * Copy the parts of mslab not in sslab.
         */
        mcurrent = mslab + reservelen;
        mcount = *mcurrent++ * 256;
        mcount += *mcurrent++;
#if DNS_RDATASET_FIXED
        mcurrent += (4 * mcount);
#endif
        for (i = 0; i < mcount; i++) {
                unsigned char *mrdatabegin = mcurrent;
#if DNS_RDATASET_FIXED
                order = mcurrent[2] * 256 + mcurrent[3];
                INSIST(order < mcount);
#endif
                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
                        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

        INSIST(tcurrent == tstart + tlength);

        *tslabp = tstart;

        return (ISC_R_SUCCESS);
}

isc_boolean_t
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 (ISC_FALSE);

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

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

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

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

                if (length1 != length2 ||
                    memcmp(current1, current2, length1) != 0)
                        return (ISC_FALSE);

                current1 += length1;
                current2 += length1;

                count1--;
        }
        return (ISC_TRUE);
}

isc_boolean_t
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 (ISC_FALSE);

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

        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 (ISC_FALSE);
                dns_rdata_reset(&rdata1);
                dns_rdata_reset(&rdata2);
        }
        return (ISC_TRUE);
}

---