lib/dns/rdataslab.c

	rdataslab.c revision 87bff9accdd711bd02fe3043800d5edd10082521
/*
 * Copyright (C) 2004-2007  Internet Systems Consortium, Inc. ("ISC")
 * Copyright (C) 1999-2003  Internet Software Consortium.
 *
 * Permission to use, copy, modify, and 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: rdataslab.c,v 1.42 2007/03/14 23:46:54 tbox Exp $ */

/*! \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>

#ifndef DNS_RDATASET_FIXED
#define DNS_RDATASET_FIXED 1
#endif

/*
 * 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)
 *      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)
 *      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 reconstuction of
 * 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)
{
    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

    buflen = reservelen + 2;

    nalloc = dns_rdataset_count(rdataset);
    nitems = nalloc;
    if (nitems == 0)
        return (ISC_R_FAILURE);

    if (nalloc > 0xffff)
        return (ISC_R_NOSPACE);

    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)
        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);
#if DNS_RDATASET_FIXED
        x[i].order = i;
#endif
        result = dns_rdataset_next(rdataset);
    }
    if (result != ISC_R_NOMORE)
        goto free_rdatas;
    if (i != nalloc) {
        /*
         * Somehow we iterated over fewer rdatas than
         * dns_rdataset_count() said there were!
         */
        result = ISC_R_FAILURE;
        goto free_rdatas;
    }

    /*
     * Put into DNSSEC order.
     */
    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 = NULL;
            x[i-1].rdata.length = 0;
#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
    }
    /*
     * 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

    /*
     * 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 == NULL)
            continue;
#if DNS_RDATASET_FIXED
        offsettable[x[i].order] = rawbuf - offsetbase;
#endif
        *rawbuf++ = (x[i].rdata.length & 0xff00) >> 8;
        *rawbuf++ = (x[i].rdata.length & 0x00ff);
#if DNS_RDATASET_FIXED
        rawbuf += 2;    /* filled in later */
#endif
        memcpy(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;

    REQUIRE(raw != NULL);

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

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
};

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;

    region.length = *tcurrent++ * 256;
    region.length += *tcurrent++;
#if DNS_RDATASET_FIXED
    tcurrent += 2;
#endif
    region.base = tcurrent;
    tcurrent += region.length;
    dns_rdata_fromregion(rdata, rdclass, type, &region);
    *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;
    unsigned int ocount, ncount, count, olength, tlength, tcount, length;
    isc_region_t nregion;
    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 {
        nregion.length = *ncurrent++ * 256;
        nregion.length += *ncurrent++;
#if DNS_RDATASET_FIXED
        ncurrent += 2;          /* Skip order. */
#endif
        nregion.base = ncurrent;
        dns_rdata_init(&nrdata);
        dns_rdata_fromregion(&nrdata, rdclass, type, &nregion);
        if (!rdata_in_slab(oslab, reservelen, rdclass, type, &nrdata))
        {
            /*
             * This rdata isn't in the old slab.
             */
#if DNS_RDATASET_FIXED
            tlength += nregion.length + 8;
#else
            tlength += nregion.length + 2;
#endif
            tcount++;
            nncount++;
            added_something = ISC_TRUE;
        }
        ncurrent += nregion.length;
        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);
    memcpy(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;
            *tcurrent++ = (length & 0xff00) >> 8;
            *tcurrent++ = (length & 0x00ff);
#if DNS_RDATASET_FIXED
            tcurrent += 2;  /* fill in later */
#endif
            memcpy(tcurrent, ordata.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;
            *tcurrent++ = (length & 0xff00) >> 8;
            *tcurrent++ = (length & 0x00ff);
#if DNS_RDATASET_FIXED
            tcurrent += 2;  /* fill in later */
#endif
            memcpy(tcurrent, nrdata.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;
#endif
    unsigned int order;

    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 += 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);
    memcpy(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 = mcurrent - mrdatabegin;
#if DNS_RDATASET_FIXED
            offsettable[order] = tcurrent - offsetbase;
#endif
            memcpy(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);
}