NTFS: Fix two nasty runlist merging bugs that had gone unnoticed so far.

[linux-2.6/cjktty.git] / fs / ntfs / runlist.c

/**
 * runlist.c - NTFS runlist handling code.  Part of the Linux-NTFS project.
 *
 * Copyright (c) 2001-2005 Anton Altaparmakov
 * Copyright (c) 2002 Richard Russon
 *
 * This program/include file is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as published
 * by the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program/include file 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 General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program (in the main directory of the Linux-NTFS
 * distribution in the file COPYING); if not, write to the Free Software
 * Foundation,Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 */

#include "debug.h"
#include "dir.h"
#include "endian.h"
#include "malloc.h"
#include "ntfs.h"

/**
 * ntfs_rl_mm - runlist memmove
 *
 * It is up to the caller to serialize access to the runlist @base.
 */
static inline void ntfs_rl_mm(runlist_element *base, int dst, int src,
                int size)
{
        if (likely((dst != src) && (size > 0)))
                memmove(base + dst, base + src, size * sizeof(*base));
}

/**
 * ntfs_rl_mc - runlist memory copy
 *
 * It is up to the caller to serialize access to the runlists @dstbase and
 * @srcbase.
 */
static inline void ntfs_rl_mc(runlist_element *dstbase, int dst,
                runlist_element *srcbase, int src, int size)
{
        if (likely(size > 0))
                memcpy(dstbase + dst, srcbase + src, size * sizeof(*dstbase));
}

/**
 * ntfs_rl_realloc - Reallocate memory for runlists
 * @rl:         original runlist
 * @old_size:   number of runlist elements in the original runlist @rl
 * @new_size:   number of runlist elements we need space for
 *
 * As the runlists grow, more memory will be required.  To prevent the
 * kernel having to allocate and reallocate large numbers of small bits of
 * memory, this function returns an entire page of memory.
 *
 * It is up to the caller to serialize access to the runlist @rl.
 *
 * N.B.  If the new allocation doesn't require a different number of pages in
 *       memory, the function will return the original pointer.
 *
 * On success, return a pointer to the newly allocated, or recycled, memory.
 * On error, return -errno. The following error codes are defined:
 *      -ENOMEM - Not enough memory to allocate runlist array.
 *      -EINVAL - Invalid parameters were passed in.
 */
static inline runlist_element *ntfs_rl_realloc(runlist_element *rl,
                int old_size, int new_size)
{
        runlist_element *new_rl;

        old_size = PAGE_ALIGN(old_size * sizeof(*rl));
        new_size = PAGE_ALIGN(new_size * sizeof(*rl));
        if (old_size == new_size)
                return rl;

        new_rl = ntfs_malloc_nofs(new_size);
        if (unlikely(!new_rl))
                return ERR_PTR(-ENOMEM);

        if (likely(rl != NULL)) {
                if (unlikely(old_size > new_size))
                        old_size = new_size;
                memcpy(new_rl, rl, old_size);
                ntfs_free(rl);
        }
        return new_rl;
}

/**
 * ntfs_rl_realloc_nofail - Reallocate memory for runlists
 * @rl:         original runlist
 * @old_size:   number of runlist elements in the original runlist @rl
 * @new_size:   number of runlist elements we need space for
 *
 * As the runlists grow, more memory will be required.  To prevent the
 * kernel having to allocate and reallocate large numbers of small bits of
 * memory, this function returns an entire page of memory.
 *
 * This function guarantees that the allocation will succeed.  It will sleep
 * for as long as it takes to complete the allocation.
 *
 * It is up to the caller to serialize access to the runlist @rl.
 *
 * N.B.  If the new allocation doesn't require a different number of pages in
 *       memory, the function will return the original pointer.
 *
 * On success, return a pointer to the newly allocated, or recycled, memory.
 * On error, return -errno. The following error codes are defined:
 *      -ENOMEM - Not enough memory to allocate runlist array.
 *      -EINVAL - Invalid parameters were passed in.
 */
static inline runlist_element *ntfs_rl_realloc_nofail(runlist_element *rl,
                int old_size, int new_size)
{
        runlist_element *new_rl;

        old_size = PAGE_ALIGN(old_size * sizeof(*rl));
        new_size = PAGE_ALIGN(new_size * sizeof(*rl));
        if (old_size == new_size)
                return rl;

        new_rl = ntfs_malloc_nofs_nofail(new_size);
        BUG_ON(!new_rl);

        if (likely(rl != NULL)) {
                if (unlikely(old_size > new_size))
                        old_size = new_size;
                memcpy(new_rl, rl, old_size);
                ntfs_free(rl);
        }
        return new_rl;
}

/**
 * ntfs_are_rl_mergeable - test if two runlists can be joined together
 * @dst:        original runlist
 * @src:        new runlist to test for mergeability with @dst
 *
 * Test if two runlists can be joined together. For this, their VCNs and LCNs
 * must be adjacent.
 *
 * It is up to the caller to serialize access to the runlists @dst and @src.
 *
 * Return: TRUE   Success, the runlists can be merged.
 *         FALSE  Failure, the runlists cannot be merged.
 */
static inline BOOL ntfs_are_rl_mergeable(runlist_element *dst,
                runlist_element *src)
{
        BUG_ON(!dst);
        BUG_ON(!src);

        if ((dst->lcn < 0) || (src->lcn < 0)) {   /* Are we merging holes? */
                if (dst->lcn == LCN_HOLE && src->lcn == LCN_HOLE)
                        return TRUE;
                return FALSE;
        }
        if ((dst->lcn + dst->length) != src->lcn) /* Are the runs contiguous? */
                return FALSE;
        if ((dst->vcn + dst->length) != src->vcn) /* Are the runs misaligned? */
                return FALSE;

        return TRUE;
}

/**
 * __ntfs_rl_merge - merge two runlists without testing if they can be merged
 * @dst:        original, destination runlist
 * @src:        new runlist to merge with @dst
 *
 * Merge the two runlists, writing into the destination runlist @dst. The
 * caller must make sure the runlists can be merged or this will corrupt the
 * destination runlist.
 *
 * It is up to the caller to serialize access to the runlists @dst and @src.
 */
static inline void __ntfs_rl_merge(runlist_element *dst, runlist_element *src)
{
        dst->length += src->length;
}

/**
 * ntfs_rl_append - append a runlist after a given element
 * @dst:        original runlist to be worked on
 * @dsize:      number of elements in @dst (including end marker)
 * @src:        runlist to be inserted into @dst
 * @ssize:      number of elements in @src (excluding end marker)
 * @loc:        append the new runlist @src after this element in @dst
 *
 * Append the runlist @src after element @loc in @dst.  Merge the right end of
 * the new runlist, if necessary. Adjust the size of the hole before the
 * appended runlist.
 *
 * It is up to the caller to serialize access to the runlists @dst and @src.
 *
 * On success, return a pointer to the new, combined, runlist. Note, both
 * runlists @dst and @src are deallocated before returning so you cannot use
 * the pointers for anything any more. (Strictly speaking the returned runlist
 * may be the same as @dst but this is irrelevant.)
 *
 * On error, return -errno. Both runlists are left unmodified. The following
 * error codes are defined:
 *      -ENOMEM - Not enough memory to allocate runlist array.
 *      -EINVAL - Invalid parameters were passed in.
 */
static inline runlist_element *ntfs_rl_append(runlist_element *dst,
                int dsize, runlist_element *src, int ssize, int loc)
{
        BOOL right;
        int magic;

        BUG_ON(!dst);
        BUG_ON(!src);

        /* First, check if the right hand end needs merging. */
        right = ntfs_are_rl_mergeable(src + ssize - 1, dst + loc + 1);

        /* Space required: @dst size + @src size, less one if we merged. */
        dst = ntfs_rl_realloc(dst, dsize, dsize + ssize - right);
        if (IS_ERR(dst))
                return dst;
        /*
         * We are guaranteed to succeed from here so can start modifying the
         * original runlists.
         */

        /* First, merge the right hand end, if necessary. */
        if (right)
                __ntfs_rl_merge(src + ssize - 1, dst + loc + 1);

        magic = loc + ssize;

        /* Move the tail of @dst out of the way, then copy in @src. */
        ntfs_rl_mm(dst, magic + 1, loc + 1 + right, dsize - loc - 1 - right);
        ntfs_rl_mc(dst, loc + 1, src, 0, ssize);

        /* Adjust the size of the preceding hole. */
        dst[loc].length = dst[loc + 1].vcn - dst[loc].vcn;

        /* We may have changed the length of the file, so fix the end marker */
        if (dst[magic + 1].lcn == LCN_ENOENT)
                dst[magic + 1].vcn = dst[magic].vcn + dst[magic].length;

        return dst;
}

/**
 * ntfs_rl_insert - insert a runlist into another
 * @dst:        original runlist to be worked on
 * @dsize:      number of elements in @dst (including end marker)
 * @src:        new runlist to be inserted
 * @ssize:      number of elements in @src (excluding end marker)
 * @loc:        insert the new runlist @src before this element in @dst
 *
 * Insert the runlist @src before element @loc in the runlist @dst. Merge the
 * left end of the new runlist, if necessary. Adjust the size of the hole
 * after the inserted runlist.
 *
 * It is up to the caller to serialize access to the runlists @dst and @src.
 *
 * On success, return a pointer to the new, combined, runlist. Note, both
 * runlists @dst and @src are deallocated before returning so you cannot use
 * the pointers for anything any more. (Strictly speaking the returned runlist
 * may be the same as @dst but this is irrelevant.)
 *
 * On error, return -errno. Both runlists are left unmodified. The following
 * error codes are defined:
 *      -ENOMEM - Not enough memory to allocate runlist array.
 *      -EINVAL - Invalid parameters were passed in.
 */
static inline runlist_element *ntfs_rl_insert(runlist_element *dst,
                int dsize, runlist_element *src, int ssize, int loc)
{
        BOOL left = FALSE;
        BOOL disc = FALSE;      /* Discontinuity */
        BOOL hole = FALSE;      /* Following a hole */
        int magic;

        BUG_ON(!dst);
        BUG_ON(!src);

        /* disc => Discontinuity between the end of @dst and the start of @src.
         *         This means we might need to insert a hole.
         * hole => @dst ends with a hole or an unmapped region which we can
         *         extend to match the discontinuity. */
        if (loc == 0)
                disc = (src[0].vcn > 0);
        else {
                s64 merged_length;

                left = ntfs_are_rl_mergeable(dst + loc - 1, src);

                merged_length = dst[loc - 1].length;
                if (left)
                        merged_length += src->length;

                disc = (src[0].vcn > dst[loc - 1].vcn + merged_length);
                if (disc)
                        hole = (dst[loc - 1].lcn == LCN_HOLE);
        }

        /* Space required: @dst size + @src size, less one if we merged, plus
         * one if there was a discontinuity, less one for a trailing hole. */
        dst = ntfs_rl_realloc(dst, dsize, dsize + ssize - left + disc - hole);
        if (IS_ERR(dst))
                return dst;
        /*
         * We are guaranteed to succeed from here so can start modifying the
         * original runlist.
         */

        if (left)
                __ntfs_rl_merge(dst + loc - 1, src);

        magic = loc + ssize - left + disc - hole;

        /* Move the tail of @dst out of the way, then copy in @src. */
        ntfs_rl_mm(dst, magic, loc, dsize - loc);
        ntfs_rl_mc(dst, loc + disc - hole, src, left, ssize - left);

        /* Adjust the VCN of the last run ... */
        if (dst[magic].lcn <= LCN_HOLE)
                dst[magic].vcn = dst[magic - 1].vcn + dst[magic - 1].length;
        /* ... and the length. */
        if (dst[magic].lcn == LCN_HOLE || dst[magic].lcn == LCN_RL_NOT_MAPPED)
                dst[magic].length = dst[magic + 1].vcn - dst[magic].vcn;

        /* Writing beyond the end of the file and there's a discontinuity. */
        if (disc) {
                if (hole)
                        dst[loc - 1].length = dst[loc].vcn - dst[loc - 1].vcn;
                else {
                        if (loc > 0) {
                                dst[loc].vcn = dst[loc - 1].vcn +
                                                dst[loc - 1].length;
                                dst[loc].length = dst[loc + 1].vcn -
                                                dst[loc].vcn;
                        } else {
                                dst[loc].vcn = 0;
                                dst[loc].length = dst[loc + 1].vcn;
                        }
                        dst[loc].lcn = LCN_RL_NOT_MAPPED;
                }

                magic += hole;

                if (dst[magic].lcn == LCN_ENOENT)
                        dst[magic].vcn = dst[magic - 1].vcn +
                                        dst[magic - 1].length;
        }
        return dst;
}

/**
 * ntfs_rl_replace - overwrite a runlist element with another runlist
 * @dst:        original runlist to be worked on
 * @dsize:      number of elements in @dst (including end marker)
 * @src:        new runlist to be inserted
 * @ssize:      number of elements in @src (excluding end marker)
 * @loc:        index in runlist @dst to overwrite with @src
 *
 * Replace the runlist element @dst at @loc with @src. Merge the left and
 * right ends of the inserted runlist, if necessary.
 *
 * It is up to the caller to serialize access to the runlists @dst and @src.
 *
 * On success, return a pointer to the new, combined, runlist. Note, both
 * runlists @dst and @src are deallocated before returning so you cannot use
 * the pointers for anything any more. (Strictly speaking the returned runlist
 * may be the same as @dst but this is irrelevant.)
 *
 * On error, return -errno. Both runlists are left unmodified. The following
 * error codes are defined:
 *      -ENOMEM - Not enough memory to allocate runlist array.
 *      -EINVAL - Invalid parameters were passed in.
 */
static inline runlist_element *ntfs_rl_replace(runlist_element *dst,
                int dsize, runlist_element *src, int ssize, int loc)
{
        BOOL left = FALSE;
        BOOL right;
        int magic;

        BUG_ON(!dst);
        BUG_ON(!src);

        /* First, merge the left and right ends, if necessary. */
        right = ntfs_are_rl_mergeable(src + ssize - 1, dst + loc + 1);
        if (loc > 0)
                left = ntfs_are_rl_mergeable(dst + loc - 1, src);

        /* Allocate some space. We'll need less if the left, right, or both
         * ends were merged. */
        dst = ntfs_rl_realloc(dst, dsize, dsize + ssize - left - right);
        if (IS_ERR(dst))
                return dst;
        /*
         * We are guaranteed to succeed from here so can start modifying the
         * original runlists.
         */
        if (right)
                __ntfs_rl_merge(src + ssize - 1, dst + loc + 1);
        if (left)
                __ntfs_rl_merge(dst + loc - 1, src);

        /* FIXME: What does this mean? (AIA) */
        magic = loc + ssize - left;

        /* Move the tail of @dst out of the way, then copy in @src. */
        ntfs_rl_mm(dst, magic, loc + right + 1, dsize - loc - right - 1);
        ntfs_rl_mc(dst, loc, src, left, ssize - left);

        /* We may have changed the length of the file, so fix the end marker */
        if (dst[magic].lcn == LCN_ENOENT)
                dst[magic].vcn = dst[magic - 1].vcn + dst[magic - 1].length;
        return dst;
}

/**
 * ntfs_rl_split - insert a runlist into the centre of a hole
 * @dst:        original runlist to be worked on
 * @dsize:      number of elements in @dst (including end marker)
 * @src:        new runlist to be inserted
 * @ssize:      number of elements in @src (excluding end marker)
 * @loc:        index in runlist @dst at which to split and insert @src
 *
 * Split the runlist @dst at @loc into two and insert @new in between the two
 * fragments. No merging of runlists is necessary. Adjust the size of the
 * holes either side.
 *
 * It is up to the caller to serialize access to the runlists @dst and @src.
 *
 * On success, return a pointer to the new, combined, runlist. Note, both
 * runlists @dst and @src are deallocated before returning so you cannot use
 * the pointers for anything any more. (Strictly speaking the returned runlist
 * may be the same as @dst but this is irrelevant.)
 *
 * On error, return -errno. Both runlists are left unmodified. The following
 * error codes are defined:
 *      -ENOMEM - Not enough memory to allocate runlist array.
 *      -EINVAL - Invalid parameters were passed in.
 */
static inline runlist_element *ntfs_rl_split(runlist_element *dst, int dsize,
                runlist_element *src, int ssize, int loc)
{
        BUG_ON(!dst);
        BUG_ON(!src);

        /* Space required: @dst size + @src size + one new hole. */
        dst = ntfs_rl_realloc(dst, dsize, dsize + ssize + 1);
        if (IS_ERR(dst))
                return dst;
        /*
         * We are guaranteed to succeed from here so can start modifying the
         * original runlists.
         */

        /* Move the tail of @dst out of the way, then copy in @src. */
        ntfs_rl_mm(dst, loc + 1 + ssize, loc, dsize - loc);
        ntfs_rl_mc(dst, loc + 1, src, 0, ssize);

        /* Adjust the size of the holes either size of @src. */
        dst[loc].length         = dst[loc+1].vcn       - dst[loc].vcn;
        dst[loc+ssize+1].vcn    = dst[loc+ssize].vcn   + dst[loc+ssize].length;
        dst[loc+ssize+1].length = dst[loc+ssize+2].vcn - dst[loc+ssize+1].vcn;

        return dst;
}

/**
 * ntfs_runlists_merge - merge two runlists into one
 * @drl:        original runlist to be worked on
 * @srl:        new runlist to be merged into @drl
 *
 * First we sanity check the two runlists @srl and @drl to make sure that they
 * are sensible and can be merged. The runlist @srl must be either after the
 * runlist @drl or completely within a hole (or unmapped region) in @drl.
 *
 * It is up to the caller to serialize access to the runlists @drl and @srl.
 *
 * Merging of runlists is necessary in two cases:
 *   1. When attribute lists are used and a further extent is being mapped.
 *   2. When new clusters are allocated to fill a hole or extend a file.
 *
 * There are four possible ways @srl can be merged. It can:
 *      - be inserted at the beginning of a hole,
 *      - split the hole in two and be inserted between the two fragments,
 *      - be appended at the end of a hole, or it can
 *      - replace the whole hole.
 * It can also be appended to the end of the runlist, which is just a variant
 * of the insert case.
 *
 * On success, return a pointer to the new, combined, runlist. Note, both
 * runlists @drl and @srl are deallocated before returning so you cannot use
 * the pointers for anything any more. (Strictly speaking the returned runlist
 * may be the same as @dst but this is irrelevant.)
 *
 * On error, return -errno. Both runlists are left unmodified. The following
 * error codes are defined:
 *      -ENOMEM - Not enough memory to allocate runlist array.
 *      -EINVAL - Invalid parameters were passed in.
 *      -ERANGE - The runlists overlap and cannot be merged.
 */
runlist_element *ntfs_runlists_merge(runlist_element *drl,
                runlist_element *srl)
{
        int di, si;             /* Current index into @[ds]rl. */
        int sstart;             /* First index with lcn > LCN_RL_NOT_MAPPED. */
        int dins;               /* Index into @drl at which to insert @srl. */
        int dend, send;         /* Last index into @[ds]rl. */
        int dfinal, sfinal;     /* The last index into @[ds]rl with
                                   lcn >= LCN_HOLE. */
        int marker = 0;
        VCN marker_vcn = 0;

#ifdef DEBUG
        ntfs_debug("dst:");
        ntfs_debug_dump_runlist(drl);
        ntfs_debug("src:");
        ntfs_debug_dump_runlist(srl);
#endif

        /* Check for silly calling... */
        if (unlikely(!srl))
                return drl;
        if (IS_ERR(srl) || IS_ERR(drl))
                return ERR_PTR(-EINVAL);

        /* Check for the case where the first mapping is being done now. */
        if (unlikely(!drl)) {
                drl = srl;
                /* Complete the source runlist if necessary. */
                if (unlikely(drl[0].vcn)) {
                        /* Scan to the end of the source runlist. */
                        for (dend = 0; likely(drl[dend].length); dend++)
                                ;
                        dend++;
                        drl = ntfs_rl_realloc(drl, dend, dend + 1);
                        if (IS_ERR(drl))
                                return drl;
                        /* Insert start element at the front of the runlist. */
                        ntfs_rl_mm(drl, 1, 0, dend);
                        drl[0].vcn = 0;
                        drl[0].lcn = LCN_RL_NOT_MAPPED;
                        drl[0].length = drl[1].vcn;
                }
                goto finished;
        }

        si = di = 0;

        /* Skip any unmapped start element(s) in the source runlist. */
        while (srl[si].length && srl[si].lcn < LCN_HOLE)
                si++;

        /* Can't have an entirely unmapped source runlist. */
        BUG_ON(!srl[si].length);

        /* Record the starting points. */
        sstart = si;

        /*
         * Skip forward in @drl until we reach the position where @srl needs to
         * be inserted. If we reach the end of @drl, @srl just needs to be
         * appended to @drl.
         */
        for (; drl[di].length; di++) {
                if (drl[di].vcn + drl[di].length > srl[sstart].vcn)
                        break;
        }
        dins = di;

        /* Sanity check for illegal overlaps. */
        if ((drl[di].vcn == srl[si].vcn) && (drl[di].lcn >= 0) &&
                        (srl[si].lcn >= 0)) {
                ntfs_error(NULL, "Run lists overlap. Cannot merge!");
                return ERR_PTR(-ERANGE);
        }

        /* Scan to the end of both runlists in order to know their sizes. */
        for (send = si; srl[send].length; send++)
                ;
        for (dend = di; drl[dend].length; dend++)
                ;

        if (srl[send].lcn == LCN_ENOENT)
                marker_vcn = srl[marker = send].vcn;

        /* Scan to the last element with lcn >= LCN_HOLE. */
        for (sfinal = send; sfinal >= 0 && srl[sfinal].lcn < LCN_HOLE; sfinal--)
                ;
        for (dfinal = dend; dfinal >= 0 && drl[dfinal].lcn < LCN_HOLE; dfinal--)
                ;

        {
        BOOL start;
        BOOL finish;
        int ds = dend + 1;              /* Number of elements in drl & srl */
        int ss = sfinal - sstart + 1;

        start  = ((drl[dins].lcn <  LCN_RL_NOT_MAPPED) ||    /* End of file   */
                  (drl[dins].vcn == srl[sstart].vcn));       /* Start of hole */
        finish = ((drl[dins].lcn >= LCN_RL_NOT_MAPPED) &&    /* End of file   */
                 ((drl[dins].vcn + drl[dins].length) <=      /* End of hole   */
                  (srl[send - 1].vcn + srl[send - 1].length)));

        /* Or we will lose an end marker. */
        if (finish && !drl[dins].length)
                ss++;
        if (marker && (drl[dins].vcn + drl[dins].length > srl[send - 1].vcn))
                finish = FALSE;
#if 0
        ntfs_debug("dfinal = %i, dend = %i", dfinal, dend);
        ntfs_debug("sstart = %i, sfinal = %i, send = %i", sstart, sfinal, send);
        ntfs_debug("start = %i, finish = %i", start, finish);
        ntfs_debug("ds = %i, ss = %i, dins = %i", ds, ss, dins);
#endif
        if (start) {
                if (finish)
                        drl = ntfs_rl_replace(drl, ds, srl + sstart, ss, dins);
                else
                        drl = ntfs_rl_insert(drl, ds, srl + sstart, ss, dins);
        } else {
                if (finish)
                        drl = ntfs_rl_append(drl, ds, srl + sstart, ss, dins);
                else
                        drl = ntfs_rl_split(drl, ds, srl + sstart, ss, dins);
        }
        if (IS_ERR(drl)) {
                ntfs_error(NULL, "Merge failed.");
                return drl;
        }
        ntfs_free(srl);
        if (marker) {
                ntfs_debug("Triggering marker code.");
                for (ds = dend; drl[ds].length; ds++)
                        ;
                /* We only need to care if @srl ended after @drl. */
                if (drl[ds].vcn <= marker_vcn) {
                        int slots = 0;

                        if (drl[ds].vcn == marker_vcn) {
                                ntfs_debug("Old marker = 0x%llx, replacing "
                                                "with LCN_ENOENT.",
                                                (unsigned long long)
                                                drl[ds].lcn);
                                drl[ds].lcn = LCN_ENOENT;
                                goto finished;
                        }
                        /*
                         * We need to create an unmapped runlist element in
                         * @drl or extend an existing one before adding the
                         * ENOENT terminator.
                         */
                        if (drl[ds].lcn == LCN_ENOENT) {
                                ds--;
                                slots = 1;
                        }
                        if (drl[ds].lcn != LCN_RL_NOT_MAPPED) {
                                /* Add an unmapped runlist element. */
                                if (!slots) {
                                        drl = ntfs_rl_realloc_nofail(drl, ds,
                                                        ds + 2);
                                        slots = 2;
                                }
                                ds++;
                                /* Need to set vcn if it isn't set already. */
                                if (slots != 1)
                                        drl[ds].vcn = drl[ds - 1].vcn +
                                                        drl[ds - 1].length;
                                drl[ds].lcn = LCN_RL_NOT_MAPPED;
                                /* We now used up a slot. */
                                slots--;
                        }
                        drl[ds].length = marker_vcn - drl[ds].vcn;
                        /* Finally add the ENOENT terminator. */
                        ds++;
                        if (!slots)
                                drl = ntfs_rl_realloc_nofail(drl, ds, ds + 1);
                        drl[ds].vcn = marker_vcn;
                        drl[ds].lcn = LCN_ENOENT;
                        drl[ds].length = (s64)0;
                }
        }
        }

finished:
        /* The merge was completed successfully. */
        ntfs_debug("Merged runlist:");
        ntfs_debug_dump_runlist(drl);
        return drl;
}

/**
 * ntfs_mapping_pairs_decompress - convert mapping pairs array to runlist
 * @vol:        ntfs volume on which the attribute resides
 * @attr:       attribute record whose mapping pairs array to decompress
 * @old_rl:     optional runlist in which to insert @attr's runlist
 *
 * It is up to the caller to serialize access to the runlist @old_rl.
 *
 * Decompress the attribute @attr's mapping pairs array into a runlist. On
 * success, return the decompressed runlist.
 *
 * If @old_rl is not NULL, decompressed runlist is inserted into the
 * appropriate place in @old_rl and the resultant, combined runlist is
 * returned. The original @old_rl is deallocated.
 *
 * On error, return -errno. @old_rl is left unmodified in that case.
 *
 * The following error codes are defined:
 *      -ENOMEM - Not enough memory to allocate runlist array.
 *      -EIO    - Corrupt runlist.
 *      -EINVAL - Invalid parameters were passed in.
 *      -ERANGE - The two runlists overlap.
 *
 * FIXME: For now we take the conceptionally simplest approach of creating the
 * new runlist disregarding the already existing one and then splicing the
 * two into one, if that is possible (we check for overlap and discard the new
 * runlist if overlap present before returning ERR_PTR(-ERANGE)).
 */
runlist_element *ntfs_mapping_pairs_decompress(const ntfs_volume *vol,
                const ATTR_RECORD *attr, runlist_element *old_rl)
{
        VCN vcn;                /* Current vcn. */
        LCN lcn;                /* Current lcn. */
        s64 deltaxcn;           /* Change in [vl]cn. */
        runlist_element *rl;    /* The output runlist. */
        u8 *buf;                /* Current position in mapping pairs array. */
        u8 *attr_end;           /* End of attribute. */
        int rlsize;             /* Size of runlist buffer. */
        u16 rlpos;              /* Current runlist position in units of
                                   runlist_elements. */
        u8 b;                   /* Current byte offset in buf. */

#ifdef DEBUG
        /* Make sure attr exists and is non-resident. */
        if (!attr || !attr->non_resident || sle64_to_cpu(
                        attr->data.non_resident.lowest_vcn) < (VCN)0) {
                ntfs_error(vol->sb, "Invalid arguments.");
                return ERR_PTR(-EINVAL);
        }
#endif
        /* Start at vcn = lowest_vcn and lcn 0. */
        vcn = sle64_to_cpu(attr->data.non_resident.lowest_vcn);
        lcn = 0;
        /* Get start of the mapping pairs array. */
        buf = (u8*)attr + le16_to_cpu(
                        attr->data.non_resident.mapping_pairs_offset);
        attr_end = (u8*)attr + le32_to_cpu(attr->length);
        if (unlikely(buf < (u8*)attr || buf > attr_end)) {
                ntfs_error(vol->sb, "Corrupt attribute.");
                return ERR_PTR(-EIO);
        }
        /* Current position in runlist array. */
        rlpos = 0;
        /* Allocate first page and set current runlist size to one page. */
        rl = ntfs_malloc_nofs(rlsize = PAGE_SIZE);
        if (unlikely(!rl))
                return ERR_PTR(-ENOMEM);
        /* Insert unmapped starting element if necessary. */
        if (vcn) {
                rl->vcn = 0;
                rl->lcn = LCN_RL_NOT_MAPPED;
                rl->length = vcn;
                rlpos++;
        }
        while (buf < attr_end && *buf) {
                /*
                 * Allocate more memory if needed, including space for the
                 * not-mapped and terminator elements. ntfs_malloc_nofs()
                 * operates on whole pages only.
                 */
                if (((rlpos + 3) * sizeof(*old_rl)) > rlsize) {
                        runlist_element *rl2;

                        rl2 = ntfs_malloc_nofs(rlsize + (int)PAGE_SIZE);
                        if (unlikely(!rl2)) {
                                ntfs_free(rl);
                                return ERR_PTR(-ENOMEM);
                        }
                        memcpy(rl2, rl, rlsize);
                        ntfs_free(rl);
                        rl = rl2;
                        rlsize += PAGE_SIZE;
                }
                /* Enter the current vcn into the current runlist element. */
                rl[rlpos].vcn = vcn;
                /*
                 * Get the change in vcn, i.e. the run length in clusters.
                 * Doing it this way ensures that we signextend negative values.
                 * A negative run length doesn't make any sense, but hey, I
                 * didn't make up the NTFS specs and Windows NT4 treats the run
                 * length as a signed value so that's how it is...
                 */
                b = *buf & 0xf;
                if (b) {
                        if (unlikely(buf + b > attr_end))
                                goto io_error;
                        for (deltaxcn = (s8)buf[b--]; b; b--)
                                deltaxcn = (deltaxcn << 8) + buf[b];
                } else { /* The length entry is compulsory. */
                        ntfs_error(vol->sb, "Missing length entry in mapping "
                                        "pairs array.");
                        deltaxcn = (s64)-1;
                }
                /*
                 * Assume a negative length to indicate data corruption and
                 * hence clean-up and return NULL.
                 */
                if (unlikely(deltaxcn < 0)) {
                        ntfs_error(vol->sb, "Invalid length in mapping pairs "
                                        "array.");
                        goto err_out;
                }
                /*
                 * Enter the current run length into the current runlist
                 * element.
                 */
                rl[rlpos].length = deltaxcn;
                /* Increment the current vcn by the current run length. */
                vcn += deltaxcn;
                /*
                 * There might be no lcn change at all, as is the case for
                 * sparse clusters on NTFS 3.0+, in which case we set the lcn
                 * to LCN_HOLE.
                 */
                if (!(*buf & 0xf0))
                        rl[rlpos].lcn = LCN_HOLE;
                else {
                        /* Get the lcn change which really can be negative. */
                        u8 b2 = *buf & 0xf;
                        b = b2 + ((*buf >> 4) & 0xf);
                        if (buf + b > attr_end)
                                goto io_error;
                        for (deltaxcn = (s8)buf[b--]; b > b2; b--)
                                deltaxcn = (deltaxcn << 8) + buf[b];
                        /* Change the current lcn to its new value. */
                        lcn += deltaxcn;
#ifdef DEBUG
                        /*
                         * On NTFS 1.2-, apparently can have lcn == -1 to
                         * indicate a hole. But we haven't verified ourselves
                         * whether it is really the lcn or the deltaxcn that is
                         * -1. So if either is found give us a message so we
                         * can investigate it further!
                         */
                        if (vol->major_ver < 3) {
                                if (unlikely(deltaxcn == (LCN)-1))
                                        ntfs_error(vol->sb, "lcn delta == -1");
                                if (unlikely(lcn == (LCN)-1))
                                        ntfs_error(vol->sb, "lcn == -1");
                        }
#endif
                        /* Check lcn is not below -1. */
                        if (unlikely(lcn < (LCN)-1)) {
                                ntfs_error(vol->sb, "Invalid LCN < -1 in "
                                                "mapping pairs array.");
                                goto err_out;
                        }
                        /* Enter the current lcn into the runlist element. */
                        rl[rlpos].lcn = lcn;
                }
                /* Get to the next runlist element. */
                rlpos++;
                /* Increment the buffer position to the next mapping pair. */
                buf += (*buf & 0xf) + ((*buf >> 4) & 0xf) + 1;
        }
        if (unlikely(buf >= attr_end))
                goto io_error;
        /*
         * If there is a highest_vcn specified, it must be equal to the final
         * vcn in the runlist - 1, or something has gone badly wrong.
         */
        deltaxcn = sle64_to_cpu(attr->data.non_resident.highest_vcn);
        if (unlikely(deltaxcn && vcn - 1 != deltaxcn)) {
mpa_err:
                ntfs_error(vol->sb, "Corrupt mapping pairs array in "
                                "non-resident attribute.");
                goto err_out;
        }
        /* Setup not mapped runlist element if this is the base extent. */
        if (!attr->data.non_resident.lowest_vcn) {
                VCN max_cluster;

                max_cluster = ((sle64_to_cpu(
                                attr->data.non_resident.allocated_size) +
                                vol->cluster_size - 1) >>
                                vol->cluster_size_bits) - 1;
                /*
                 * A highest_vcn of zero means this is a single extent
                 * attribute so simply terminate the runlist with LCN_ENOENT).
                 */
                if (deltaxcn) {
                        /*
                         * If there is a difference between the highest_vcn and
                         * the highest cluster, the runlist is either corrupt
                         * or, more likely, there are more extents following
                         * this one.
                         */
                        if (deltaxcn < max_cluster) {
                                ntfs_debug("More extents to follow; deltaxcn "
                                                "= 0x%llx, max_cluster = "
                                                "0x%llx",
                                                (unsigned long long)deltaxcn,
                                                (unsigned long long)
                                                max_cluster);
                                rl[rlpos].vcn = vcn;
                                vcn += rl[rlpos].length = max_cluster -
                                                deltaxcn;
                                rl[rlpos].lcn = LCN_RL_NOT_MAPPED;
                                rlpos++;
                        } else if (unlikely(deltaxcn > max_cluster)) {
                                ntfs_error(vol->sb, "Corrupt attribute.  "
                                                "deltaxcn = 0x%llx, "
                                                "max_cluster = 0x%llx",
                                                (unsigned long long)deltaxcn,
                                                (unsigned long long)
                                                max_cluster);
                                goto mpa_err;
                        }
                }
                rl[rlpos].lcn = LCN_ENOENT;
        } else /* Not the base extent. There may be more extents to follow. */
                rl[rlpos].lcn = LCN_RL_NOT_MAPPED;

        /* Setup terminating runlist element. */
        rl[rlpos].vcn = vcn;
        rl[rlpos].length = (s64)0;
        /* If no existing runlist was specified, we are done. */
        if (!old_rl) {
                ntfs_debug("Mapping pairs array successfully decompressed:");
                ntfs_debug_dump_runlist(rl);
                return rl;
        }
        /* Now combine the new and old runlists checking for overlaps. */
        old_rl = ntfs_runlists_merge(old_rl, rl);
        if (likely(!IS_ERR(old_rl)))
                return old_rl;
        ntfs_free(rl);
        ntfs_error(vol->sb, "Failed to merge runlists.");
        return old_rl;
io_error:
        ntfs_error(vol->sb, "Corrupt attribute.");
err_out:
        ntfs_free(rl);
        return ERR_PTR(-EIO);
}

/**
 * ntfs_rl_vcn_to_lcn - convert a vcn into a lcn given a runlist
 * @rl:         runlist to use for conversion
 * @vcn:        vcn to convert
 *
 * Convert the virtual cluster number @vcn of an attribute into a logical
 * cluster number (lcn) of a device using the runlist @rl to map vcns to their
 * corresponding lcns.
 *
 * It is up to the caller to serialize access to the runlist @rl.
 *
 * Since lcns must be >= 0, we use negative return codes with special meaning:
 *
 * Return code          Meaning / Description
 * ==================================================
 *  LCN_HOLE            Hole / not allocated on disk.
 *  LCN_RL_NOT_MAPPED   This is part of the runlist which has not been
 *                      inserted into the runlist yet.
 *  LCN_ENOENT          There is no such vcn in the attribute.
 *
 * Locking: - The caller must have locked the runlist (for reading or writing).
 *          - This function does not touch the lock, nor does it modify the
 *            runlist.
 */
LCN ntfs_rl_vcn_to_lcn(const runlist_element *rl, const VCN vcn)
{
        int i;

        BUG_ON(vcn < 0);
        /*
         * If rl is NULL, assume that we have found an unmapped runlist. The
         * caller can then attempt to map it and fail appropriately if
         * necessary.
         */
        if (unlikely(!rl))
                return LCN_RL_NOT_MAPPED;

        /* Catch out of lower bounds vcn. */
        if (unlikely(vcn < rl[0].vcn))
                return LCN_ENOENT;

        for (i = 0; likely(rl[i].length); i++) {
                if (unlikely(vcn < rl[i+1].vcn)) {
                        if (likely(rl[i].lcn >= (LCN)0))
                                return rl[i].lcn + (vcn - rl[i].vcn);
                        return rl[i].lcn;
                }
        }
        /*
         * The terminator element is setup to the correct value, i.e. one of
         * LCN_HOLE, LCN_RL_NOT_MAPPED, or LCN_ENOENT.
         */
        if (likely(rl[i].lcn < (LCN)0))
                return rl[i].lcn;
        /* Just in case... We could replace this with BUG() some day. */
        return LCN_ENOENT;
}

#ifdef NTFS_RW

/**
 * ntfs_rl_find_vcn_nolock - find a vcn in a runlist
 * @rl:         runlist to search
 * @vcn:        vcn to find
 *
 * Find the virtual cluster number @vcn in the runlist @rl and return the
 * address of the runlist element containing the @vcn on success.
 *
 * Return NULL if @rl is NULL or @vcn is in an unmapped part/out of bounds of
 * the runlist.
 *
 * Locking: The runlist must be locked on entry.
 */
runlist_element *ntfs_rl_find_vcn_nolock(runlist_element *rl, const VCN vcn)
{
        BUG_ON(vcn < 0);
        if (unlikely(!rl || vcn < rl[0].vcn))
                return NULL;
        while (likely(rl->length)) {
                if (unlikely(vcn < rl[1].vcn)) {
                        if (likely(rl->lcn >= LCN_HOLE))
                                return rl;
                        return NULL;
                }
                rl++;
        }
        if (likely(rl->lcn == LCN_ENOENT))
                return rl;
        return NULL;
}

/**
 * ntfs_get_nr_significant_bytes - get number of bytes needed to store a number
 * @n:          number for which to get the number of bytes for
 *
 * Return the number of bytes required to store @n unambiguously as
 * a signed number.
 *
 * This is used in the context of the mapping pairs array to determine how
 * many bytes will be needed in the array to store a given logical cluster
 * number (lcn) or a specific run length.
 *
 * Return the number of bytes written.  This function cannot fail.
 */
static inline int ntfs_get_nr_significant_bytes(const s64 n)
{
        s64 l = n;
        int i;
        s8 j;

        i = 0;
        do {
                l >>= 8;
                i++;
        } while (l != 0 && l != -1);
        j = (n >> 8 * (i - 1)) & 0xff;
        /* If the sign bit is wrong, we need an extra byte. */
        if ((n < 0 && j >= 0) || (n > 0 && j < 0))
                i++;
        return i;
}

/**
 * ntfs_get_size_for_mapping_pairs - get bytes needed for mapping pairs array
 * @vol:        ntfs volume (needed for the ntfs version)
 * @rl:         locked runlist to determine the size of the mapping pairs of
 * @first_vcn:  first vcn which to include in the mapping pairs array
 * @last_vcn:   last vcn which to include in the mapping pairs array
 *
 * Walk the locked runlist @rl and calculate the size in bytes of the mapping
 * pairs array corresponding to the runlist @rl, starting at vcn @first_vcn and
 * finishing with vcn @last_vcn.
 *
 * A @last_vcn of -1 means end of runlist and in that case the size of the
 * mapping pairs array corresponding to the runlist starting at vcn @first_vcn
 * and finishing at the end of the runlist is determined.
 *
 * This for example allows us to allocate a buffer of the right size when
 * building the mapping pairs array.
 *
 * If @rl is NULL, just return 1 (for the single terminator byte).
 *
 * Return the calculated size in bytes on success.  On error, return -errno.
 * The following error codes are defined:
 *      -EINVAL - Run list contains unmapped elements.  Make sure to only pass
 *                fully mapped runlists to this function.
 *      -EIO    - The runlist is corrupt.
 *
 * Locking: @rl must be locked on entry (either for reading or writing), it
 *          remains locked throughout, and is left locked upon return.
 */
int ntfs_get_size_for_mapping_pairs(const ntfs_volume *vol,
                const runlist_element *rl, const VCN first_vcn,
                const VCN last_vcn)
{
        LCN prev_lcn;
        int rls;
        BOOL the_end = FALSE;

        BUG_ON(first_vcn < 0);
        BUG_ON(last_vcn < -1);
        BUG_ON(last_vcn >= 0 && first_vcn > last_vcn);
        if (!rl) {
                BUG_ON(first_vcn);
                BUG_ON(last_vcn > 0);
                return 1;
        }
        /* Skip to runlist element containing @first_vcn. */
        while (rl->length && first_vcn >= rl[1].vcn)
                rl++;
        if (unlikely((!rl->length && first_vcn > rl->vcn) ||
                        first_vcn < rl->vcn))
                return -EINVAL;
        prev_lcn = 0;
        /* Always need the termining zero byte. */
        rls = 1;
        /* Do the first partial run if present. */
        if (first_vcn > rl->vcn) {
                s64 delta, length = rl->length;

                /* We know rl->length != 0 already. */
                if (unlikely(length < 0 || rl->lcn < LCN_HOLE))
                        goto err_out;
                /*
                 * If @stop_vcn is given and finishes inside this run, cap the
                 * run length.
                 */
                if (unlikely(last_vcn >= 0 && rl[1].vcn > last_vcn)) {
                        s64 s1 = last_vcn + 1;
                        if (unlikely(rl[1].vcn > s1))
                                length = s1 - rl->vcn;
                        the_end = TRUE;
                }
                delta = first_vcn - rl->vcn;
                /* Header byte + length. */
                rls += 1 + ntfs_get_nr_significant_bytes(length - delta);
                /*
                 * If the logical cluster number (lcn) denotes a hole and we
                 * are on NTFS 3.0+, we don't store it at all, i.e. we need
                 * zero space.  On earlier NTFS versions we just store the lcn.
                 * Note: this assumes that on NTFS 1.2-, holes are stored with
                 * an lcn of -1 and not a delta_lcn of -1 (unless both are -1).
                 */
                if (likely(rl->lcn >= 0 || vol->major_ver < 3)) {
                        prev_lcn = rl->lcn;
                        if (likely(rl->lcn >= 0))
                                prev_lcn += delta;
                        /* Change in lcn. */
                        rls += ntfs_get_nr_significant_bytes(prev_lcn);
                }
                /* Go to next runlist element. */
                rl++;
        }
        /* Do the full runs. */
        for (; rl->length && !the_end; rl++) {
                s64 length = rl->length;

                if (unlikely(length < 0 || rl->lcn < LCN_HOLE))
                        goto err_out;
                /*
                 * If @stop_vcn is given and finishes inside this run, cap the
                 * run length.
                 */
                if (unlikely(last_vcn >= 0 && rl[1].vcn > last_vcn)) {
                        s64 s1 = last_vcn + 1;
                        if (unlikely(rl[1].vcn > s1))
                                length = s1 - rl->vcn;
                        the_end = TRUE;
                }
                /* Header byte + length. */
                rls += 1 + ntfs_get_nr_significant_bytes(length);
                /*
                 * If the logical cluster number (lcn) denotes a hole and we
                 * are on NTFS 3.0+, we don't store it at all, i.e. we need
                 * zero space.  On earlier NTFS versions we just store the lcn.
                 * Note: this assumes that on NTFS 1.2-, holes are stored with
                 * an lcn of -1 and not a delta_lcn of -1 (unless both are -1).
                 */
                if (likely(rl->lcn >= 0 || vol->major_ver < 3)) {
                        /* Change in lcn. */
                        rls += ntfs_get_nr_significant_bytes(rl->lcn -
                                        prev_lcn);
                        prev_lcn = rl->lcn;
                }
        }
        return rls;
err_out:
        if (rl->lcn == LCN_RL_NOT_MAPPED)
                rls = -EINVAL;
        else
                rls = -EIO;
        return rls;
}

/**
 * ntfs_write_significant_bytes - write the significant bytes of a number
 * @dst:        destination buffer to write to
 * @dst_max:    pointer to last byte of destination buffer for bounds checking
 * @n:          number whose significant bytes to write
 *
 * Store in @dst, the minimum bytes of the number @n which are required to
 * identify @n unambiguously as a signed number, taking care not to exceed
 * @dest_max, the maximum position within @dst to which we are allowed to
 * write.
 *
 * This is used when building the mapping pairs array of a runlist to compress
 * a given logical cluster number (lcn) or a specific run length to the minumum
 * size possible.
 *
 * Return the number of bytes written on success.  On error, i.e. the
 * destination buffer @dst is too small, return -ENOSPC.
 */
static inline int ntfs_write_significant_bytes(s8 *dst, const s8 *dst_max,
                const s64 n)
{
        s64 l = n;
        int i;
        s8 j;

        i = 0;
        do {
                if (unlikely(dst > dst_max))
                        goto err_out;
                *dst++ = l & 0xffll;
                l >>= 8;
                i++;
        } while (l != 0 && l != -1);
        j = (n >> 8 * (i - 1)) & 0xff;
        /* If the sign bit is wrong, we need an extra byte. */
        if (n < 0 && j >= 0) {
                if (unlikely(dst > dst_max))
                        goto err_out;
                i++;
                *dst = (s8)-1;
        } else if (n > 0 && j < 0) {
                if (unlikely(dst > dst_max))
                        goto err_out;
                i++;
                *dst = (s8)0;
        }
        return i;
err_out:
        return -ENOSPC;
}

/**
 * ntfs_mapping_pairs_build - build the mapping pairs array from a runlist
 * @vol:        ntfs volume (needed for the ntfs version)
 * @dst:        destination buffer to which to write the mapping pairs array
 * @dst_len:    size of destination buffer @dst in bytes
 * @rl:         locked runlist for which to build the mapping pairs array
 * @first_vcn:  first vcn which to include in the mapping pairs array
 * @last_vcn:   last vcn which to include in the mapping pairs array
 * @stop_vcn:   first vcn outside destination buffer on success or -ENOSPC
 *
 * Create the mapping pairs array from the locked runlist @rl, starting at vcn
 * @first_vcn and finishing with vcn @last_vcn and save the array in @dst.
 * @dst_len is the size of @dst in bytes and it should be at least equal to the
 * value obtained by calling ntfs_get_size_for_mapping_pairs().
 *
 * A @last_vcn of -1 means end of runlist and in that case the mapping pairs
 * array corresponding to the runlist starting at vcn @first_vcn and finishing
 * at the end of the runlist is created.
 *
 * If @rl is NULL, just write a single terminator byte to @dst.
 *
 * On success or -ENOSPC error, if @stop_vcn is not NULL, *@stop_vcn is set to
 * the first vcn outside the destination buffer.  Note that on error, @dst has
 * been filled with all the mapping pairs that will fit, thus it can be treated
 * as partial success, in that a new attribute extent needs to be created or
 * the next extent has to be used and the mapping pairs build has to be
 * continued with @first_vcn set to *@stop_vcn.
 *
 * Return 0 on success and -errno on error.  The following error codes are
 * defined:
 *      -EINVAL - Run list contains unmapped elements.  Make sure to only pass
 *                fully mapped runlists to this function.
 *      -EIO    - The runlist is corrupt.
 *      -ENOSPC - The destination buffer is too small.
 *
 * Locking: @rl must be locked on entry (either for reading or writing), it
 *          remains locked throughout, and is left locked upon return.
 */
int ntfs_mapping_pairs_build(const ntfs_volume *vol, s8 *dst,
                const int dst_len, const runlist_element *rl,
                const VCN first_vcn, const VCN last_vcn, VCN *const stop_vcn)
{
        LCN prev_lcn;
        s8 *dst_max, *dst_next;
        int err = -ENOSPC;
        BOOL the_end = FALSE;
        s8 len_len, lcn_len;

        BUG_ON(first_vcn < 0);
        BUG_ON(last_vcn < -1);
        BUG_ON(last_vcn >= 0 && first_vcn > last_vcn);
        BUG_ON(dst_len < 1);
        if (!rl) {
                BUG_ON(first_vcn);
                BUG_ON(last_vcn > 0);
                if (stop_vcn)
                        *stop_vcn = 0;
                /* Terminator byte. */
                *dst = 0;
                return 0;
        }
        /* Skip to runlist element containing @first_vcn. */
        while (rl->length && first_vcn >= rl[1].vcn)
                rl++;
        if (unlikely((!rl->length && first_vcn > rl->vcn) ||
                        first_vcn < rl->vcn))
                return -EINVAL;
        /*
         * @dst_max is used for bounds checking in
         * ntfs_write_significant_bytes().
         */
        dst_max = dst + dst_len - 1;
        prev_lcn = 0;
        /* Do the first partial run if present. */
        if (first_vcn > rl->vcn) {
                s64 delta, length = rl->length;

                /* We know rl->length != 0 already. */
                if (unlikely(length < 0 || rl->lcn < LCN_HOLE))
                        goto err_out;
                /*
                 * If @stop_vcn is given and finishes inside this run, cap the
                 * run length.
                 */
                if (unlikely(last_vcn >= 0 && rl[1].vcn > last_vcn)) {
                        s64 s1 = last_vcn + 1;
                        if (unlikely(rl[1].vcn > s1))
                                length = s1 - rl->vcn;
                        the_end = TRUE;
                }
                delta = first_vcn - rl->vcn;
                /* Write length. */
                len_len = ntfs_write_significant_bytes(dst + 1, dst_max,
                                length - delta);
                if (unlikely(len_len < 0))
                        goto size_err;
                /*
                 * If the logical cluster number (lcn) denotes a hole and we
                 * are on NTFS 3.0+, we don't store it at all, i.e. we need
                 * zero space.  On earlier NTFS versions we just write the lcn
                 * change.  FIXME: Do we need to write the lcn change or just
                 * the lcn in that case?  Not sure as I have never seen this
                 * case on NT4. - We assume that we just need to write the lcn
                 * change until someone tells us otherwise... (AIA)
                 */
                if (likely(rl->lcn >= 0 || vol->major_ver < 3)) {
                        prev_lcn = rl->lcn;
                        if (likely(rl->lcn >= 0))
                                prev_lcn += delta;
                        /* Write change in lcn. */
                        lcn_len = ntfs_write_significant_bytes(dst + 1 +
                                        len_len, dst_max, prev_lcn);
                        if (unlikely(lcn_len < 0))
                                goto size_err;
                } else
                        lcn_len = 0;
                dst_next = dst + len_len + lcn_len + 1;
                if (unlikely(dst_next > dst_max))
                        goto size_err;
                /* Update header byte. */
                *dst = lcn_len << 4 | len_len;
                /* Position at next mapping pairs array element. */
                dst = dst_next;
                /* Go to next runlist element. */
                rl++;
        }
        /* Do the full runs. */
        for (; rl->length && !the_end; rl++) {
                s64 length = rl->length;

                if (unlikely(length < 0 || rl->lcn < LCN_HOLE))
                        goto err_out;
                /*
                 * If @stop_vcn is given and finishes inside this run, cap the
                 * run length.
                 */
                if (unlikely(last_vcn >= 0 && rl[1].vcn > last_vcn)) {
                        s64 s1 = last_vcn + 1;
                        if (unlikely(rl[1].vcn > s1))
                                length = s1 - rl->vcn;
                        the_end = TRUE;
                }
                /* Write length. */
                len_len = ntfs_write_significant_bytes(dst + 1, dst_max,
                                length);
                if (unlikely(len_len < 0))
                        goto size_err;
                /*
                 * If the logical cluster number (lcn) denotes a hole and we
                 * are on NTFS 3.0+, we don't store it at all, i.e. we need
                 * zero space.  On earlier NTFS versions we just write the lcn
                 * change.  FIXME: Do we need to write the lcn change or just
                 * the lcn in that case?  Not sure as I have never seen this
                 * case on NT4. - We assume that we just need to write the lcn
                 * change until someone tells us otherwise... (AIA)
                 */
                if (likely(rl->lcn >= 0 || vol->major_ver < 3)) {
                        /* Write change in lcn. */
                        lcn_len = ntfs_write_significant_bytes(dst + 1 +
                                        len_len, dst_max, rl->lcn - prev_lcn);
                        if (unlikely(lcn_len < 0))
                                goto size_err;
                        prev_lcn = rl->lcn;
                } else
                        lcn_len = 0;
                dst_next = dst + len_len + lcn_len + 1;
                if (unlikely(dst_next > dst_max))
                        goto size_err;
                /* Update header byte. */
                *dst = lcn_len << 4 | len_len;
                /* Position at next mapping pairs array element. */
                dst = dst_next;
        }
        /* Success. */
        err = 0;
size_err:
        /* Set stop vcn. */
        if (stop_vcn)
                *stop_vcn = rl->vcn;
        /* Add terminator byte. */
        *dst = 0;
        return err;
err_out:
        if (rl->lcn == LCN_RL_NOT_MAPPED)
                err = -EINVAL;
        else
                err = -EIO;
        return err;
}

/**
 * ntfs_rl_truncate_nolock - truncate a runlist starting at a specified vcn
 * @runlist:    runlist to truncate
 * @new_length: the new length of the runlist in VCNs
 *
 * Truncate the runlist described by @runlist as well as the memory buffer
 * holding the runlist elements to a length of @new_length VCNs.
 *
 * If @new_length lies within the runlist, the runlist elements with VCNs of
 * @new_length and above are discarded.
 *
 * If @new_length lies beyond the runlist, a sparse runlist element is added to
 * the end of the runlist @runlist or if the last runlist element is a sparse
 * one already, this is extended.
 *
 * Return 0 on success and -errno on error.
 *
 * Locking: The caller must hold @runlist->lock for writing.
 */
int ntfs_rl_truncate_nolock(const ntfs_volume *vol, runlist *const runlist,
                const s64 new_length)
{
        runlist_element *rl;
        int old_size;

        ntfs_debug("Entering for new_length 0x%llx.", (long long)new_length);
        BUG_ON(!runlist);
        BUG_ON(new_length < 0);
        rl = runlist->rl;
        if (unlikely(!rl)) {
                /*
                 * Create a runlist consisting of a sparse runlist element of
                 * length @new_length followed by a terminator runlist element.
                 */
                rl = ntfs_malloc_nofs(PAGE_SIZE);
                if (unlikely(!rl)) {
                        ntfs_error(vol->sb, "Not enough memory to allocate "
                                        "runlist element buffer.");
                        return -ENOMEM;
                }
                runlist->rl = rl;
                rl[1].length = rl->vcn = 0;
                rl->lcn = LCN_HOLE;
                rl[1].vcn = rl->length = new_length;
                rl[1].lcn = LCN_ENOENT;
                return 0;
        }
        BUG_ON(new_length < rl->vcn);
        /* Find @new_length in the runlist. */
        while (likely(rl->length && new_length >= rl[1].vcn))
                rl++;
        /*
         * If not at the end of the runlist we need to shrink it.
         * If at the end of the runlist we need to expand it.
         */
        if (rl->length) {
                runlist_element *trl;
                BOOL is_end;

                ntfs_debug("Shrinking runlist.");
                /* Determine the runlist size. */
                trl = rl + 1;
                while (likely(trl->length))
                        trl++;
                old_size = trl - runlist->rl + 1;
                /* Truncate the run. */
                rl->length = new_length - rl->vcn;
                /*
                 * If a run was partially truncated, make the following runlist
                 * element a terminator.
                 */
                is_end = FALSE;
                if (rl->length) {
                        rl++;
                        if (!rl->length)
                                is_end = TRUE;
                        rl->vcn = new_length;
                        rl->length = 0;
                }
                rl->lcn = LCN_ENOENT;
                /* Reallocate memory if necessary. */
                if (!is_end) {
                        int new_size = rl - runlist->rl + 1;
                        rl = ntfs_rl_realloc(runlist->rl, old_size, new_size);
                        if (IS_ERR(rl))
                                ntfs_warning(vol->sb, "Failed to shrink "
                                                "runlist buffer.  This just "
                                                "wastes a bit of memory "
                                                "temporarily so we ignore it "
                                                "and return success.");
                        else
                                runlist->rl = rl;
                }
        } else if (likely(/* !rl->length && */ new_length > rl->vcn)) {
                ntfs_debug("Expanding runlist.");
                /*
                 * If there is a previous runlist element and it is a sparse
                 * one, extend it.  Otherwise need to add a new, sparse runlist
                 * element.
                 */
                if ((rl > runlist->rl) && ((rl - 1)->lcn == LCN_HOLE))
                        (rl - 1)->length = new_length - (rl - 1)->vcn;
                else {
                        /* Determine the runlist size. */
                        old_size = rl - runlist->rl + 1;
                        /* Reallocate memory if necessary. */
                        rl = ntfs_rl_realloc(runlist->rl, old_size,
                                        old_size + 1);
                        if (IS_ERR(rl)) {
                                ntfs_error(vol->sb, "Failed to expand runlist "
                                                "buffer, aborting.");
                                return PTR_ERR(rl);
                        }
                        runlist->rl = rl;
                        /*
                         * Set @rl to the same runlist element in the new
                         * runlist as before in the old runlist.
                         */
                        rl += old_size - 1;
                        /* Add a new, sparse runlist element. */
                        rl->lcn = LCN_HOLE;
                        rl->length = new_length - rl->vcn;
                        /* Add a new terminator runlist element. */
                        rl++;
                        rl->length = 0;
                }
                rl->vcn = new_length;
                rl->lcn = LCN_ENOENT;
        } else /* if (unlikely(!rl->length && new_length == rl->vcn)) */ {
                /* Runlist already has same size as requested. */
                rl->lcn = LCN_ENOENT;
        }
        ntfs_debug("Done.");
        return 0;
}

#endif /* NTFS_RW */