FreeBSD/Linux Kernel Cross Reference
sys/fs/ntfs/runlist.c

     /**
      * runlist.c - NTFS runlist handling code.  Part of the Linux-NTFS project.
      *
      * Copyright (c) 2001-2007 Anton Altaparmakov
      * Copyright (c) 2002-2005 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);
   
           /* We can merge unmapped regions even if they are misaligned. */
           if ((dst->lcn == LCN_RL_NOT_MAPPED) && (src->lcn == LCN_RL_NOT_MAPPED))
                   return true;
           /* If the runs are misaligned, we cannot merge them. */
           if ((dst->vcn + dst->length) != src->vcn)
                   return false;
           /* If both runs are non-sparse and contiguous, we can merge them. */
           if ((dst->lcn >= 0) && (src->lcn >= 0) &&
                           ((dst->lcn + dst->length) == src->lcn))
                   return true;
           /* If we are merging two holes, we can merge them. */
           if ((dst->lcn == LCN_HOLE) && (src->lcn == LCN_HOLE))
                   return true;
           /* Cannot merge. */
           return false;
   }
   
   /**
    * __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 = false;     /* Right end of @src needs merging. */
           int marker;             /* End of the inserted runs. */
   
           BUG_ON(!dst);
           BUG_ON(!src);
   
           /* First, check if the right hand end needs merging. */
           if ((loc + 1) < dsize)
                   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);
   
           /* First run after the @src runs that have been inserted. */
           marker = loc + ssize + 1;
   
           /* Move the tail of @dst out of the way, then copy in @src. */
           ntfs_rl_mm(dst, marker, 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[marker].lcn == LCN_ENOENT)
                   dst[marker].vcn = dst[marker - 1].vcn + dst[marker - 1].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;      /* Left end of @src needs merging. */
           bool disc = false;      /* Discontinuity between @dst and @src. */
           int marker;             /* End of the inserted runs. */
   
           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 "not mapped" run.
            */
           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);
           }
           /*
            * Space required: @dst size + @src size, less one if we merged, plus
            * one if there was a discontinuity.
            */
           dst = ntfs_rl_realloc(dst, dsize, dsize + ssize - left + disc);
           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);
           /*
            * First run after the @src runs that have been inserted.
            * Nominally,  @marker equals @loc + @ssize, i.e. location + number of
            * runs in @src.  However, if @left, then the first run in @src has
            * been merged with one in @dst.  And if @disc, then @dst and @src do
            * not meet and we need an extra run to fill the gap.
            */
           marker = loc + ssize - left + disc;
   
           /* Move the tail of @dst out of the way, then copy in @src. */
           ntfs_rl_mm(dst, marker, loc, dsize - loc);
           ntfs_rl_mc(dst, loc + disc, src, left, ssize - left);
   
           /* Adjust the VCN of the first run after the insertion... */
           dst[marker].vcn = dst[marker - 1].vcn + dst[marker - 1].length;
           /* ... and the length. */
           if (dst[marker].lcn == LCN_HOLE || dst[marker].lcn == LCN_RL_NOT_MAPPED)
                   dst[marker].length = dst[marker + 1].vcn - dst[marker].vcn;
   
           /* Writing beyond the end of the file and there is a discontinuity. */
           if (disc) {
                   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;
           }
           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)
   {
           signed delta;
           bool left = false;      /* Left end of @src needs merging. */
           bool right = false;     /* Right end of @src needs merging. */
           int tail;               /* Start of tail of @dst. */
           int marker;             /* End of the inserted runs. */
   
           BUG_ON(!dst);
           BUG_ON(!src);
   
           /* First, see if the left and right ends need merging. */
           if ((loc + 1) < dsize)
                   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 will need less if the left, right, or both
            * ends get merged.  The -1 accounts for the run being replaced.
            */
           delta = ssize - 1 - left - right;
           if (delta > 0) {
                   dst = ntfs_rl_realloc(dst, dsize, dsize + delta);
                   if (IS_ERR(dst))
                           return dst;
           }
           /*
            * We are guaranteed to succeed from here so can start modifying the
            * original runlists.
            */
   
           /* First, merge the left and right ends, if necessary. */
           if (right)
                   __ntfs_rl_merge(src + ssize - 1, dst + loc + 1);
           if (left)
                   __ntfs_rl_merge(dst + loc - 1, src);
           /*
            * Offset of the tail of @dst.  This needs to be moved out of the way
            * to make space for the runs to be copied from @src, i.e. the first
            * run of the tail of @dst.
            * Nominally, @tail equals @loc + 1, i.e. location, skipping the
            * replaced run.  However, if @right, then one of @dst's runs is
            * already merged into @src.
            */
           tail = loc + right + 1;
           /*
            * First run after the @src runs that have been inserted, i.e. where
            * the tail of @dst needs to be moved to.
            * Nominally, @marker equals @loc + @ssize, i.e. location + number of
            * runs in @src.  However, if @left, then the first run in @src has
            * been merged with one in @dst.
            */
           marker = loc + ssize - left;
   
           /* Move the tail of @dst out of the way, then copy in @src. */
           ntfs_rl_mm(dst, marker, tail, dsize - tail);
           ntfs_rl_mc(dst, loc, src, left, ssize - left);
   
           /* We may have changed the length of the file, so fix the end marker. */
           if (dsize - tail > 0 && dst[marker].lcn == LCN_ENOENT)
                   dst[marker].vcn = dst[marker - 1].vcn + dst[marker - 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);
           }
           /* If the mapping pairs array is valid but empty, nothing to do. */
           if (!vcn && !*buf)
                   return old_rl;
           /* 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 minimum
   * 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
   * @vol:        ntfs volume (needed for error output)
   * @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.  As a special case if @new_length is
   * zero, the runlist is discarded and set to NULL.
   *
   * 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.
   *
   * Note, no checking is done for unmapped runlist elements.  It is assumed that
   * the caller has mapped any elements that need to be mapped already.
   *
   * 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 (!new_length) {
                  ntfs_debug("Freeing runlist.");
                  runlist->rl = NULL;
                  if (rl)
                          ntfs_free(rl);
                  return 0;
          }
          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;
  }
  
  /**
   * ntfs_rl_punch_nolock - punch a hole into a runlist
   * @vol:        ntfs volume (needed for error output)
   * @runlist:    runlist to punch a hole into
   * @start:      starting VCN of the hole to be created
   * @length:     size of the hole to be created in units of clusters
   *
   * Punch a hole into the runlist @runlist starting at VCN @start and of size
   * @length clusters.
   *
   * Return 0 on success and -errno on error, in which case @runlist has not been
   * modified.
   *
   * If @start and/or @start + @length are outside the runlist return error code
   * -ENOENT.
   *
   * If the runlist contains unmapped or error elements between @start and @start
   * + @length return error code -EINVAL.
   *
   * Locking: The caller must hold @runlist->lock for writing.
   */
  int ntfs_rl_punch_nolock(const ntfs_volume *vol, runlist *const runlist,
                  const VCN start, const s64 length)
  {
          const VCN end = start + length;
          s64 delta;
          runlist_element *rl, *rl_end, *rl_real_end, *trl;
          int old_size;
          bool lcn_fixup = false;
  
          ntfs_debug("Entering for start 0x%llx, length 0x%llx.",
                          (long long)start, (long long)length);
          BUG_ON(!runlist);
          BUG_ON(start < 0);
          BUG_ON(length < 0);
          BUG_ON(end < 0);
          rl = runlist->rl;
          if (unlikely(!rl)) {
                  if (likely(!start && !length))
                          return 0;
                  return -ENOENT;
          }
          /* Find @start in the runlist. */
          while (likely(rl->length && start >= rl[1].vcn))
                  rl++;
          rl_end = rl;
          /* Find @end in the runlist. */
          while (likely(rl_end->length && end >= rl_end[1].vcn)) {
                  /* Verify there are no unmapped or error elements. */
                  if (unlikely(rl_end->lcn < LCN_HOLE))
                          return -EINVAL;
                  rl_end++;
          }
          /* Check the last element. */
          if (unlikely(rl_end->length && rl_end->lcn < LCN_HOLE))
                  return -EINVAL;
          /* This covers @start being out of bounds, too. */
          if (!rl_end->length && end > rl_end->vcn)
                  return -ENOENT;
          if (!length)
                  return 0;
          if (!rl->length)
                  return -ENOENT;
          rl_real_end = rl_end;
          /* Determine the runlist size. */
          while (likely(rl_real_end->length))
                  rl_real_end++;
          old_size = rl_real_end - runlist->rl + 1;
          /* If @start is in a hole simply extend the hole. */
          if (rl->lcn == LCN_HOLE) {
                  /*
                   * If both @start and @end are in the same sparse run, we are
                   * done.
                   */
                  if (end <= rl[1].vcn) {
                          ntfs_debug("Done (requested hole is already sparse).");
                          return 0;
                  }
  extend_hole:
                  /* Extend the hole. */
                  rl->length = end - rl->vcn;
                  /* If @end is in a hole, merge it with the current one. */
                  if (rl_end->lcn == LCN_HOLE) {
                          rl_end++;
                          rl->length = rl_end->vcn - rl->vcn;
                  }
                  /* We have done the hole.  Now deal with the remaining tail. */
                  rl++;
                  /* Cut out all runlist elements up to @end. */
                  if (rl < rl_end)
                          memmove(rl, rl_end, (rl_real_end - rl_end + 1) *
                                          sizeof(*rl));
                  /* Adjust the beginning of the tail if necessary. */
                  if (end > rl->vcn) {
                          delta = end - rl->vcn;
                          rl->vcn = end;
                          rl->length -= delta;
                          /* Only adjust the lcn if it is real. */
                          if (rl->lcn >= 0)
                                  rl->lcn += delta;
                  }
  shrink_allocation:
                  /* Reallocate memory if the allocation changed. */
                  if (rl < rl_end) {
                          rl = ntfs_rl_realloc(runlist->rl, old_size,
                                          old_size - (rl_end - rl));
                          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;
                  }
                  ntfs_debug("Done (extend hole).");
                  return 0;
          }
          /*
           * If @start is at the beginning of a run things are easier as there is
           * no need to split the first run.
           */
          if (start == rl->vcn) {
                  /*
                   * @start is at the beginning of a run.
                   *
                   * If the previous run is sparse, extend its hole.
                   *
                   * If @end is not in the same run, switch the run to be sparse
                   * and extend the newly created hole.
                   *
                   * Thus both of these cases reduce the problem to the above
                   * case of "@start is in a hole".
                   */
                  if (rl > runlist->rl && (rl - 1)->lcn == LCN_HOLE) {
                          rl--;
                          goto extend_hole;
                  }
                  if (end >= rl[1].vcn) {
                          rl->lcn = LCN_HOLE;
                          goto extend_hole;
                  }
                  /*
                   * The final case is when @end is in the same run as @start.
                   * For this need to split the run into two.  One run for the
                   * sparse region between the beginning of the old run, i.e.
                   * @start, and @end and one for the remaining non-sparse
                   * region, i.e. between @end and the end of the old run.
                   */
                  trl = ntfs_rl_realloc(runlist->rl, old_size, old_size + 1);
                  if (IS_ERR(trl))
                          goto enomem_out;
                  old_size++;
                  if (runlist->rl != trl) {
                          rl = trl + (rl - runlist->rl);
                          rl_end = trl + (rl_end - runlist->rl);
                          rl_real_end = trl + (rl_real_end - runlist->rl);
                          runlist->rl = trl;
                  }
  split_end:
                  /* Shift all the runs up by one. */
                  memmove(rl + 1, rl, (rl_real_end - rl + 1) * sizeof(*rl));
                  /* Finally, setup the two split runs. */
                  rl->lcn = LCN_HOLE;
                  rl->length = length;
                  rl++;
                  rl->vcn += length;
                  /* Only adjust the lcn if it is real. */
                  if (rl->lcn >= 0 || lcn_fixup)
                          rl->lcn += length;
                  rl->length -= length;
                  ntfs_debug("Done (split one).");
                  return 0;
          }
          /*
           * @start is neither in a hole nor at the beginning of a run.
           *
           * If @end is in a hole, things are easier as simply truncating the run
           * @start is in to end at @start - 1, deleting all runs after that up
           * to @end, and finally extending the beginning of the run @end is in
           * to be @start is all that is needed.
           */
          if (rl_end->lcn == LCN_HOLE) {
                  /* Truncate the run containing @start. */
                  rl->length = start - rl->vcn;
                  rl++;
                  /* Cut out all runlist elements up to @end. */
                  if (rl < rl_end)
                          memmove(rl, rl_end, (rl_real_end - rl_end + 1) *
                                          sizeof(*rl));
                  /* Extend the beginning of the run @end is in to be @start. */
                  rl->vcn = start;
                  rl->length = rl[1].vcn - start;
                  goto shrink_allocation;
          }
          /* 
           * If @end is not in a hole there are still two cases to distinguish.
           * Either @end is or is not in the same run as @start.
           *
           * The second case is easier as it can be reduced to an already solved
           * problem by truncating the run @start is in to end at @start - 1.
           * Then, if @end is in the next run need to split the run into a sparse
           * run followed by a non-sparse run (already covered above) and if @end
           * is not in the next run switching it to be sparse, again reduces the
           * problem to the already covered case of "@start is in a hole".
           */
          if (end >= rl[1].vcn) {
                  /*
                   * If @end is not in the next run, reduce the problem to the
                   * case of "@start is in a hole".
                   */
                  if (rl[1].length && end >= rl[2].vcn) {
                          /* Truncate the run containing @start. */
                          rl->length = start - rl->vcn;
                          rl++;
                          rl->vcn = start;
                          rl->lcn = LCN_HOLE;
                          goto extend_hole;
                  }
                  trl = ntfs_rl_realloc(runlist->rl, old_size, old_size + 1);
                  if (IS_ERR(trl))
                          goto enomem_out;
                  old_size++;
                  if (runlist->rl != trl) {
                          rl = trl + (rl - runlist->rl);
                          rl_end = trl + (rl_end - runlist->rl);
                          rl_real_end = trl + (rl_real_end - runlist->rl);
                          runlist->rl = trl;
                  }
                  /* Truncate the run containing @start. */
                  rl->length = start - rl->vcn;
                  rl++;
                  /*
                   * @end is in the next run, reduce the problem to the case
                   * where "@start is at the beginning of a run and @end is in
                   * the same run as @start".
                   */
                  delta = rl->vcn - start;
                  rl->vcn = start;
                  if (rl->lcn >= 0) {
                          rl->lcn -= delta;
                          /* Need this in case the lcn just became negative. */
                          lcn_fixup = true;
                  }
                  rl->length += delta;
                  goto split_end;
          }
          /*
           * The first case from above, i.e. @end is in the same run as @start.
           * We need to split the run into three.  One run for the non-sparse
           * region between the beginning of the old run and @start, one for the
           * sparse region between @start and @end, and one for the remaining
           * non-sparse region, i.e. between @end and the end of the old run.
           */
          trl = ntfs_rl_realloc(runlist->rl, old_size, old_size + 2);
          if (IS_ERR(trl))
                  goto enomem_out;
          old_size += 2;
          if (runlist->rl != trl) {
                  rl = trl + (rl - runlist->rl);
                  rl_end = trl + (rl_end - runlist->rl);
                  rl_real_end = trl + (rl_real_end - runlist->rl);
                  runlist->rl = trl;
          }
          /* Shift all the runs up by two. */
          memmove(rl + 2, rl, (rl_real_end - rl + 1) * sizeof(*rl));
          /* Finally, setup the three split runs. */
          rl->length = start - rl->vcn;
          rl++;
          rl->vcn = start;
          rl->lcn = LCN_HOLE;
          rl->length = length;
          rl++;
          delta = end - rl->vcn;
          rl->vcn = end;
          rl->lcn += delta;
          rl->length -= delta;
          ntfs_debug("Done (split both).");
          return 0;
  enomem_out:
          ntfs_error(vol->sb, "Not enough memory to extend runlist buffer.");
          return -ENOMEM;
  }
  
  #endif /* NTFS_RW */

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