hfs-366.1.1.tar.gz

[apple/hfs.git] / core / hfs_extents.c

/*
 * Copyright (c) 2014-2015 Apple Inc. All rights reserved.
 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_START@
 *
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. The rights granted to you under the License
 * may not be used to create, or enable the creation or redistribution of,
 * unlawful or unlicensed copies of an Apple operating system, or to
 * circumvent, violate, or enable the circumvention or violation of, any
 * terms of an Apple operating system software license agreement.
 *
 * Please obtain a copy of the License at
 * http://www.opensource.apple.com/apsl/ and read it before using this file.
 *
 * The Original Code and all software distributed under the License are
 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
 * Please see the License for the specific language governing rights and
 * limitations under the License.
 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_END@
 */

#if HFS_EXTENTS_TEST

#include "../tests/hfs_extents_test.h"
#include "hfs_extents.h"

#else

#include "hfs_extents.h"

// In this file, group refers to a set of 8 extents

static uint32_t hfs_total_blocks(const HFSPlusExtentDescriptor *ext, int count);
static errno_t hfs_ext_iter_next_group(struct hfs_ext_iter *iter);
static errno_t hfs_ext_iter_update(struct hfs_ext_iter *iter,
                                                                   HFSPlusExtentDescriptor *extents,
                                                                   int count,
                                                                   HFSPlusExtentRecord cat_extents);
static errno_t hfs_ext_iter_check_group(hfs_ext_iter_t *iter);

#endif

#define CHECK(x, var, goto_label)                                                                       \
        do {                                                                                                                    \
                var = (x);                                                                                                      \
                if (var) {                                                                                                      \
                        printf("%s:%u error: %d\n", __func__, __LINE__, var);   \
                        goto goto_label;                                                                                \
                }                                                                                                                       \
        } while (0)

#define min(a,b) \
        ({ typeof (a) _a = (a); typeof (b) _b = (b); _a < _b ? _a : _b; })

static __attribute__((pure))
const HFSPlusExtentKey *hfs_ext_iter_key(const hfs_ext_iter_t *iter)
{
        return (const HFSPlusExtentKey *)&iter->bt_iter.key;
}

static __attribute__((pure))
HFSPlusExtentKey *hfs_ext_iter_key_mut(hfs_ext_iter_t *iter)
{
        return (HFSPlusExtentKey *)&iter->bt_iter.key;
}

// Returns the total number of blocks for the @count extents provided
uint32_t hfs_total_blocks(const HFSPlusExtentDescriptor *extents, int count)
{
        uint32_t block_count = 0;
        for (int i = 0; i < count; ++i)
                block_count += extents[i].blockCount;
        return block_count;
}

/*
 * Checks a group of extents: makes sure that if it's the last group
 * for a fork, that all the remaining extents are properly zeroed and
 * if it's not then checks that all extents are set.  This also sets
 * @group_block_count and @last_in_fork.  Returns ESTALE if
 * inconsistent.
 */
errno_t hfs_ext_iter_check_group(hfs_ext_iter_t *iter)
{
        filefork_t *ff = VTOF(iter->vp);
        const HFSPlusExtentKey *key = hfs_ext_iter_key(iter);
        uint32_t count = 0;
        int i;

        for (i = 0; i < kHFSPlusExtentDensity; ++i) {
                if (!iter->group[i].blockCount)
                        break;
                count += iter->group[i].blockCount;
        }

        if (i < kHFSPlusExtentDensity) {
                iter->last_in_fork = true;
                if (key->startBlock + count != ff_allocblocks(ff))
                        goto bad;

                // Check remainder of extents
                for (++i; i < kHFSPlusExtentDensity; ++i) {
                        if (iter->group[i].blockCount)
                                goto bad;
                }
        } else {
                if (key->startBlock + count > ff_allocblocks(ff))
                        goto bad;

                iter->last_in_fork = (key->startBlock + count == ff_allocblocks(ff));
        }

        iter->group_block_count = count;

        return 0;

bad:

#if DEBUG
        printf("hfs_ext_iter_check_group: bad group; start: %u, total blocks: %u\n",
                   key->startBlock, ff_allocblocks(ff));

        for (int j = 0; j < kHFSPlusExtentDensity; ++j) {
                printf("%s<%u, %u>", j ? ", " : "",
                           iter->group[j].startBlock, iter->group[j].blockCount);
        }

        printf("\n");
#endif

        return ESTALE;
}

// NOTE: doesn't copy group data
static void hfs_ext_iter_copy(const hfs_ext_iter_t *src, hfs_ext_iter_t *dst)
{
        dst->vp = src->vp;
        memcpy(&dst->bt_iter.key, &src->bt_iter.key, sizeof(HFSPlusExtentKey));

        dst->file_block = src->file_block;
        dst->ndx = src->ndx;

        dst->bt_iter.hint                       = src->bt_iter.hint;
        dst->bt_iter.version            = 0;
        dst->bt_iter.reserved           = 0;
        dst->bt_iter.hitCount           = 0;
        dst->bt_iter.maxLeafRecs        = 0;
}

bool hfs_ext_iter_is_catalog_extents(hfs_ext_iter_t *iter)
{
        return hfs_ext_iter_key(iter)->startBlock == 0;
}

#if !HFS_EXTENTS_TEST

/*
 * Finds the extent for offset.  It might be in the catalog or the extents
 * file.
 */
errno_t hfs_ext_find(vnode_t vp, off_t offset, hfs_ext_iter_t *iter)
{
        errno_t ret;
        hfsmount_t *hfsmp = VTOHFS(vp);

        iter->vp = vp;

        uint32_t end_block, index;
        HFSPlusExtentKey *key = hfs_ext_iter_key_mut(iter);

        filefork_t *ff = VTOF(vp);

        CHECK(SearchExtentFile(hfsmp, ff, offset,
                                                   key, iter->group, &index,
                                                   &iter->bt_iter.hint.nodeNum, &end_block), ret, exit);

        iter->ndx = index;
        iter->file_block = end_block - iter->group[index].blockCount;

        if (!key->keyLength) {
                // We're pointing at the catalog record extents so fix up the key
                key->keyLength  = kHFSPlusExtentKeyMaximumLength;
                key->forkType   = (VNODE_IS_RSRC(iter->vp)
                                                   ? kHFSResourceForkType : kHFSDataForkType);
                key->pad                = 0;
                key->fileID             = VTOC(iter->vp)->c_fileid;
                key->startBlock = 0;
        }

        CHECK(hfs_ext_iter_check_group(iter), ret, exit);

        ret = 0;

exit:

        return MacToVFSError(ret);
}

static uint32_t hfs_ext_iter_next_group_block(const hfs_ext_iter_t *iter)
{
        const HFSPlusExtentKey *key = hfs_ext_iter_key(iter);

        return key->startBlock + iter->group_block_count;
}

/*
 * Move the iterator to the next group.  Don't call if there's a chance
 * there is no entry; the caller should check last_in_fork instead.
 */
static errno_t hfs_ext_iter_next_group(hfs_ext_iter_t *iter)
{
        errno_t ret;
        hfsmount_t *hfsmp = VTOHFS(iter->vp);
        filefork_t * const tree = hfsmp->hfs_extents_cp->c_datafork;
        HFSPlusExtentKey *key = hfs_ext_iter_key_mut(iter);
        const bool catalog_extents = hfs_ext_iter_is_catalog_extents(iter);
        const uint32_t next_block = hfs_ext_iter_next_group_block(iter);

        FSBufferDescriptor fbd = {
                .bufferAddress = &iter->group,
                .itemCount = 1,
                .itemSize = sizeof(iter->group)
        };

        if (catalog_extents) {
                key->startBlock = next_block;

                CHECK(BTSearchRecord(tree, &iter->bt_iter, &fbd, NULL,
                                                         &iter->bt_iter), ret, exit);
        } else {
                const uint32_t   file_id = key->fileID;
                const uint8_t    fork_type = key->forkType;

                CHECK(BTIterateRecord(tree, kBTreeNextRecord, &iter->bt_iter,
                                                          &fbd, NULL), ret, exit);

                if (key->fileID != file_id
                        || key->forkType != fork_type
                        || key->startBlock != next_block) {
                        // This indicates an inconsistency
                        ret = ESTALE;
                        goto exit;
                }
        }

        iter->file_block = key->startBlock;
        iter->ndx = 0;

        CHECK(hfs_ext_iter_check_group(iter), ret, exit);

        ret = 0;

exit:

        return MacToVFSError(ret);
}

/*
 * Updates with the extents provided and sets the key up for the next group.
 * It is assumed that any previous record that might collide has been deleted.
 * NOTE: @extents must point to a buffer that can be zero padded to multiple
 * of 8 extents.
 */
errno_t hfs_ext_iter_update(hfs_ext_iter_t *iter,
                                                        HFSPlusExtentDescriptor *extents,
                                                        int count,
                                                        HFSPlusExtentRecord cat_extents)
{
        errno_t                          ret;
        hfsmount_t                      *hfsmp  = VTOHFS(iter->vp);
        cnode_t                         *cp             = VTOC(iter->vp);
        HFSPlusExtentKey        *key    = hfs_ext_iter_key_mut(iter);
        int                                      ndx    = 0;

        if (!extents)
                extents = iter->group;

        if (count % kHFSPlusExtentDensity) {
                // Zero out last group
                bzero(&extents[count], (kHFSPlusExtentDensity
                                                                - (count % 8)) * sizeof(*extents));
        }

        if (hfs_ext_iter_is_catalog_extents(iter)) {
                // Caller is responsible for in-memory updates

                if (cat_extents)
                        hfs_ext_copy_rec(extents, cat_extents);

                struct cat_fork fork;

                hfs_fork_copy(&fork, &VTOF(iter->vp)->ff_data, extents);
                hfs_prepare_fork_for_update(VTOF(iter->vp), &fork, &fork, hfsmp->blockSize);

                bool is_rsrc = VNODE_IS_RSRC(iter->vp);
                CHECK(cat_update(hfsmp, &cp->c_desc, &cp->c_attr,
                                                 is_rsrc ? NULL : &fork,
                                                 is_rsrc ? &fork : NULL), ret, exit);

                // Set the key to the next group
                key->startBlock = hfs_total_blocks(extents, kHFSPlusExtentDensity);

                ndx += 8;
        }

        // Deal with the remainder which must be overflow extents
        for (; ndx < count; ndx += 8) {
                filefork_t * const tree = hfsmp->hfs_extents_cp->c_datafork;

                FSBufferDescriptor fbd = {
                        .bufferAddress = &extents[ndx],
                        .itemCount = 1,
                        .itemSize = sizeof(HFSPlusExtentRecord)
                };

                CHECK(BTInsertRecord(tree, &iter->bt_iter, &fbd,
                                                         sizeof(HFSPlusExtentRecord)), ret, exit);

                // Set the key to the next group
                key->startBlock += hfs_total_blocks(&extents[ndx], kHFSPlusExtentDensity);
        }

        ret = 0;

exit:

        return ret;
}

#endif // !HFS_EXTENTS_TEST

static void push_ext(HFSPlusExtentDescriptor *extents, int *count,
                                         const HFSPlusExtentDescriptor *ext)
{
        if (!ext->blockCount)
                return;

        if (*count && hfs_ext_end(&extents[*count - 1]) == ext->startBlock)
                extents[*count - 1].blockCount += ext->blockCount;
        else
                extents[(*count)++] = *ext;
}

/*
 * NOTE: Here we rely on the replacement extents not being too big as
 * otherwise the number of BTree records that we have to delete could be
 * too large.
 */
errno_t hfs_ext_replace(hfsmount_t *hfsmp, vnode_t vp,
                                                uint32_t file_block,
                                                const HFSPlusExtentDescriptor *repl,
                                                int repl_count,
                                                HFSPlusExtentRecord catalog_extents)
{
        errno_t                                          ret;
        filefork_t * const                       tree = hfsmp->hfs_extents_cp->c_datafork;
        hfs_ext_iter_t                          *iter_in = NULL, *iter_out;
        HFSPlusExtentDescriptor         *extents = NULL;
        int                                                      buffered_extents = 0;
        const int                                        max_roll_back_extents = 16384; // 128k
        HFSPlusExtentDescriptor         *roll_back_extents = NULL;
        int                                                      roll_back_count = 0;
        const uint32_t                           end_file_block = file_block + hfs_total_blocks(repl, repl_count);
        filefork_t                                      *ff = VTOF(vp);
        uint32_t                                         start_group_block = 0, block = 0;

        // Indicate we haven't touched catalog extents
        catalog_extents[0].blockCount = 0;

        if (end_file_block > ff_allocblocks(ff))
                return EINVAL;

        iter_in = hfs_malloc(sizeof(*iter_in) * 2);
        iter_out = iter_in + 1;
        HFSPlusExtentKey *key_in = hfs_ext_iter_key_mut(iter_in);

        // Get to where we want to start
        off_t offset = hfs_blk_to_bytes(file_block, hfsmp->blockSize);

        /*
         * If the replacement is at the start of a group, we want to pull in the
         * group before so that we tidy up any padding that we might have done
         * in a prior hfs_ext_replace call.
         */
        if (offset > 0)
                --offset;

        CHECK(hfs_ext_find(vp, offset, iter_in), ret, exit);

        start_group_block = key_in->startBlock;

        roll_back_extents = hfs_malloc(max_roll_back_extents
                                                                   * sizeof(HFSPlusExtentDescriptor));

        // Move to the first extent in this group
        iter_in->ndx = 0;

        hfs_ext_iter_copy(iter_in, iter_out);

        // Create a buffer for our extents
        buffered_extents = roundup(3 * kHFSPlusExtentDensity + repl_count,
                                                           kHFSPlusExtentDensity);
        extents = hfs_malloc(sizeof(*extents) * buffered_extents);
        int count = 0;

        /*
         * Iterate through the extents that are affected by this replace operation.
         * We cannot push more than 16 + repl_count extents here; 8 for the group
         * containing the replacement start, repl_count for the replacements and 8
         * for the group containing the end.  If we went back a group due to
         * decrementing the offset above, it's still the same because we know in 
         * that case the replacement starts at the beginning of the next group.
         */
        block = start_group_block;
        for (;;) {
                if (!iter_in->ndx) {
                        hfs_ext_copy_rec(iter_in->group, &roll_back_extents[roll_back_count]);
                        roll_back_count += kHFSPlusExtentDensity;

                        if (!hfs_ext_iter_is_catalog_extents(iter_in)) {
                                // Delete this extent group; we're going to replace it
                                CHECK(BTDeleteRecord(tree, &iter_in->bt_iter), ret, exit);
                        }
                }

                HFSPlusExtentDescriptor *ext = &iter_in->group[iter_in->ndx];
                if (!ext->blockCount) {
                    /*
                         * We ran out of existing extents so we just write the
                         * extents and we're done.
                         */
                        goto finish;
                }

                // If the current extent does not overlap replacement...
                if (block + ext->blockCount <= file_block || block >= end_file_block) {
                        // Keep the current extent exactly as it is
                        push_ext(extents, &count, ext);
                } else {
                        HFSPlusExtentDescriptor dealloc_ext = *ext;

                        if (block <= file_block) {
                                /*
                                 * The middle or tail of the current extent overlaps
                                 * the replacement extents.  Keep the non-overlapping
                                 * head of the current extent.
                                 */
                                uint32_t trimmed_len = file_block - block;

                                if (trimmed_len) {
                                        // Push (keep) non-overlapping head of current extent
                                        push_ext(extents, &count,
                                                         &(HFSPlusExtentDescriptor){ ext->startBlock,
                                                                 trimmed_len });

                                        /*
                                         * Deallocate the part of the current extent that
                                         * overlaps the replacement extents.  That starts
                                         * at @file_block.  For now, assume it goes
                                         * through the end of the current extent.  (If the
                                         * current extent extends beyond the end of the
                                         * replacement extents, we'll update the
                                         * blockCount below.)
                                         */
                                        dealloc_ext.startBlock += trimmed_len;
                                        dealloc_ext.blockCount -= trimmed_len;
                                }

                                // Insert the replacements
                                for (int i = 0; i < repl_count; ++i)
                                        push_ext(extents, &count, &repl[i]);
                        }

                        if (block + ext->blockCount > end_file_block) {
                                /*
                                 * The head or middle of the current extent overlaps
                                 * the replacement extents.  Keep the non-overlapping
                                 * tail of the current extent.
                                 */
                                uint32_t overlap = end_file_block - block;

                                // Push (keep) non-overlapping tail of current extent
                                push_ext(extents, &count,
                                                 &(HFSPlusExtentDescriptor){ ext->startBlock + overlap,
                                                         ext->blockCount - overlap });

                                /*
                                 * Deallocate the part of current extent that overlaps
                                 * the replacements.
                                 */
                                dealloc_ext.blockCount = (ext->startBlock + overlap
                                                                                  - dealloc_ext.startBlock);
                        }

                        CHECK(BlockDeallocate(hfsmp, dealloc_ext.startBlock,
                                                                  dealloc_ext.blockCount, 0), ret, exit);
                }

                // Move to next (existing) extent from iterator
                block += ext->blockCount;

                if (++iter_in->ndx >= kHFSPlusExtentDensity) {
                        if (block >= end_file_block) {
                                if (iter_in->last_in_fork || !(count % kHFSPlusExtentDensity)) {
                                        /*
                                         * This is the easy case.  We've hit the end or we have a 
                                         * multiple of 8, so we can just write out the extents we 
                                         * have and it should all fit within a transaction.
                                         */

                                        goto finish;
                                }

                                if (count + kHFSPlusExtentDensity > buffered_extents
                                        || (roll_back_count
                                                + kHFSPlusExtentDensity > max_roll_back_extents)) {
                                        /*
                                         * We've run out of room for the next group, so drop out
                                         * and take a different strategy.
                                         */
                                        break;
                                }
                        }

                        CHECK(hfs_ext_iter_next_group(iter_in), ret, exit);
                }
        } // for (;;)

        /*
         * We're not at the end so we need to try and pad to a multiple of 8
         * so that we don't have to touch all the subsequent records.  We pad
         * by stealing single blocks.
         */

        int stop_at = 0;

        for (;;) {
                // @in points to the record we're stealing from
                int in = count - 1;

                count = roundup(count, kHFSPlusExtentDensity);

                // @out is where we put the stolen single blocks
                int out = count - 1;

                do {
                        if (out <= in) {
                                // We suceeded in padding; we're done
                                goto finish;
                        }

                        /*
                         * "Steal" a block, or move a one-block extent within the
                         * @extents array.
                         *
                         * If the extent we're "stealing" from (@in) is only one
                         * block long, we'll end up copying it to @out, setting
                         * @in's blockCount to zero, and decrementing @in.  So, we
                         * either split a multi-block extent; or move it within
                         * the @extents array.
                         */
                        extents[out].blockCount = 1;
                        extents[out].startBlock = (extents[in].startBlock
                                                                           + extents[in].blockCount - 1);
                        --out;
                } while (--extents[in].blockCount || --in >= stop_at);

                // We ran out of extents
                if (roll_back_count + kHFSPlusExtentDensity > max_roll_back_extents) {
                        ret = ENOSPC;
                        goto exit;
                }

                // Need to shift extents starting at out + 1
                ++out;
                memmove(&extents[stop_at], &extents[out],
                                (count - out) * sizeof(*extents));
                count -= out - stop_at;

                // Pull in the next group
                CHECK(hfs_ext_iter_next_group(iter_in), ret, exit);

                // Take a copy of these extents for roll back purposes
                hfs_ext_copy_rec(iter_in->group, &roll_back_extents[roll_back_count]);
                roll_back_count += kHFSPlusExtentDensity;

                // Delete this group; we're going to replace it
                CHECK(BTDeleteRecord(tree, &iter_in->bt_iter), ret, exit);

                if (iter_in->last_in_fork) {
                        // Great!  We've hit the end.  Coalesce and write out.
                        int old_count = count;
                        count = 0;

                        /*
                         * First coalesce the extents we already have.  Takes
                         * advantage of push_ext coalescing the input extent with
                         * the last extent in @extents.  If the extents are not
                         * contiguous, then this just copies the extents over
                         * themselves and sets @count back to @old_count.
                         */
                        for (int i = 0; i < old_count; ++i)
                                push_ext(extents, &count, &extents[i]);

                        // Make room if necessary
                        const int flush_count = buffered_extents - kHFSPlusExtentDensity;
                        if (count > flush_count) {
                                CHECK(hfs_ext_iter_update(iter_out, extents,
                                                                                  flush_count, catalog_extents), ret, exit);

                                memmove(&extents[0], &extents[flush_count],
                                                (count - flush_count) * sizeof(*extents));

                                count -= flush_count;
                        }

                        // Add in the extents we just read in
                        for (int i = 0; i < kHFSPlusExtentDensity; ++i) {
                                HFSPlusExtentDescriptor *ext = &iter_in->group[i];
                                if (!ext->blockCount)
                                        break;
                                push_ext(extents, &count, ext);
                        }

                        goto finish;
                } // if (iter_in->last_in_fork)

                /*
                 * Otherwise, we're not at the end, so we add these extents and then
                 * try and pad out again to a multiple of 8.  We start by making room.
                 */
                if (count > buffered_extents - kHFSPlusExtentDensity) {
                        // Only write out one group here
                        CHECK(hfs_ext_iter_update(iter_out, extents,
                                                                          kHFSPlusExtentDensity,
                                                                          catalog_extents), ret, exit);

                        memmove(&extents[0], &extents[kHFSPlusExtentDensity],
                                        (count - kHFSPlusExtentDensity) * sizeof(*extents));

                        count -= kHFSPlusExtentDensity;
                }

                // Record where to stop when padding above
                stop_at = count;

                // Copy in the new extents
                hfs_ext_copy_rec(iter_in->group, &extents[count]);
                count += kHFSPlusExtentDensity;
        } // for (;;)

finish:

        // Write the remaining extents
        CHECK(hfs_ext_iter_update(iter_out, extents, count,
                                                          catalog_extents), ret, exit);

        CHECK(BTFlushPath(hfsmp->hfs_catalog_cp->c_datafork), ret, exit);
        CHECK(BTFlushPath(hfsmp->hfs_extents_cp->c_datafork), ret, exit);

exit:

        if (ret && roll_back_count) {

#define RB_FAILED                                                                                                               \
        do {                                                                                                                            \
                printf("hfs_ext_replace:%u: roll back failed\n", __LINE__);             \
                hfs_mark_inconsistent(hfsmp, HFS_ROLLBACK_FAILED);                              \
                goto roll_back_failed;                                                                                  \
        } while (0)

                // First delete any groups we inserted
                HFSPlusExtentKey *key_out = hfs_ext_iter_key_mut(iter_out);

                key_in->startBlock = start_group_block;
                if (!key_in->startBlock && key_out->startBlock > key_in->startBlock) {
                        key_in->startBlock += hfs_total_blocks(catalog_extents,
                                                                                                   kHFSPlusExtentDensity);
                }

                if (key_out->startBlock > key_in->startBlock) {
                        FSBufferDescriptor fbd = {
                                .bufferAddress = &iter_in->group,
                                .itemCount = 1,
                                .itemSize = sizeof(iter_in->group)
                        };

                        if (BTSearchRecord(tree, &iter_in->bt_iter, &fbd, NULL,
                                                           &iter_in->bt_iter)) {
                                RB_FAILED;
                        }

                        for (;;) {
                                if (BTDeleteRecord(tree, &iter_in->bt_iter))
                                        RB_FAILED;

                                key_in->startBlock += hfs_total_blocks(iter_in->group,
                                                                                                           kHFSPlusExtentDensity);

                                if (key_in->startBlock >= key_out->startBlock)
                                        break;

                                if (BTSearchRecord(tree, &iter_in->bt_iter, &fbd, NULL,
                                                                   &iter_in->bt_iter)) {
                                        RB_FAILED;
                                }
                        }
                }

                // Position iter_out
                key_out->startBlock = start_group_block;

                // Roll back all the extents
                if (hfs_ext_iter_update(iter_out, roll_back_extents, roll_back_count,
                                                                catalog_extents)) {
                        RB_FAILED;
                }

                // And we need to reallocate the blocks we deallocated
                const uint32_t end_block = min(block, end_file_block);
                block = start_group_block;
                for (int i = 0; i < roll_back_count && block < end_block; ++i) {
                        HFSPlusExtentDescriptor *ext = &roll_back_extents[i];

                        if (block + ext->blockCount <= file_block)
                                continue;

                        HFSPlusExtentDescriptor alloc_ext = *ext;

                        if (block <= file_block) {
                                uint32_t trimmed_len = file_block - block;

                                alloc_ext.startBlock += trimmed_len;
                                alloc_ext.blockCount -= trimmed_len;
                        }

                        if (block + ext->blockCount > end_file_block) {
                                uint32_t overlap = end_file_block - block;

                                alloc_ext.blockCount = (ext->startBlock + overlap
                                                                                - alloc_ext.startBlock);
                        }

                        if (hfs_block_alloc(hfsmp, &alloc_ext, HFS_ALLOC_ROLL_BACK, NULL))
                                RB_FAILED;

                        block += ext->blockCount;
                }

                if (BTFlushPath(hfsmp->hfs_catalog_cp->c_datafork)
                        || BTFlushPath(hfsmp->hfs_extents_cp->c_datafork)) {
                        RB_FAILED;
                }
        } // if (ret && roll_back_count)

roll_back_failed:

        hfs_free(iter_in, sizeof(*iter_in) * 2);
        hfs_free(extents, sizeof(*extents) * buffered_extents);
        hfs_free(roll_back_extents, (max_roll_back_extents
                                                                 * sizeof(HFSPlusExtentDescriptor)));

        return MacToVFSError(ret);
}