vhdx-log.c 28.9 KB
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/*
 * Block driver for Hyper-V VHDX Images
 *
 * Copyright (c) 2013 Red Hat, Inc.,
 *
 * Authors:
 *  Jeff Cody <jcody@redhat.com>
 *
 *  This is based on the "VHDX Format Specification v1.00", published 8/25/2012
 *  by Microsoft:
 *      https://www.microsoft.com/en-us/download/details.aspx?id=34750
 *
 * This file covers the functionality of the metadata log writing, parsing, and
 * replay.
 *
 * This work is licensed under the terms of the GNU LGPL, version 2 or later.
 * See the COPYING.LIB file in the top-level directory.
 *
 */
#include "qemu-common.h"
#include "block/block_int.h"
#include "qemu/module.h"
#include "block/vhdx.h"


typedef struct VHDXLogSequence {
    bool valid;
    uint32_t count;
    VHDXLogEntries log;
    VHDXLogEntryHeader hdr;
} VHDXLogSequence;

typedef struct VHDXLogDescEntries {
    VHDXLogEntryHeader hdr;
    VHDXLogDescriptor desc[];
} VHDXLogDescEntries;

static const MSGUID zero_guid = { 0 };

/* The log located on the disk is circular buffer containing
 * sectors of 4096 bytes each.
 *
 * It is assumed for the read/write functions below that the
 * circular buffer scheme uses a 'one sector open' to indicate
 * the buffer is full.  Given the validation methods used for each
 * sector, this method should be compatible with other methods that
 * do not waste a sector.
 */


/* Allow peeking at the hdr entry at the beginning of the current
 * read index, without advancing the read index */
static int vhdx_log_peek_hdr(BlockDriverState *bs, VHDXLogEntries *log,
                             VHDXLogEntryHeader *hdr)
{
    int ret = 0;
    uint64_t offset;
    uint32_t read;

    assert(hdr != NULL);

    /* peek is only supported on sector boundaries */
    if (log->read % VHDX_LOG_SECTOR_SIZE) {
        ret = -EFAULT;
        goto exit;
    }

    read = log->read;
    /* we are guaranteed that a) log sectors are 4096 bytes,
     * and b) the log length is a multiple of 1MB. So, there
     * is always a round number of sectors in the buffer */
    if ((read + sizeof(VHDXLogEntryHeader)) > log->length) {
        read = 0;
    }

    if (read == log->write) {
        ret = -EINVAL;
        goto exit;
    }

    offset = log->offset + read;

    ret = bdrv_pread(bs->file, offset, hdr, sizeof(VHDXLogEntryHeader));
    if (ret < 0) {
        goto exit;
    }

exit:
    return ret;
}

/* Index increment for log, based on sector boundaries */
static int vhdx_log_inc_idx(uint32_t idx, uint64_t length)
{
    idx += VHDX_LOG_SECTOR_SIZE;
    /* we are guaranteed that a) log sectors are 4096 bytes,
     * and b) the log length is a multiple of 1MB. So, there
     * is always a round number of sectors in the buffer */
    return idx >= length ? 0 : idx;
}


/* Reset the log to empty */
static void vhdx_log_reset(BlockDriverState *bs, BDRVVHDXState *s)
{
    MSGUID guid = { 0 };
    s->log.read = s->log.write = 0;
    /* a log guid of 0 indicates an empty log to any parser of v0
     * VHDX logs */
    vhdx_update_headers(bs, s, false, &guid);
}

/* Reads num_sectors from the log (all log sectors are 4096 bytes),
 * into buffer 'buffer'.  Upon return, *sectors_read will contain
 * the number of sectors successfully read.
 *
 * It is assumed that 'buffer' is already allocated, and of sufficient
 * size (i.e. >= 4096*num_sectors).
 *
 * If 'peek' is true, then the tail (read) pointer for the circular buffer is
 * not modified.
 *
 * 0 is returned on success, -errno otherwise.  */
static int vhdx_log_read_sectors(BlockDriverState *bs, VHDXLogEntries *log,
                                 uint32_t *sectors_read, void *buffer,
                                 uint32_t num_sectors, bool peek)
{
    int ret = 0;
    uint64_t offset;
    uint32_t read;

    read = log->read;

    *sectors_read = 0;
    while (num_sectors) {
        if (read == log->write) {
            /* empty */
            break;
        }
        offset = log->offset + read;

        ret = bdrv_pread(bs->file, offset, buffer, VHDX_LOG_SECTOR_SIZE);
        if (ret < 0) {
            goto exit;
        }
        read = vhdx_log_inc_idx(read, log->length);

        *sectors_read = *sectors_read + 1;
        num_sectors--;
    }

exit:
    if (!peek) {
        log->read = read;
    }
    return ret;
}

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/* Writes num_sectors to the log (all log sectors are 4096 bytes),
 * from buffer 'buffer'.  Upon return, *sectors_written will contain
 * the number of sectors successfully written.
 *
 * It is assumed that 'buffer' is at least 4096*num_sectors large.
 *
 * 0 is returned on success, -errno otherwise */
static int vhdx_log_write_sectors(BlockDriverState *bs, VHDXLogEntries *log,
                                  uint32_t *sectors_written, void *buffer,
                                  uint32_t num_sectors)
{
    int ret = 0;
    uint64_t offset;
    uint32_t write;
    void *buffer_tmp;
    BDRVVHDXState *s = bs->opaque;

    ret = vhdx_user_visible_write(bs, s);
    if (ret < 0) {
        goto exit;
    }

    write = log->write;

    buffer_tmp = buffer;
    while (num_sectors) {

        offset = log->offset + write;
        write = vhdx_log_inc_idx(write, log->length);
        if (write == log->read) {
            /* full */
            break;
        }
        ret = bdrv_pwrite(bs->file, offset, buffer_tmp, VHDX_LOG_SECTOR_SIZE);
        if (ret < 0) {
            goto exit;
        }
        buffer_tmp += VHDX_LOG_SECTOR_SIZE;

        log->write = write;
        *sectors_written = *sectors_written + 1;
        num_sectors--;
    }

exit:
    return ret;
}


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/* Validates a log entry header */
static bool vhdx_log_hdr_is_valid(VHDXLogEntries *log, VHDXLogEntryHeader *hdr,
                                  BDRVVHDXState *s)
{
    int valid = false;

    if (memcmp(&hdr->signature, "loge", 4)) {
        goto exit;
    }

    /* if the individual entry length is larger than the whole log
     * buffer, that is obviously invalid */
    if (log->length < hdr->entry_length) {
        goto exit;
    }

    /* length of entire entry must be in units of 4KB (log sector size) */
    if (hdr->entry_length % (VHDX_LOG_SECTOR_SIZE)) {
        goto exit;
    }

    /* per spec, sequence # must be > 0 */
    if (hdr->sequence_number == 0) {
        goto exit;
    }

    /* log entries are only valid if they match the file-wide log guid
     * found in the active header */
    if (!guid_eq(hdr->log_guid, s->headers[s->curr_header]->log_guid)) {
        goto exit;
    }

    if (hdr->descriptor_count * sizeof(VHDXLogDescriptor) > hdr->entry_length) {
        goto exit;
    }

    valid = true;

exit:
    return valid;
}

/*
 * Given a log header, this will validate that the descriptors and the
 * corresponding data sectors (if applicable)
 *
 * Validation consists of:
 *      1. Making sure the sequence numbers matches the entry header
 *      2. Verifying a valid signature ('zero' or 'desc' for descriptors)
 *      3. File offset field is a multiple of 4KB
 *      4. If a data descriptor, the corresponding data sector
 *         has its signature ('data') and matching sequence number
 *
 * @desc: the data buffer containing the descriptor
 * @hdr:  the log entry header
 *
 * Returns true if valid
 */
static bool vhdx_log_desc_is_valid(VHDXLogDescriptor *desc,
                                   VHDXLogEntryHeader *hdr)
{
    bool ret = false;

    if (desc->sequence_number != hdr->sequence_number) {
        goto exit;
    }
    if (desc->file_offset % VHDX_LOG_SECTOR_SIZE) {
        goto exit;
    }

    if (!memcmp(&desc->signature, "zero", 4)) {
        if (desc->zero_length % VHDX_LOG_SECTOR_SIZE == 0) {
            /* valid */
            ret = true;
        }
    } else if (!memcmp(&desc->signature, "desc", 4)) {
            /* valid */
            ret = true;
    }

exit:
    return ret;
}


/* Prior to sector data for a log entry, there is the header
 * and the descriptors referenced in the header:
 *
 * [] = 4KB sector
 *
 * [ hdr, desc ][   desc   ][ ... ][ data ][ ... ]
 *
 * The first sector in a log entry has a 64 byte header, and
 * up to 126 32-byte descriptors.  If more descriptors than
 * 126 are required, then subsequent sectors can have up to 128
 * descriptors.  Each sector is 4KB.  Data follows the descriptor
 * sectors.
 *
 * This will return the number of sectors needed to encompass
 * the passed number of descriptors in desc_cnt.
 *
 * This will never return 0, even if desc_cnt is 0.
 */
static int vhdx_compute_desc_sectors(uint32_t desc_cnt)
{
    uint32_t desc_sectors;

    desc_cnt += 2; /* account for header in first sector */
    desc_sectors = desc_cnt / 128;
    if (desc_cnt % 128) {
        desc_sectors++;
    }

    return desc_sectors;
}


/* Reads the log header, and subsequent descriptors (if any).  This
 * will allocate all the space for buffer, which must be NULL when
 * passed into this function. Each descriptor will also be validated,
 * and error returned if any are invalid. */
static int vhdx_log_read_desc(BlockDriverState *bs, BDRVVHDXState *s,
                              VHDXLogEntries *log, VHDXLogDescEntries **buffer)
{
    int ret = 0;
    uint32_t desc_sectors;
    uint32_t sectors_read;
    VHDXLogEntryHeader hdr;
    VHDXLogDescEntries *desc_entries = NULL;
    int i;

    assert(*buffer == NULL);

    ret = vhdx_log_peek_hdr(bs, log, &hdr);
    if (ret < 0) {
        goto exit;
    }
    vhdx_log_entry_hdr_le_import(&hdr);
    if (vhdx_log_hdr_is_valid(log, &hdr, s) == false) {
        ret = -EINVAL;
        goto exit;
    }

    desc_sectors = vhdx_compute_desc_sectors(hdr.descriptor_count);
    desc_entries = qemu_blockalign(bs, desc_sectors * VHDX_LOG_SECTOR_SIZE);

    ret = vhdx_log_read_sectors(bs, log, &sectors_read, desc_entries,
                                desc_sectors, false);
    if (ret < 0) {
        goto free_and_exit;
    }
    if (sectors_read != desc_sectors) {
        ret = -EINVAL;
        goto free_and_exit;
    }

    /* put in proper endianness, and validate each desc */
    for (i = 0; i < hdr.descriptor_count; i++) {
        vhdx_log_desc_le_import(&desc_entries->desc[i]);
        if (vhdx_log_desc_is_valid(&desc_entries->desc[i], &hdr) == false) {
            ret = -EINVAL;
            goto free_and_exit;
        }
    }

    *buffer = desc_entries;
    goto exit;

free_and_exit:
    qemu_vfree(desc_entries);
exit:
    return ret;
}


/* Flushes the descriptor described by desc to the VHDX image file.
 * If the descriptor is a data descriptor, than 'data' must be non-NULL,
 * and >= 4096 bytes (VHDX_LOG_SECTOR_SIZE), containing the data to be
 * written.
 *
 * Verification is performed to make sure the sequence numbers of a data
 * descriptor match the sequence number in the desc.
 *
 * For a zero descriptor, it may describe multiple sectors to fill with zeroes.
 * In this case, it should be noted that zeroes are written to disk, and the
 * image file is not extended as a sparse file.  */
static int vhdx_log_flush_desc(BlockDriverState *bs, VHDXLogDescriptor *desc,
                               VHDXLogDataSector *data)
{
    int ret = 0;
    uint64_t seq, file_offset;
    uint32_t offset = 0;
    void *buffer = NULL;
    uint64_t count = 1;
    int i;

    buffer = qemu_blockalign(bs, VHDX_LOG_SECTOR_SIZE);

    if (!memcmp(&desc->signature, "desc", 4)) {
        /* data sector */
        if (data == NULL) {
            ret = -EFAULT;
            goto exit;
        }

        /* The sequence number of the data sector must match that
         * in the descriptor */
        seq = data->sequence_high;
        seq <<= 32;
        seq |= data->sequence_low & 0xffffffff;

        if (seq != desc->sequence_number) {
            ret = -EINVAL;
            goto exit;
        }

        /* Each data sector is in total 4096 bytes, however the first
         * 8 bytes, and last 4 bytes, are located in the descriptor */
        memcpy(buffer, &desc->leading_bytes, 8);
        offset += 8;

        memcpy(buffer+offset, data->data, 4084);
        offset += 4084;

        memcpy(buffer+offset, &desc->trailing_bytes, 4);

    } else if (!memcmp(&desc->signature, "zero", 4)) {
        /* write 'count' sectors of sector */
        memset(buffer, 0, VHDX_LOG_SECTOR_SIZE);
        count = desc->zero_length / VHDX_LOG_SECTOR_SIZE;
    }

    file_offset = desc->file_offset;

    /* count is only > 1 if we are writing zeroes */
    for (i = 0; i < count; i++) {
        ret = bdrv_pwrite_sync(bs->file, file_offset, buffer,
                               VHDX_LOG_SECTOR_SIZE);
        if (ret < 0) {
            goto exit;
        }
        file_offset += VHDX_LOG_SECTOR_SIZE;
    }

exit:
    qemu_vfree(buffer);
    return ret;
}

/* Flush the entire log (as described by 'logs') to the VHDX image
 * file, and then set the log to 'empty' status once complete.
 *
 * The log entries should be validate prior to flushing */
static int vhdx_log_flush(BlockDriverState *bs, BDRVVHDXState *s,
                          VHDXLogSequence *logs)
{
    int ret = 0;
    int i;
    uint32_t cnt, sectors_read;
    uint64_t new_file_size;
    void *data = NULL;
    VHDXLogDescEntries *desc_entries = NULL;
    VHDXLogEntryHeader hdr_tmp = { 0 };

    cnt = logs->count;

    data = qemu_blockalign(bs, VHDX_LOG_SECTOR_SIZE);

    ret = vhdx_user_visible_write(bs, s);
    if (ret < 0) {
        goto exit;
    }

    /* each iteration represents one log sequence, which may span multiple
     * sectors */
    while (cnt--) {
        ret = vhdx_log_peek_hdr(bs, &logs->log, &hdr_tmp);
        if (ret < 0) {
            goto exit;
        }
        /* if the log shows a FlushedFileOffset larger than our current file
         * size, then that means the file has been truncated / corrupted, and
         * we must refused to open it / use it */
        if (hdr_tmp.flushed_file_offset > bdrv_getlength(bs->file)) {
            ret = -EINVAL;
            goto exit;
        }

        ret = vhdx_log_read_desc(bs, s, &logs->log, &desc_entries);
        if (ret < 0) {
            goto exit;
        }

        for (i = 0; i < desc_entries->hdr.descriptor_count; i++) {
            if (!memcmp(&desc_entries->desc[i].signature, "desc", 4)) {
                /* data sector, so read a sector to flush */
                ret = vhdx_log_read_sectors(bs, &logs->log, &sectors_read,
                                            data, 1, false);
                if (ret < 0) {
                    goto exit;
                }
                if (sectors_read != 1) {
                    ret = -EINVAL;
                    goto exit;
                }
            }

            ret = vhdx_log_flush_desc(bs, &desc_entries->desc[i], data);
            if (ret < 0) {
                goto exit;
            }
        }
        if (bdrv_getlength(bs->file) < desc_entries->hdr.last_file_offset) {
            new_file_size = desc_entries->hdr.last_file_offset;
            if (new_file_size % (1024*1024)) {
                /* round up to nearest 1MB boundary */
                new_file_size = ((new_file_size >> 20) + 1) << 20;
                bdrv_truncate(bs->file, new_file_size);
            }
        }
        qemu_vfree(desc_entries);
        desc_entries = NULL;
    }

    bdrv_flush(bs);
    /* once the log is fully flushed, indicate that we have an empty log
     * now.  This also sets the log guid to 0, to indicate an empty log */
    vhdx_log_reset(bs, s);

exit:
    qemu_vfree(data);
    qemu_vfree(desc_entries);
    return ret;
}

static int vhdx_validate_log_entry(BlockDriverState *bs, BDRVVHDXState *s,
                                   VHDXLogEntries *log, uint64_t seq,
                                   bool *valid, VHDXLogEntryHeader *entry)
{
    int ret = 0;
    VHDXLogEntryHeader hdr;
    void *buffer = NULL;
    uint32_t i, desc_sectors, total_sectors, crc;
    uint32_t sectors_read = 0;
    VHDXLogDescEntries *desc_buffer = NULL;

    *valid = false;

    ret = vhdx_log_peek_hdr(bs, log, &hdr);
    if (ret < 0) {
        goto inc_and_exit;
    }

    vhdx_log_entry_hdr_le_import(&hdr);


    if (vhdx_log_hdr_is_valid(log, &hdr, s) == false) {
        goto inc_and_exit;
    }

    if (seq > 0) {
        if (hdr.sequence_number != seq + 1) {
            goto inc_and_exit;
        }
    }

    desc_sectors = vhdx_compute_desc_sectors(hdr.descriptor_count);

    /* Read desc sectors, and calculate log checksum */

    total_sectors = hdr.entry_length / VHDX_LOG_SECTOR_SIZE;


    /* read_desc() will incrememnt the read idx */
    ret = vhdx_log_read_desc(bs, s, log, &desc_buffer);
    if (ret < 0) {
        goto free_and_exit;
    }

    crc = vhdx_checksum_calc(0xffffffff, (void *)desc_buffer,
                            desc_sectors * VHDX_LOG_SECTOR_SIZE, 4);
    crc ^= 0xffffffff;

    buffer = qemu_blockalign(bs, VHDX_LOG_SECTOR_SIZE);
    if (total_sectors > desc_sectors) {
        for (i = 0; i < total_sectors - desc_sectors; i++) {
            sectors_read = 0;
            ret = vhdx_log_read_sectors(bs, log, &sectors_read, buffer,
                                        1, false);
            if (ret < 0 || sectors_read != 1) {
                goto free_and_exit;
            }
            crc = vhdx_checksum_calc(crc, buffer, VHDX_LOG_SECTOR_SIZE, -1);
            crc ^= 0xffffffff;
        }
    }
    crc ^= 0xffffffff;
    if (crc != desc_buffer->hdr.checksum) {
        goto free_and_exit;
    }

    *valid = true;
    *entry = hdr;
    goto free_and_exit;

inc_and_exit:
    log->read = vhdx_log_inc_idx(log->read, log->length);

free_and_exit:
    qemu_vfree(buffer);
    qemu_vfree(desc_buffer);
    return ret;
}

/* Search through the log circular buffer, and find the valid, active
 * log sequence, if any exists
 * */
static int vhdx_log_search(BlockDriverState *bs, BDRVVHDXState *s,
                           VHDXLogSequence *logs)
{
    int ret = 0;
    uint32_t tail;
    bool seq_valid = false;
    VHDXLogSequence candidate = { 0 };
    VHDXLogEntryHeader hdr = { 0 };
    VHDXLogEntries curr_log;

    memcpy(&curr_log, &s->log, sizeof(VHDXLogEntries));
    curr_log.write = curr_log.length;   /* assume log is full */
    curr_log.read = 0;


    /* now we will go through the whole log sector by sector, until
     * we find a valid, active log sequence, or reach the end of the
     * log buffer */
    for (;;) {
        uint64_t curr_seq = 0;
        VHDXLogSequence current = { 0 };

        tail = curr_log.read;

        ret = vhdx_validate_log_entry(bs, s, &curr_log, curr_seq,
                                      &seq_valid, &hdr);
        if (ret < 0) {
            goto exit;
        }

        if (seq_valid) {
            current.valid     = true;
            current.log       = curr_log;
            current.log.read  = tail;
            current.log.write = curr_log.read;
            current.count     = 1;
            current.hdr       = hdr;


            for (;;) {
                ret = vhdx_validate_log_entry(bs, s, &curr_log, curr_seq,
                                              &seq_valid, &hdr);
                if (ret < 0) {
                    goto exit;
                }
                if (seq_valid == false) {
                    break;
                }
                current.log.write = curr_log.read;
                current.count++;

                curr_seq = hdr.sequence_number;
            }
        }

        if (current.valid) {
            if (candidate.valid == false ||
                current.hdr.sequence_number > candidate.hdr.sequence_number) {
                candidate = current;
            }
        }

        if (curr_log.read < tail) {
            break;
        }
    }

    *logs = candidate;

    if (candidate.valid) {
        /* this is the next sequence number, for writes */
        s->log.sequence = candidate.hdr.sequence_number + 1;
    }


exit:
    return ret;
}

/* Parse the replay log.  Per the VHDX spec, if the log is present
 * it must be replayed prior to opening the file, even read-only.
 *
 * If read-only, we must replay the log in RAM (or refuse to open
 * a dirty VHDX file read-only) */
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int vhdx_parse_log(BlockDriverState *bs, BDRVVHDXState *s, bool *flushed,
                   Error **errp)
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{
    int ret = 0;
    VHDXHeader *hdr;
    VHDXLogSequence logs = { 0 };

    hdr = s->headers[s->curr_header];

    *flushed = false;

    /* s->log.hdr is freed in vhdx_close() */
    if (s->log.hdr == NULL) {
        s->log.hdr = qemu_blockalign(bs, sizeof(VHDXLogEntryHeader));
    }

    s->log.offset = hdr->log_offset;
    s->log.length = hdr->log_length;

    if (s->log.offset < VHDX_LOG_MIN_SIZE ||
        s->log.offset % VHDX_LOG_MIN_SIZE) {
        ret = -EINVAL;
        goto exit;
    }

    /* per spec, only log version of 0 is supported */
    if (hdr->log_version != 0) {
        ret = -EINVAL;
        goto exit;
    }

    /* If either the log guid, or log length is zero,
     * then a replay log is not present */
    if (guid_eq(hdr->log_guid, zero_guid)) {
        goto exit;
    }

    if (hdr->log_length == 0) {
        goto exit;
    }

    if (hdr->log_length % VHDX_LOG_MIN_SIZE) {
        ret = -EINVAL;
        goto exit;
    }


    /* The log is present, we need to find if and where there is an active
     * sequence of valid entries present in the log.  */

    ret = vhdx_log_search(bs, s, &logs);
    if (ret < 0) {
        goto exit;
    }

    if (logs.valid) {
765 766 767 768 769 770 771 772 773 774
        if (bs->read_only) {
            ret = -EPERM;
            error_setg_errno(errp, EPERM,
                             "VHDX image file '%s' opened read-only, but "
                             "contains a log that needs to be replayed.  To "
                             "replay the log, execute:\n qemu-img check -r "
                             "all '%s'",
                             bs->filename, bs->filename);
            goto exit;
        }
775 776 777 778 779 780 781 782 783 784 785 786 787 788
        /* now flush the log */
        ret = vhdx_log_flush(bs, s, &logs);
        if (ret < 0) {
            goto exit;
        }
        *flushed = true;
    }


exit:
    return ret;
}


789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021

static void vhdx_log_raw_to_le_sector(VHDXLogDescriptor *desc,
                                      VHDXLogDataSector *sector, void *data,
                                      uint64_t seq)
{
    /* 8 + 4084 + 4 = 4096, 1 log sector */
    memcpy(&desc->leading_bytes, data, 8);
    data += 8;
    cpu_to_le64s(&desc->leading_bytes);
    memcpy(sector->data, data, 4084);
    data += 4084;
    memcpy(&desc->trailing_bytes, data, 4);
    cpu_to_le32s(&desc->trailing_bytes);
    data += 4;

    sector->sequence_high  = (uint32_t) (seq >> 32);
    sector->sequence_low   = (uint32_t) (seq & 0xffffffff);
    sector->data_signature = VHDX_LOG_DATA_SIGNATURE;

    vhdx_log_desc_le_export(desc);
    vhdx_log_data_le_export(sector);
}


static int vhdx_log_write(BlockDriverState *bs, BDRVVHDXState *s,
                          void *data, uint32_t length, uint64_t offset)
{
    int ret = 0;
    void *buffer = NULL;
    void *merged_sector = NULL;
    void *data_tmp, *sector_write;
    unsigned int i;
    int sector_offset;
    uint32_t desc_sectors, sectors, total_length;
    uint32_t sectors_written = 0;
    uint32_t aligned_length;
    uint32_t leading_length = 0;
    uint32_t trailing_length = 0;
    uint32_t partial_sectors = 0;
    uint32_t bytes_written = 0;
    uint64_t file_offset;
    VHDXHeader *header;
    VHDXLogEntryHeader new_hdr;
    VHDXLogDescriptor *new_desc = NULL;
    VHDXLogDataSector *data_sector = NULL;
    MSGUID new_guid = { 0 };

    header = s->headers[s->curr_header];

    /* need to have offset read data, and be on 4096 byte boundary */

    if (length > header->log_length) {
        /* no log present.  we could create a log here instead of failing */
        ret = -EINVAL;
        goto exit;
    }

    if (guid_eq(header->log_guid, zero_guid)) {
        vhdx_guid_generate(&new_guid);
        vhdx_update_headers(bs, s, false, &new_guid);
    } else {
        /* currently, we require that the log be flushed after
         * every write. */
        ret = -ENOTSUP;
        goto exit;
    }

    /* 0 is an invalid sequence number, but may also represent the first
     * log write (or a wrapped seq) */
    if (s->log.sequence == 0) {
        s->log.sequence = 1;
    }

    sector_offset = offset % VHDX_LOG_SECTOR_SIZE;
    file_offset = (offset / VHDX_LOG_SECTOR_SIZE) * VHDX_LOG_SECTOR_SIZE;

    aligned_length = length;

    /* add in the unaligned head and tail bytes */
    if (sector_offset) {
        leading_length = (VHDX_LOG_SECTOR_SIZE - sector_offset);
        leading_length = leading_length > length ? length : leading_length;
        aligned_length -= leading_length;
        partial_sectors++;
    }

    sectors = aligned_length / VHDX_LOG_SECTOR_SIZE;
    trailing_length = aligned_length - (sectors * VHDX_LOG_SECTOR_SIZE);
    if (trailing_length) {
        partial_sectors++;
    }

    sectors += partial_sectors;

    /* sectors is now how many sectors the data itself takes, not
     * including the header and descriptor metadata */

    new_hdr = (VHDXLogEntryHeader) {
                .signature           = VHDX_LOG_SIGNATURE,
                .tail                = s->log.tail,
                .sequence_number     = s->log.sequence,
                .descriptor_count    = sectors,
                .reserved            = 0,
                .flushed_file_offset = bdrv_getlength(bs->file),
                .last_file_offset    = bdrv_getlength(bs->file),
              };

    new_hdr.log_guid = header->log_guid;

    desc_sectors = vhdx_compute_desc_sectors(new_hdr.descriptor_count);

    total_length = (desc_sectors + sectors) * VHDX_LOG_SECTOR_SIZE;
    new_hdr.entry_length = total_length;

    vhdx_log_entry_hdr_le_export(&new_hdr);

    buffer = qemu_blockalign(bs, total_length);
    memcpy(buffer, &new_hdr, sizeof(new_hdr));

    new_desc = (VHDXLogDescriptor *) (buffer + sizeof(new_hdr));
    data_sector = buffer + (desc_sectors * VHDX_LOG_SECTOR_SIZE);
    data_tmp = data;

    /* All log sectors are 4KB, so for any partial sectors we must
     * merge the data with preexisting data from the final file
     * destination */
    merged_sector = qemu_blockalign(bs, VHDX_LOG_SECTOR_SIZE);

    for (i = 0; i < sectors; i++) {
        new_desc->signature       = VHDX_LOG_DESC_SIGNATURE;
        new_desc->sequence_number = s->log.sequence;
        new_desc->file_offset     = file_offset;

        if (i == 0 && leading_length) {
            /* partial sector at the front of the buffer */
            ret = bdrv_pread(bs->file, file_offset, merged_sector,
                             VHDX_LOG_SECTOR_SIZE);
            if (ret < 0) {
                goto exit;
            }
            memcpy(merged_sector + sector_offset, data_tmp, leading_length);
            bytes_written = leading_length;
            sector_write = merged_sector;
        } else if (i == sectors - 1 && trailing_length) {
            /* partial sector at the end of the buffer */
            ret = bdrv_pread(bs->file,
                            file_offset,
                            merged_sector + trailing_length,
                            VHDX_LOG_SECTOR_SIZE - trailing_length);
            if (ret < 0) {
                goto exit;
            }
            memcpy(merged_sector, data_tmp, trailing_length);
            bytes_written = trailing_length;
            sector_write = merged_sector;
        } else {
            bytes_written = VHDX_LOG_SECTOR_SIZE;
            sector_write = data_tmp;
        }

        /* populate the raw sector data into the proper structures,
         * as well as update the descriptor, and convert to proper
         * endianness */
        vhdx_log_raw_to_le_sector(new_desc, data_sector, sector_write,
                                  s->log.sequence);

        data_tmp += bytes_written;
        data_sector++;
        new_desc++;
        file_offset += VHDX_LOG_SECTOR_SIZE;
    }

    /* checksum covers entire entry, from the log header through the
     * last data sector */
    vhdx_update_checksum(buffer, total_length,
                         offsetof(VHDXLogEntryHeader, checksum));
    cpu_to_le32s((uint32_t *)(buffer + 4));

    /* now write to the log */
    vhdx_log_write_sectors(bs, &s->log, &sectors_written, buffer,
                           desc_sectors + sectors);
    if (ret < 0) {
        goto exit;
    }

    if (sectors_written != desc_sectors + sectors) {
        /* instead of failing, we could flush the log here */
        ret = -EINVAL;
        goto exit;
    }

    s->log.sequence++;
    /* write new tail */
    s->log.tail = s->log.write;

exit:
    qemu_vfree(buffer);
    qemu_vfree(merged_sector);
    return ret;
}

/* Perform a log write, and then immediately flush the entire log */
int vhdx_log_write_and_flush(BlockDriverState *bs, BDRVVHDXState *s,
                             void *data, uint32_t length, uint64_t offset)
{
    int ret = 0;
    VHDXLogSequence logs = { .valid = true,
                             .count = 1,
                             .hdr = { 0 } };


    /* Make sure data written (new and/or changed blocks) is stable
     * on disk, before creating log entry */
    bdrv_flush(bs);
    ret = vhdx_log_write(bs, s, data, length, offset);
    if (ret < 0) {
        goto exit;
    }
    logs.log = s->log;

    /* Make sure log is stable on disk */
    bdrv_flush(bs);
    ret = vhdx_log_flush(bs, s, &logs);
    if (ret < 0) {
        goto exit;
    }

    s->log = logs.log;

exit:
    return ret;
}