Re: [Qemu-devel] [RFC PATCH v1] Support vhd type VHD_DIFFERENCING

[Stefan Hajnoczi]

On Tue, Jul 01, 2014 at 05:45:19PM +0800, Ding xiao wrote:

Sorry for the delay, I forgot about this patch.

> +typedef struct vhd_tdbatmap_header {
> +    char    magic[8]; /* "tdbatmap"*/
> +
> +    /* byte offset to batmap*/
> +    uint64_t    batmap_offset;
> +
> +    /* Offset of the Block Allocation Table (BAT)*/

This comment describes the batmap_offset field?  Maybe this should be
dropped since that field already has a comment.

> +        /* read backend file*/
> +        if (dyndisk_header->parent_name[0] || 
> dyndisk_header->parent_name[1]) {
> +            for (i = 0; i < PARENT_LOCATOR_NUM; i++) {
> +                data_offset = be64_to_cpu(
> +                                
> dyndisk_header->parent_locator[i].data_offset);
> +                data_length = be32_to_cpu(
> +                                
> dyndisk_header->parent_locator[i].data_length);
> +                if (dyndisk_header->parent_locator[i].platform == MACX) {

Missing be32_to_cpu()?

> +                    ret = bdrv_pread(bs->file, data_offset + 7,
> +                              bs->backing_file, data_length - 7);

Buffer overflow: char bs->backing_file[1024].

All input must be validated!

> +                    if (ret < 0) {
> +                        goto fail;
> +                    }
> +                    bs->backing_file[data_length - 7] = '\0';

Memory corruption if data_length < 7.  Missing input validation.

> +                }
> +                if (data_offset > parent_locator_offset) {
> +                    parent_locator_offset = data_offset;
> +                }
> +            }
> +        }
> +
> +        if (parent_locator_offset + 512 > s->free_data_block_offset) {
> +            s->free_data_block_offset = parent_locator_offset + 512;
> +        }
> +
>          for (i = 0; i < s->max_table_entries; i++) {
>              be32_to_cpus(&s->pagetable[i]);
>              if (s->pagetable[i] != 0xFFFFFFFF) {
> @@ -364,6 +425,9 @@ static inline int64_t get_sector_offset(BlockDriverState 
> *bs,
>      // bitmap each time we write to a new block. This might cause Virtual PC 
> to
>      // miss sparse read optimization, but it's not a problem in terms of
>      // correctness.
> +
> +    /*this will not use*/
> +#if 0

Delete the code if it is no longer used.

> @@ -433,7 +498,7 @@ static int rewrite_footer(BlockDriverState* bs)
>   *
>   * Returns the sectors' offset in the image file on success and < 0 on error
>   */
> -static int64_t alloc_block(BlockDriverState* bs, int64_t sector_num)
> +static int64_t alloc_block(BlockDriverState *bs, int64_t sector_num, int 
> diff)

Please use the C99 bool type since it communicates more clearly that
diff is either true or false (not a counter or bitmap).

It would be clearest to check footer->type for VHD_DIFFERENCING in this
function instead of adding a new argument to the function.

> @@ -501,33 +611,64 @@ static int vpc_read(BlockDriverState *bs, int64_t 
> sector_num,
>      int64_t offset;
>      int64_t sectors, sectors_per_block;
>      VHDFooter *footer = (VHDFooter *) s->footer_buf;
> +    QEMUIOVector hd_qiov;
> +    struct iovec qiov;
>  
>      if (cpu_to_be32(footer->type) == VHD_FIXED) {
>          return bdrv_read(bs->file, sector_num, buf, nb_sectors);
> -    }
> -    while (nb_sectors > 0) {
> -        offset = get_sector_offset(bs, sector_num, 0);
> +    } else if (cpu_to_be32(footer->type) == VHD_DYNAMIC) {

cpu_to_be32() is wrong since VHD_DYNAMIC is an enum constant (just a
regular CPU-endian integer).

Please add a separate patch before this one that cleans up incorrect
cpu_to_be*() usage.  For example, cpu_to_be32(footer->type) == VHD_FIXED
a few lines above is wrong too.

> +        while (nb_sectors > 0) {
> +            offset = get_sector_offset(bs, sector_num, 0);
> +
> +            sectors_per_block = s->block_size >> BDRV_SECTOR_BITS;
> +            sectors = sectors_per_block - (sector_num % sectors_per_block);
> +            if (sectors > nb_sectors) {
> +                sectors = nb_sectors;
> +            }
>  
> -        sectors_per_block = s->block_size >> BDRV_SECTOR_BITS;
> -        sectors = sectors_per_block - (sector_num % sectors_per_block);
> -        if (sectors > nb_sectors) {
> -            sectors = nb_sectors;
> +            if (offset == -1) {
> +                memset(buf, 0, sectors * BDRV_SECTOR_SIZE);
> +            } else {
> +                ret = bdrv_pread(bs->file, offset, buf,
> +                    sectors * BDRV_SECTOR_SIZE);
> +               if (ret != sectors * BDRV_SECTOR_SIZE) {

Indentation is off

> +                    return -1;
> +                }
> +            }
> +
> +            nb_sectors -= sectors;
> +            sector_num += sectors;
> +            buf += sectors * BDRV_SECTOR_SIZE;
>          }
> +    } else {
> +         while (nb_sectors > 0) {
> +            offset = get_sector_offset_diff(bs, sector_num);
> +            if (offset == -1) {
> +                memset(buf, 0, BDRV_SECTOR_SIZE);
> +            } else if (offset == -2) {
> +                qiov.iov_base = (void *)buf;

This cast is unnecessary.  The compiler does not warn about pointer
casts to or from void*.

> +                qiov.iov_len = 512;
> +                hd_qiov.iov = &qiov;
> +                hd_qiov.niov = 1;
> +                hd_qiov.nalloc = -1;
> +                hd_qiov.size = 512;

This is not idiomatic.  Normally 'qiov' is a QEMUIOVector, not a struct
iovec.  The qemu_iovec_*() functions should be used instead of manually
setting QEMUIOVector fields:

iov.iov_base = buf;
iov.iov_len = 512;
qemu_iovec_init_external(&hd_qiov, &iov, 1);

> +                ret = bdrv_co_readv(bs->backing_hd, sector_num, 1, &hd_qiov);
> +                if (ret < 0) {
> +                    return -1;
> +                }

Why are you using bdrv_co_readv() instead of bdrv_pread() like the rest
of this file?

It would be simpler to use bdrv_pread().  Then you don't need the struct
iovec and QEMUIOVector.  This function also hasn't been marked
coroutine_fn yet, so it is cleaner to stick with bdrv_pread() until the
file is properly converted to coroutines.

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