Re: [Qemu-devel] [PATCH 07/21] qcow2: Helper for refcount array size calculation

[Eric Blake <eblake@redhat.com>]

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On 11/10/2014 06:45 AM, Max Reitz wrote:
> Add a helper function which correctly calculates the byte size of a
> refcount array for any refcount order, and use that function.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  block/qcow2-refcount.c | 39 ++++++++++++++++++++++++++++-----------
>  1 file changed, 28 insertions(+), 11 deletions(-)
> 
> diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
> index 16652da..cfb4807 100644
> --- a/block/qcow2-refcount.c
> +++ b/block/qcow2-refcount.c
> @@ -1132,6 +1132,20 @@ fail:
>  /* refcount checking functions */
>  
>  
> +static size_t refcount_array_byte_size(BDRVQcowState *s, uint64_t entries)
> +{
> +    if (s->refcount_order < 3) {
> +        /* sub-byte width */
> +        int shift = 3 - s->refcount_order;
> +        return (entries + (1 << shift) - 1) >> shift;
> +    } else if (s->refcount_order == 3) {
> +        /* byte width */
> +        return entries;
> +    } else {
> +        /* multiple bytes wide */
> +        return entries << (s->refcount_order - 3);
> +    }

A comment proving why this can't overflow might be nice (if I analyzed
correctly, entries will be computed by file size / clusters, and in the
worst case, the smallest cluster and largest refcount_order results in
'(size >> 9) << (6 - 3)' which is still safe).

> @@ -1161,12 +1175,13 @@ static int inc_refcounts(BlockDriverState *bs,
>          k = cluster_offset >> s->cluster_bits;
>          if (k >= *refcount_table_size) {
>              int64_t old_refcount_table_size = *refcount_table_size;
> +            size_t new_byte_size;
>              void *new_refcount_table;
>  
>              *refcount_table_size = k + 1;
> -            new_refcount_table = g_try_realloc(*refcount_table,
> -                                               *refcount_table_size *
> -                                               s->refcount_bits / 8);
> +            new_byte_size = refcount_array_byte_size(s, *refcount_table_size);
> +
> +            new_refcount_table = g_try_realloc(*refcount_table, new_byte_size);

Yay - this addresses one of my possible overflow comments on 6/21.

I wonder if the series would have less churn if you rearranged this
patch to come before 6/21.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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