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Re: [Qemu-devel] [PATCH] docs: Define refcount_bits value

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH] docs: Define refcount_bits value
Date: Fri, 23 May 2014 07:56:16 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0

On 05/23/2014 07:41 AM, Maria Kustova wrote:

>           96 -  99:  refcount_order
>                      Describes the width of a reference count block entry 
> (width
> -                    in bits = 1 << refcount_order). For version 2 images, the
> -                    order is always assumed to be 4 (i.e. the width is 16 
> bits).
> +                    in bits: refcount_bits = 1 << refcount_order). For 
> version 2
> +                    images, the order is always assumed to be 4
> +                    (i.e. refcount_bits = 16).

In light of all the recent CVE fixes (and possibly a separate patch if
any code is broken), I wonder if we need more work to ensure that
refcount_order is capped to a worthwhile maximum rather than causing
undefined behavior.  That is, a refcount_order of 0x10004 should be an
error, and not a synonym of refcount_order of 4, since '1 << 0x10004' is

Furthermore, this raises some questions in my mind. Later on, we document:

    refcount_block_entries = (cluster_size / sizeof(uint16_t))

which implies a hard cap of refcount_bits=16 as the maximum, which in
turn implies a hard cap of refcount_order of 4 as the maximum.  Or is it
possible to specify a larger refcount_order, in which case
refcount_block_entries is dynamically sized to uint32_t, and in which
case the rest of the docs need to be fixed to accommodate that?


Refcount block entry (x = refcount_bits - 1):

    Bit  0 -  x:    Reference count of the cluster. If refcount_bits
implies a
                    sub-byte width, note that bit 0 means the least
                    bit in this context.

but nothing is said about bits x+1 - 15 (which only exist when
refcount_order < 4, but which presumably must be all 0 bits for the file
to be valid).

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

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