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Re: [Qemu-devel] [PATCH] docs: Define refcount_bits value
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [PATCH] docs: Define refcount_bits value |
Date: |
Fri, 23 May 2014 17:20:54 +0200 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
On Fri, May 23, 2014 at 07:56:16AM -0600, Eric Blake wrote:
> 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
> undefined.
>
> 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?
>
> Also,
>
> 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
> significant
> 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).
Only refcount_order = 4 is supported by QEMU at the moment.
I agree the spec could be made a bit clearer though. Maybe Kevin wants
to send a patch to explain the details of refcount entry sizing.
Stefan