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
