Re: [Qemu-devel] [PATCH 08/21] qcow2: More helpers for refcount modification

[Max Reitz]

On 2014-11-11 at 01:29, Eric Blake wrote:
> On 11/10/2014 06:45 AM, Max Reitz wrote:
> > Add helper functions for getting and setting refcounts in a refcount
> > array for any possible refcount order, and choose the correct one during
> > refcount initialization.
> >
> > Signed-off-by: Max Reitz <address@hidden>
> > ---
> >   block/qcow2-refcount.c | 143 ++++++++++++++++++++++++++++++++++++++++++++++++-
> >   1 file changed, 141 insertions(+), 2 deletions(-)
> >
> > +
> > +static uint64_t get_refcount_ro6(const void *refcount_array, uint64_t index)
> > +{
> > +    return be64_to_cpu(((const uint64_t *)refcount_array)[index]);
> > +}
> Should this return int64_t and error out if the user ever exceeded 2**63
> via image fuzzing?

I don't know. It's nice that these helper functions cannot return an 
error and thus we don't have to check for a error. I think checking that 
the value didn't overflow in qcow2_get_refcount() should be sufficient 
and relieves the other callers (mainly the image check for its in-memory 
refcount table/array) which know that the value cannot overflow from 
error checking.

Although I did forget an overflow check after the call to get_refcount() 
in update_refcount_discard().

> > +
> > +static void set_refcount_ro6(void *refcount_array, uint64_t index,
> > +                             uint64_t value)
> > +{
> > +    ((uint64_t *)refcount_array)[index] = cpu_to_be64(value);
> > +}
> Should this assert that value <= INT64_MAX, since that's what the rest
> of the code caps it to?

Yes, that it should most certainly do.

Max