Re: [PATCH 2/2] migration: Define BLK_MIG_BLOCK_SIZE as unsigned long long

[Peter Maydell]

On Thu, 21 Jul 2022 at 13:07, Dr. David Alan Gilbert
<dgilbert@redhat.com> wrote:
>
> * Peter Maydell (peter.maydell@linaro.org) wrote:
> > When we use BLK_MIG_BLOCK_SIZE in expressions like
> > block_mig_state.submitted * BLK_MIG_BLOCK_SIZE, this multiplication
> > is done as 32 bits, because both operands are 32 bits.  Coverity
> > complains about possible overflows because we then accumulate that
> > into a 64 bit variable.
> >
> > Define BLK_MIG_BLOCK_SIZE as unsigned long long using the ULL suffix.
> > The only two current uses of it with this problem are both in
> > block_save_pending(), so we could just cast to uint64_t there, but
> > using the ULL suffix is simpler and ensures that we don't
> > accidentally introduce new variants of the same issue in future.
> >
> > Resolves: Coverity CID 1487136, 1487175
> > Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> > ---
> > I haven't tried to analyse the code to see if the multiplications
> > could ever actually end up overflowing, but I would assume
> > probably not.
> >
> >  migration/block.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/migration/block.c b/migration/block.c
> > index 9e5aae58982..3577c815a94 100644
> > --- a/migration/block.c
> > +++ b/migration/block.c
> > @@ -28,7 +28,7 @@
> >  #include "sysemu/block-backend.h"
> >  #include "trace.h"
> >
> > -#define BLK_MIG_BLOCK_SIZE           (1 << 20)
> > +#define BLK_MIG_BLOCK_SIZE           (1ULL << 20)
>
> Is it a problem that this is passed to bdrv_create_dirty_bitmap that
> takes a uint32_t ?

Shouldn't be -- the constant value still fits within 32 bits.

-- PMM