Re: dm raid: pointer math issue in super_sync()

[Mike Snitzer <snitzer@redhat.com>]

On Tue, Oct 21 2014 at  8:57am -0400,
Dan Carpenter <dan.carpenter@oracle.com> wrote:

> On Tue, Oct 21, 2014 at 08:48:26AM -0400, Mike Snitzer wrote:
> > > -	memset(sb + sizeof(*sb), 0, rdev->sb_size - sizeof(*sb));
> > > +	memset(sb + 1, 0, rdev->sb_size - sizeof(*sb));
> > >  
> > >  	sb->magic = cpu_to_le32(DM_RAID_MAGIC);
> > >  	sb->features = cpu_to_le32(0);	/* No features yet */
> > 
> > Not following... sizeof(*sb) != sizeof(sb).  So I'm not seeing a
> > problem.
> > 
> > Nor am I seeing how you think sb + 1 is equivalent to what Heinz
> > intended (zero the memory following the sizeof(struct dm_raid_superblock)).
> 
> It's pointer math.

Yes, I see that now..

> sizeof(*sb) is 512.
> 
> "sb + sizeof(*sb)" is the same as (void *)sb + 512 * 512.
> "sb + 1" is the same as (void *)sb + 512.

Actually, Heinz removed the 452 bytes of padding from struct
dm_raid_superblock, so it is more like:

sizeof(*sb) == sizeof(struct dm_raid_super_block) == 60

"sb + sizeof(*sb)" is the same as (void *)sb + 60 * 60
"sb + 1" is the same as (void *)sb + 60.

But regardless, your broader point on the math stands.  I'll get this
fixed up, thanks!

Mike