Re: [RFC][PATCH] fix sign extension with 1.5TB usb-storage LBD=y

Matthew Wilcox
2009-04-21T21:31:54Z

On Tue, Apr 21, 2009 at 02:29:54PM -0700, Randy Dunlap wrote:
> > Fix READ CAPACITY 10 with drives over 1TB
> >
> > Shifting an unsigned char implicitly casts it to a signed int. This
> > caused 'lba' to sign-extend and Linux would then try READ CAPACITY 16
> > which was not supported by at least one drive. Making 'lba' an unsigned
> > int ensures that sign extension will not occur.
> >
> > Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
> >
> > diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
> > index 3fcb64b..c856b1b 100644
> > --- a/drivers/scsi/sd.c
> > +++ b/drivers/scsi/sd.c
> > @@ -1373,7 +1373,7 @@ static int read_capacity_10(struct scsi_disk *sdkp, struct scsi_device *sdp,
> > int sense_valid = 0;
> > int the_result;
> > int retries = 3;
> > - sector_t lba;
> > + unsigned lba;
>
> sector_t is either unsigned long or u64, depending on CONFIG_LBD.
> Are you saying (implying) that the higher-order bits of it don't matter here?
> If so, I'd just like for that to be clear(er).

We're reading 32 bits of information from the drive response. The top
32 bits are implicitly zero.

--
Matthew Wilcox Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours. We can't possibly take such
a retrograde step."