Re: [PATCH v2] fix sign extension with 1.5TB usb-storage LBD=y

Matthew Wilcox
2009-04-21T23:03:24Z

On Tue, Apr 21, 2009 at 03:00:10PM -0700, Dave Hansen wrote:
> Here's a patch implementing Al's suggestion. Not quite as trivial as
> Matthew's, but even nicer aesthetically. (Description stolen from
> Matthew's patch). I have verified that this fixes my issue.

I prefer this approach, but I'm equally happy with whichever patch
James chooses.

> 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. Using the
> get_unaligned_be*() helpers keeps us from having to worry about how the
> extension might occur.
>
> Signed-off-by: Dave Hansen <dave@linux.vnet.ibm.com>

Reviewed-by: Matthew Wilcox <willy@linux.intel.com>

> @@ -1344,12 +1345,8 @@ static int read_capacity_16(struct scsi_disk *sdkp, struct scsi_device *sdp,
> return -EINVAL;
> }
>
> - sector_size = (buffer[8] << 24) | (buffer[9] << 16) |
> - (buffer[10] << 8) | buffer[11];
> - lba = (((u64)buffer[0] << 56) | ((u64)buffer[1] << 48) |
> - ((u64)buffer[2] << 40) | ((u64)buffer[3] << 32) |
> - ((u64)buffer[4] << 24) | ((u64)buffer[5] << 16) |
> - ((u64)buffer[6] << 8) | (u64)buffer[7]);
> + sector_size = get_unaligned_be32(&buffer[8]);
> + lba = get_unaligned_be64(&buffer[0]);

The smallest of quibbles ... you have an excess space after the = ...

--
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."