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

Matthew Wilcox
2009-04-21T21:19:20Z

On Tue, Apr 21, 2009 at 01:52:54PM -0700, Dave Hansen wrote:
> This is with current git as of this morning, which is at v2.6.30-rc2.
>
> I have a 1.5TB USB device which gets a bit angry when I plug it in. It
> ends up with a scsi_disk->capacity of ffffffffaea87b30. I tracked it
> down to the lba calculation in read_capacity_10():
>
> lba = (buffer[0] << 24) | (buffer[1] << 16) |
> (buffer[2] << 8) | buffer[3];
>
> lba is getting all 0xf's in its high 32 bits. It seems odd that this
> would happen since 'buffer' is an 'unsigned char', but that is
> apparently what is going on. Note that this isn't an issue 32-bit
> kernels compiled with CONFIG_LBD=n since there's no more bits into which
> the sign could be extended.

I think I know ... unsigned char gets promoted to signed int since it will
fit. then signed int gets cast to unsigned long long, sign-extending. C's
promotion rules have always felt a bit wacky to me.

> diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
> index 3fcb64b..db60e96 100644
> --- a/drivers/scsi/sd.c
> +++ b/drivers/scsi/sd.c
> @@ -1402,7 +1402,7 @@ static int read_capacity_10(struct scsi_disk *sdkp, struct scsi_device *sdp,
>
> sector_size = (buffer[4] << 24) | (buffer[5] << 16) |
> (buffer[6] << 8) | buffer[7];
> - lba = (buffer[0] << 24) | (buffer[1] << 16) |
> + lba = ((sector_t)buffer[0] << 24) | (buffer[1] << 16) |
> (buffer[2] << 8) | buffer[3];

this certainly fixes your problem. I'd prefer this patch instead, just
because I find the cast unaesthetic ...

----

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;
unsigned sector_size;

do {
@@ -1413,7 +1413,7 @@ static int read_capacity_10(struct scsi_disk *sdkp, struct scsi_device *sdp,
return -EOVERFLOW;
}

- sdkp->capacity = lba + 1;
+ sdkp->capacity = (sector_t)lba + 1;
return sector_size;
}

--
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."