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SubjectRe: [PATCH v2] fix sign extension with 1.5TB usb-storage LBD=y
On 04/22/2009 01:00 AM, 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.
>
> --
>
> 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>
>
> diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
> index 3fcb64b..c50142b 100644
> --- a/drivers/scsi/sd.c
> +++ b/drivers/scsi/sd.c
> @@ -50,6 +50,7 @@
> #include <linux/string_helpers.h>
> #include <linux/async.h>
> #include <asm/uaccess.h>
> +#include <asm/unaligned.h>
>
> #include <scsi/scsi.h>
> #include <scsi/scsi_cmnd.h>
> @@ -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]);

These are actually aligned access it might be worth sacrificing a cast
to be32/64 for sake of speed.

>
> sd_read_protection_type(sdkp, buffer);
>
> @@ -1400,10 +1397,8 @@ static int read_capacity_10(struct scsi_disk *sdkp, struct scsi_device *sdp,
> return -EINVAL;
> }
>
> - sector_size = (buffer[4] << 24) | (buffer[5] << 16) |
> - (buffer[6] << 8) | buffer[7];
> - lba = (buffer[0] << 24) | (buffer[1] << 16) |
> - (buffer[2] << 8) | buffer[3];
> + sector_size = get_unaligned_be32(&buffer[4]);
> + lba = get_unaligned_be32(&buffer[0]);
>

Here too, both are actually aligned.

> if ((sizeof(sdkp->capacity) == 4) && (lba == 0xffffffff)) {
> sd_printk(KERN_ERR, sdkp, "Too big for this kernel. Use a "
>
>
> -- Dave
>

I'm not 100% convinced just a thought.
Boaz


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