Bugs in scsi_vpd_inquiry()

Alan Stern
2009-08-10T14:41:43Z

Martin and Matthew:

Since you guys added scsi_vpd_inquiry() and scsi_get_vpd_page() plus
sd_read_block_limits() and sd_read_block_characteristics(), I'm
directing these questions to you.

Is there some reason for not accounting for the 4 header bytes in the
allocation length value stored in the CDB? Or is this simply a bug?

Were you aware that SCSI-2 defines the allocation length to be a single
byte? cmd[3] is specified as "Reserved" in the spec. Hence the value
of "len" should be capped at 255 if sdev->scsi_level <= SCSI_2, right?

Why does scsi_get_vpd_page() retrieve page 0 first, rather than
directly asking for the page in question? Is this some sort of
play-it-safe approach, to avoid sending devices commands they may not
support?

Have you considered that plenty of low-budget USB mass-storage devices
don't implement VPD properly? I've got a flash drive right here which
totally ignores the "page" byte in the INQUIRY command; it always
responds with the normal INQUIRY data. Thus expecting the response
data always to be accurate may not be a good idea. I'm considering
adding a "restrict_to_MS_usb" flag to the host template, to indicate
that commands shouldn't be sent unless some version of Windows uses
them when talking to USB devices -- do you think that could work?

Finally, what's your opinion on the proposed patch below?

Alan Stern

Index: usb-2.6/drivers/scsi/scsi.c
===================================================================
--- usb-2.6.orig/drivers/scsi/scsi.c
+++ usb-2.6/drivers/scsi/scsi.c
@@ -969,7 +969,7 @@ EXPORT_SYMBOL(scsi_track_queue_full);
* @sdev: The device to ask
* @buffer: Where to put the result
* @page: Which Vital Product Data to return
- * @len: The length of the buffer
+ * @len: The length of the data (= buffer length - 4)
*
* This is an internal helper function. You probably want to use
* scsi_get_vpd_page instead.
@@ -982,6 +982,12 @@ static int scsi_vpd_inquiry(struct scsi_
int result;
unsigned char cmd[16];

+ len += 4; /* Include room for the header bytes */
+
+ /* SCSI-2 and earlier allow only 1 byte for the allocation length */
+ if (sdev->scsi_level <= SCSI_2)
+ len = min(len, 255u);
+
cmd[0] = INQUIRY;
cmd[1] = 1; /* EVPD */
cmd[2] = page;
@@ -994,7 +1000,7 @@ static int scsi_vpd_inquiry(struct scsi_
* all the existing users tried this hard.
*/
result = scsi_execute_req(sdev, cmd, DMA_FROM_DEVICE, buffer,
- len + 4, NULL, 30 * HZ, 3, NULL);
+ len, NULL, 30 * HZ, 3, NULL);
if (result)
return result;