sd.c: be more cautious in asking for mode page 8 data,

sanity-checking the information more carefully. 

The old code basically just used a random-number approach
to determine how much to read.

drivers/scsi/sd.c

@@ -1108,14 +1108,26 @@ sd_read_cache_type(struct scsi_disk *sdkp, char *diskname,
 	/* cautiously ask */
 	res = sd_do_mode_sense(SRpnt, dbd, modepage, buffer, 4, &data);
 
-	if (scsi_status_is_good(res)) {
+	if (!scsi_status_is_good(res))
+		goto bad_sense;
+
 	/* that went OK, now ask for the proper length */
 	len = data.length;
-		if (len > 128)
-			len = 128;
-		res = sd_do_mode_sense(SRpnt, dbd, modepage, buffer,
-				       len, &data);
-	}
+
+	/*
+	 * We're only interested in the first three bytes, actually.
+	 * But the data cache page is defined for the first 20.
+	 */
+	if (len < 3)
+		goto bad_sense;
+	if (len > 20)
+		len = 20;
+
+	/* Take headers and block descriptors into account */
+	len += data.header_length + data.block_descriptor_length;
+
+	/* Get the data */
+	res = sd_do_mode_sense(SRpnt, dbd, modepage, buffer, len, &data);
 
 	if (scsi_status_is_good(res)) {
 		const char *types[] = {
@@ -1133,7 +1145,11 @@ sd_read_cache_type(struct scsi_disk *sdkp, char *diskname,
 
 		printk(KERN_NOTICE "SCSI device %s: drive cache: %s\n",
 		       diskname, types[ct]);
-	} else {
+
+		return;
+	}
+
+bad_sense:
 	if ((SRpnt->sr_sense_buffer[0] & 0x70) == 0x70
 	     && (SRpnt->sr_sense_buffer[2] & 0x0f) == ILLEGAL_REQUEST
 	     /* ASC 0x24 ASCQ 0x00: Invalid field in CDB */
@@ -1149,7 +1165,6 @@ sd_read_cache_type(struct scsi_disk *sdkp, char *diskname,
 	       diskname);
 	sdkp->WCE = 0;
 	sdkp->RCD = 0;
-	}
 }
 
 /**