xen-blkfront: properly name all devices

- devices beyond xvdzz didn't get proper names assigned at all
- extended devices with minors not representable within the kernel's
  major/minor bit split spilled into foreign majors
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/block/xen-blkfront.c

@@ -526,6 +526,14 @@ static int xen_translate_vdev(int vdevice, int *minor, unsigned int *offset)
 	return 0;
 }
 
+static char *encode_disk_name(char *ptr, unsigned int n)
+{
+	if (n >= 26)
+		ptr = encode_disk_name(ptr, n / 26 - 1);
+	*ptr = 'a' + n % 26;
+	return ptr + 1;
+}
+
 static int xlvbd_alloc_gendisk(blkif_sector_t capacity,
 			       struct blkfront_info *info,
 			       u16 vdisk_info, u16 sector_size)
@@ -536,6 +544,7 @@ static int xlvbd_alloc_gendisk(blkif_sector_t capacity,
 	unsigned int offset;
 	int minor;
 	int nr_parts;
+	char *ptr;
 
 	BUG_ON(info->gd != NULL);
 	BUG_ON(info->rq != NULL);
@@ -560,7 +569,11 @@ static int xlvbd_alloc_gendisk(blkif_sector_t capacity,
 					"emulated IDE disks,\n\t choose an xvd device name"
 					"from xvde on\n", info->vdevice);
 	}
-	err = -ENODEV;
+	if (minor >> MINORBITS) {
+		pr_warn("blkfront: %#x's minor (%#x) out of range; ignoring\n",
+			info->vdevice, minor);
+		return -ENODEV;
+	}
 
 	if ((minor % nr_parts) == 0)
 		nr_minors = nr_parts;
@@ -574,23 +587,14 @@ static int xlvbd_alloc_gendisk(blkif_sector_t capacity,
 	if (gd == NULL)
 		goto release;
 
-	if (nr_minors > 1) {
-		if (offset < 26)
-			sprintf(gd->disk_name, "%s%c", DEV_NAME, 'a' + offset);
-		else
-			sprintf(gd->disk_name, "%s%c%c", DEV_NAME,
-				'a' + ((offset / 26)-1), 'a' + (offset % 26));
-	} else {
-		if (offset < 26)
-			sprintf(gd->disk_name, "%s%c%d", DEV_NAME,
-				'a' + offset,
-				minor & (nr_parts - 1));
+	strcpy(gd->disk_name, DEV_NAME);
+	ptr = encode_disk_name(gd->disk_name + sizeof(DEV_NAME) - 1, offset);
+	BUG_ON(ptr >= gd->disk_name + DISK_NAME_LEN);
+	if (nr_minors > 1)
+		*ptr = 0;
 	else
-			sprintf(gd->disk_name, "%s%c%c%d", DEV_NAME,
-				'a' + ((offset / 26) - 1),
-				'a' + (offset % 26),
-				minor & (nr_parts - 1));
-	}
+		snprintf(ptr, gd->disk_name + DISK_NAME_LEN - ptr,
+			 "%d", minor & (nr_parts - 1));
 
 	gd->major = XENVBD_MAJOR;
 	gd->first_minor = minor;