block: implement CONFIG_DEBUG_BLOCK_EXT_DEVT

Extended devt introduces non-contiguos device numbers.  This patch
implements a debug option which forces most devt allocations to be
from the extended area and spreads them out.  This is enabled by
default if DEBUG_KERNEL is set and achieves...

1. Detects code paths in kernel or userland which expect predetermined
   consecutive device numbers.

2. When something goes wrong, avoid corruption as adding to the minor
   of earlier partition won't lead to the wrong but valid device.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 35 insertions, 3 deletions

diff --git a/block/genhd.c b/block/genhd.c
index ee4b13520e5..67e5a59ced2 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -299,6 +299,38 @@ EXPORT_SYMBOL(unregister_blkdev);
 static struct kobj_map *bdev_map;
 
 /**
+ * blk_mangle_minor - scatter minor numbers apart
+ * @minor: minor number to mangle
+ *
+ * Scatter consecutively allocated @minor number apart if MANGLE_DEVT
+ * is enabled.  Mangling twice gives the original value.
+ *
+ * RETURNS:
+ * Mangled value.
+ *
+ * CONTEXT:
+ * Don't care.
+ */
+static int blk_mangle_minor(int minor)
+{
+#ifdef CONFIG_DEBUG_BLOCK_EXT_DEVT
+        int i;
+
+        for (i = 0; i < MINORBITS / 2; i++) {
+                int low = minor & (1 << i);
+                int high = minor & (1 << (MINORBITS - 1 - i));
+                int distance = MINORBITS - 1 - 2 * i;
+
+                minor ^= low | high;    /* clear both bits */
+                low <<= distance;       /* swap the positions */
+                high >>= distance;
+                minor |= low | high;    /* and set */
+        }
+#endif
+        return minor;
+}
+
+/**
  * blk_alloc_devt - allocate a dev_t for a partition
  * @part: partition to allocate dev_t for
  * @gfp_mask: memory allocation flag
@@ -339,7 +371,7 @@ int blk_alloc_devt(struct hd_struct *part, dev_t *devt)
                 return -EBUSY;
         }
 
-        *devt = MKDEV(BLOCK_EXT_MAJOR, idx);
+        *devt = MKDEV(BLOCK_EXT_MAJOR, blk_mangle_minor(idx));
         return 0;
 }
 
@@ -361,7 +393,7 @@ void blk_free_devt(dev_t devt)
 
         if (MAJOR(devt) == BLOCK_EXT_MAJOR) {
                 mutex_lock(&ext_devt_mutex);
-                idr_remove(&ext_devt_idr, MINOR(devt));
+                idr_remove(&ext_devt_idr, blk_mangle_minor(MINOR(devt)));
                 mutex_unlock(&ext_devt_mutex);
         }
 }
@@ -473,7 +505,7 @@ struct gendisk *get_gendisk(dev_t devt, int *partno)
                 struct hd_struct *part;
 
                 mutex_lock(&ext_devt_mutex);
-                part = idr_find(&ext_devt_idr, MINOR(devt));
+                part = idr_find(&ext_devt_idr, blk_mangle_minor(MINOR(devt)));
                 if (part && get_disk(part_to_disk(part))) {
                         *partno = part->partno;
                         disk = part_to_disk(part);