dm: table detect io beyond device

This patch fixes a panic on shrinking a DM device if there is
outstanding I/O to the part of the device that is being removed.
(Normally this doesn't happen - a filesystem would be resized first,
for example.)

The bug is that __clone_and_map() assumes dm_table_find_target()
always returns a valid pointer.  It may fail if a bio arrives from the
block layer but its target sector is no longer included in the DM
btree.

This patch appends an empty entry to table->targets[] which will
be returned by a lookup beyond the end of the device.

After calling dm_table_find_target(), __clone_and_map() and target_message()
check for this condition using
dm_target_is_valid().

Sample test script to trigger oops:

1 files changed, 6 insertions, 1 deletions

diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index e298d8d11f24..f3f952e347ed 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -189,8 +189,10 @@ static int alloc_targets(struct dm_table *t, unsigned int num)
 
         /*
          * Allocate both the target array and offset array at once.
+         * Append an empty entry to catch sectors beyond the end of
+         * the device.
          */
-        n_highs = (sector_t *) dm_vcalloc(num, sizeof(struct dm_target) +
+        n_highs = (sector_t *) dm_vcalloc(num + 1, sizeof(struct dm_target) +
                                           sizeof(sector_t));
         if (!n_highs)
                 return -ENOMEM;
@@ -867,6 +869,9 @@ struct dm_target *dm_table_get_target(struct dm_table *t, unsigned int index)
 
 /*
  * Search the btree for the correct target.
+ *
+ * Caller should check returned pointer with dm_target_is_valid()
+ * to trap I/O beyond end of device.
  */
 struct dm_target *dm_table_find_target(struct dm_table *t, sector_t sector)
 {