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, 18 insertions, 6 deletions

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 07cbbb8eb3e0..cff2a714c107 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -672,13 +672,19 @@ static struct bio *clone_bio(struct bio *bio, sector_t sector,
         return clone;
 }
 
-static void __clone_and_map(struct clone_info *ci)
+static int __clone_and_map(struct clone_info *ci)
 {
         struct bio *clone, *bio = ci->bio;
-        struct dm_target *ti = dm_table_find_target(ci->map, ci->sector);
-        sector_t len = 0, max = max_io_len(ci->md, ci->sector, ti);
+        struct dm_target *ti;
+        sector_t len = 0, max;
         struct dm_target_io *tio;
 
+        ti = dm_table_find_target(ci->map, ci->sector);
+        if (!dm_target_is_valid(ti))
+                return -EIO;
+
+        max = max_io_len(ci->md, ci->sector, ti);
+
         /*
          * Allocate a target io object.
          */
@@ -736,6 +742,9 @@ static void __clone_and_map(struct clone_info *ci)
                 do {
                         if (offset) {
                                 ti = dm_table_find_target(ci->map, ci->sector);
+                                if (!dm_target_is_valid(ti))
+                                        return -EIO;
+
                                 max = max_io_len(ci->md, ci->sector, ti);
 
                                 tio = alloc_tio(ci->md);
@@ -759,6 +768,8 @@ static void __clone_and_map(struct clone_info *ci)
 
                 ci->idx++;
         }
+
+        return 0;
 }
 
 /*
@@ -767,6 +778,7 @@ static void __clone_and_map(struct clone_info *ci)
 static int __split_bio(struct mapped_device *md, struct bio *bio)
 {
         struct clone_info ci;
+        int error = 0;
 
         ci.map = dm_get_table(md);
         if (unlikely(!ci.map))
@@ -784,11 +796,11 @@ static int __split_bio(struct mapped_device *md, struct bio *bio)
         ci.idx = bio->bi_idx;
 
         start_io_acct(ci.io);
-        while (ci.sector_count)
-                __clone_and_map(&ci);
+        while (ci.sector_count && !error)
+                error = __clone_and_map(&ci);
 
         /* drop the extra reference count */
-        dec_pending(ci.io, 0);
+        dec_pending(ci.io, error);
         dm_table_put(ci.map);
 
         return 0;