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 07cbbb8..cff2a714 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;