dm: fix casting bug in dm_merge_bvec()

dm_merge_bvec() was originally added in f6fccb ("dm: introduce
merge_bvec_fn").  In that commit a value in sectors is converted to
bytes using << 9, and then assigned to an int.  This code made
assumptions about the value of BIO_MAX_SECTORS.

A later commit 148e51 ("dm: improve documentation and code clarity in
dm_merge_bvec") was meant to have no functional change but it removed
the use of BIO_MAX_SECTORS in favor of using queue_max_sectors().  At
this point the cast from sector_t to int resulted in a zero value.  The
fallout being dm_merge_bvec() would only allow a single page to be added
to a bio.

This interim fix is minimal for the benefit of stable@ because the more
comprehensive cleanup of passing a sector_t to all DM targets' merge
function will impact quite a few DM targets.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org # 3.19+

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index e24069aaeb1872d1bb5b29e6dc8fdc0b9f561568..2caf492890d64b27a0a88f24f4f04d1778448d9a 100644 (file)
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -1723,8 +1723,7 @@ static int dm_merge_bvec(struct request_queue *q,
        struct mapped_device *md = q->queuedata;
        struct dm_table *map = dm_get_live_table_fast(md);
        struct dm_target *ti;
-       sector_t max_sectors;
-       int max_size = 0;
+       sector_t max_sectors, max_size = 0;
 
        if (unlikely(!map))
                goto out;
@@ -1739,8 +1738,16 @@ static int dm_merge_bvec(struct request_queue *q,
        max_sectors = min(max_io_len(bvm->bi_sector, ti),
                          (sector_t) queue_max_sectors(q));
        max_size = (max_sectors << SECTOR_SHIFT) - bvm->bi_size;
-       if (unlikely(max_size < 0)) /* this shouldn't _ever_ happen */
-               max_size = 0;
+
+       /*
+        * FIXME: this stop-gap fix _must_ be cleaned up (by passing a sector_t
+        * to the targets' merge function since it holds sectors not bytes).
+        * Just doing this as an interim fix for stable@ because the more
+        * comprehensive cleanup of switching to sector_t will impact every
+        * DM target that implements a ->merge hook.
+        */
+       if (max_size > INT_MAX)
+               max_size = INT_MAX;
 
        /*
         * merge_bvec_fn() returns number of bytes
@@ -1748,7 +1755,7 @@ static int dm_merge_bvec(struct request_queue *q,
         * max is precomputed maximal io size
         */
        if (max_size && ti->type->merge)
-               max_size = ti->type->merge(ti, bvm, biovec, max_size);
+               max_size = ti->type->merge(ti, bvm, biovec, (int) max_size);
        /*
         * If the target doesn't support merge method and some of the devices
         * provided their merge_bvec method (we know this by looking for the