Patch "dm: correctly handle chained bios in dec_pending()" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

dm: correctly handle chained bios in dec_pending()

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
dm-correctly-handle-chained-bios-in-dec_pending.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: NeilBrown <[email protected]>
Date: Thu, 15 Feb 2018 20:00:15 +1100
Subject: dm: correctly handle chained bios in dec_pending()

From: NeilBrown <[email protected]>

commit 8dd601fa8317243be887458c49f6c29c2f3d719f upstream.

dec_pending() is given an error status (possibly 0) to be recorded against a bio. It can be called several times on the one 'struct dm_io', and it is careful to only assign a non-zero error to io->status. However when it then assigned io->status to bio->bi_status, it is not careful and could overwrite a genuine error status with 0.

This can happen when chained bios are in use. If a bio is chained beneath the bio that this dm_io is handling, the child bio might complete and set bio->bi_status before the dm_io completes.

This has been possible since chained bios were introduced in 3.14, and has become a lot easier to trigger with commit 18a25da84354 ("dm: ensure bio submission follows a depth-first tree walk") as that commit caused dm to start using chained bios itself.

A particular failure mode is that if a bio spans an 'error' target and a working target, the 'error' fragment will complete instantly and set the ->bi_status, and the other fragment will normally complete a little later, and will clear ->bi_status.

The fix is simply to only assign io_error to bio->bi_status when io_error is not zero.

---
 drivers/md/dm.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

@@ -809,7 +809,8 @@ static void dec_pending(struct dm_io *io
 		} else {
 			/* done with normal IO or empty flush */
 			trace_block_bio_complete(md->queue, bio, io_error);
-			bio->bi_error = io_error;
+			if (io_error)
+				bio->bi_error = io_error;
 			bio_endio(bio);
 		}
 	}

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/dm-correctly-handle-chained-bios-in-dec_pending.patch