[PATCH] btrfs: handle missing chunk mapping more gracefully

[Qu Wenruo <wqu@suse.com>]

[BUG]
During my scrub rework, I did a stupid thing like this:

        bio->bi_iter.bi_sector = stripe->logical;
        btrfs_submit_bio(fs_info, bio, stripe->mirror_num);

Above bi_sector assignment is using logical address directly, which
lacks ">> SECTOR_SHIFT".

This results a read on a range which has no chunk mapping.

This results the following crash:

 BTRFS critical (device dm-1): unable to find logical 11274289152 length 65536
 assertion failed: !IS_ERR(em), in fs/btrfs/volumes.c:6387
 ------------[ cut here ]------------

Sure this is all my fault, but this shows a possible problem in real
world, that some bitflip in file extents/tree block can point to
unmapped ranges, and trigger above ASSERT().

[PROBLEMS]
In above call chain, there are 2 locations not properly handling the
errors:

- __btrfs_map_block()
  If btrfs_get_chunk_map() returned error, we just trigger an ASSERT().

- btrfs_submit_bio()
  If the returned mapped length is smaller than expected, we just BUG().

[FIX]
This patch will fix the problems by:

- Add extra WARN_ON() if btrfs_get_chunk_map() failed
  I know syzbot will complain, but it's better noisy for fstests.

- Replace the ASSERT()
  By returning the error.

- Handle the error when mapped length is smaller than expected length

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/bio.c     | 6 +++++-
 fs/btrfs/volumes.c | 5 ++++-
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/bio.c b/fs/btrfs/bio.c
index b8fb7ef6b520..8f7b56a0290f 100644
--- a/fs/btrfs/bio.c
+++ b/fs/btrfs/bio.c
@@ -246,7 +246,11 @@ void btrfs_submit_bio(struct btrfs_fs_info *fs_info, struct bio *bio, int mirror
 		btrfs_crit(fs_info,
 			   "mapping failed logical %llu bio len %llu len %llu",
 			   logical, length, map_length);
-		BUG();
+		WARN_ON(1);
+		ret = -EINVAL;
+		btrfs_bio_counter_dec(fs_info);
+		btrfs_bio_end_io(btrfs_bio(bio), errno_to_blk_status(ret));
+		return;
 	}
 
 	if (!bioc) {
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index aa25fa335d3e..f69475fb1bc1 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -3012,6 +3012,7 @@ struct extent_map *btrfs_get_chunk_map(struct btrfs_fs_info *fs_info,
 	if (!em) {
 		btrfs_crit(fs_info, "unable to find logical %llu length %llu",
 			   logical, length);
+		WARN_ON(1);
 		return ERR_PTR(-EINVAL);
 	}
 
@@ -3020,6 +3021,7 @@ struct extent_map *btrfs_get_chunk_map(struct btrfs_fs_info *fs_info,
 			   "found a bad mapping, wanted %llu-%llu, found %llu-%llu",
 			   logical, length, em->start, em->start + em->len);
 		free_extent_map(em);
+		WARN_ON(1);
 		return ERR_PTR(-EINVAL);
 	}
 
@@ -6384,7 +6386,8 @@ int __btrfs_map_block(struct btrfs_fs_info *fs_info, enum btrfs_map_op op,
 	ASSERT(op != BTRFS_MAP_DISCARD);
 
 	em = btrfs_get_chunk_map(fs_info, logical, *length);
-	ASSERT(!IS_ERR(em));
+	if (IS_ERR(em))
+		return PTR_ERR(em);
 
 	ret = btrfs_get_io_geometry(fs_info, em, op, logical, &geom);
 	if (ret < 0)
-- 
2.38.1