[PATCH] fs: gracefully handle ->get_block not mapping bh in __mpage_writepage

[Jan Kara <jack@suse.cz>]

When filesystem's ->get_block function does not map the buffer head when
called from __mpage_writepage(), the function will happily go and pass
bogus bdev and block number to bio allocation routines which leads to
crashes sooner or later. E.g. UDF can do this because it doesn't want to
allocate blocks from ->writepages callbacks. It allocates blocks on
write or page fault but writeback can still spot dirty buffers without
underlying blocks allocated e.g. if blocksize < pagesize, the tail page
is dirtied (which means all its buffers are dirtied), and truncate
extends the file so that some buffer starts to be within i_size.

Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/mpage.c | 2 ++
 1 file changed, 2 insertions(+)

I'd like to get this patch merged through my tree as other UDF fixes depend on
it and without this change the kernel crashes in unfortunate ways.

diff --git a/fs/mpage.c b/fs/mpage.c
index 9f040c1d5912..8bd77a8e2627 100644
--- a/fs/mpage.c
+++ b/fs/mpage.c
@@ -538,6 +538,8 @@ static int __mpage_writepage(struct page *page, struct writeback_control *wbc,
 		map_bh.b_size = 1 << blkbits;
 		if (mpd->get_block(inode, block_in_file, &map_bh, 1))
 			goto confused;
+		if (!buffer_mapped(&map_bh))
+			goto confused;
 		if (buffer_new(&map_bh))
 			clean_bdev_bh_alias(&map_bh);
 		if (buffer_boundary(&map_bh)) {
-- 
2.35.3