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

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.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>

diff --git a/fs/mpage.c b/fs/mpage.c
index 0f8ae954a579035253109dba0aa9592527be0964..ce53179428db4dd4ff4ac6366a9307a3b33021c0 100644 (file)
--- a/fs/mpage.c
+++ b/fs/mpage.c
@@ -532,6 +532,8 @@ 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)) {