[PATCH] ext4: Prevent race while waling extent tree

[Lukas Czerner <lczerner@redhat.com>]

Currently ext4_ext_walk_space() only takes i_data_sem for read when
searching for the extent at given block with ext4_ext_find_extent().
Then it drops the lock and the extent tree can be changed at will.
However later on we're searching for the 'next' extent, but the extent
tree might already have changed, so the information might not be
accurate.

In fact we can hit BUG_ON(end <= start) if the extent got inserted into
the tree after the one we found and before the block we were searching
for. This has been reproduced by running xfstests 225 in loop on s390x
architecture, but theoretically we could hit this on any other
architecture as well, but probably not as often.

ext4_ext_walk_space() is currently only used from ext4_fiemap() and even
if we do not hit the BUG_ON() fiemap might return scrambled information
to the user.

Fix this by requiring ext4_ext_walk_space() to be called with i_data_sem
held. By calling it from ext4_fiemap() we can only take the i_data_sem
for read, but possibly other users might want to modify the extents so
they will be able to take write lock.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
---
 fs/ext4/extents.c |    9 +++++++--
 1 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 7011ac9..f1aca06 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -1959,6 +1959,11 @@ cleanup:
 	return err;
 }
 
+/*
+ * ext4_ext_walk_space() should be called with i_data_sem locked. If we're
+ * not modifying found extents, or extent tree in callback function, then
+ * read lock is ok.
+ */
 static int ext4_ext_walk_space(struct inode *inode, ext4_lblk_t block,
 			       ext4_lblk_t num, ext_prepare_callback func,
 			       void *cbdata)
@@ -1976,9 +1981,7 @@ static int ext4_ext_walk_space(struct inode *inode, ext4_lblk_t block,
 	while (block < last && block != EXT_MAX_BLOCKS) {
 		num = last - block;
 		/* find extent for this block */
-		down_read(&EXT4_I(inode)->i_data_sem);
 		path = ext4_ext_find_extent(inode, block, path);
-		up_read(&EXT4_I(inode)->i_data_sem);
 		if (IS_ERR(path)) {
 			err = PTR_ERR(path);
 			path = NULL;
@@ -5021,8 +5024,10 @@ int ext4_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
 		 * Walk the extent tree gathering extent information.
 		 * ext4_ext_fiemap_cb will push extents back to user.
 		 */
+		down_read(&EXT4_I(inode)->i_data_sem);
 		error = ext4_ext_walk_space(inode, start_blk, len_blks,
 					  ext4_ext_fiemap_cb, fieinfo);
+		up_read(&EXT4_I(inode)->i_data_sem);
 	}
 
 	return error;
-- 
1.7.7.6