jbd2: Avoid possible NULL dereference in jbd2_journal_begin_ordered_truncate()

If we race with commit code setting i_transaction to NULL, we could
possibly dereference it.  Proper locking requires the journal pointer
(to access journal->j_list_lock), which we don't have.  So we have to
change the prototype of the function so that filesystem passes us the
journal pointer.  Also add a more detailed comment about why the
function jbd2_journal_begin_ordered_truncate() does what it does and
how it should be used.

Thanks to Dan Carpenter <error27@gmail.com> for pointing to the
suspitious code.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Acked-by: Joel Becker <joel.becker@oracle.com>
CC: linux-ext4@vger.kernel.org
CC: ocfs2-devel@oss.oracle.com
CC: mfasheh@suse.de
CC: Dan Carpenter <error27@gmail.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
index b28b37eb11c6..4d248b3f1323 100644
--- a/include/linux/jbd2.h
+++ b/include/linux/jbd2.h
@@ -1150,7 +1150,8 @@ extern int	   jbd2_journal_clear_err  (journal_t *);
 extern int         jbd2_journal_bmap(journal_t *, unsigned long, unsigned long long *);
 extern int         jbd2_journal_force_commit(journal_t *);
 extern int         jbd2_journal_file_inode(handle_t *handle, struct jbd2_inode *inode);
-extern int         jbd2_journal_begin_ordered_truncate(struct jbd2_inode *inode, loff_t new_size);
+extern int         jbd2_journal_begin_ordered_truncate(journal_t *journal,
+                                struct jbd2_inode *inode, loff_t new_size);
 extern void        jbd2_journal_init_jbd_inode(struct jbd2_inode *jinode, struct inode *inode);
 extern void        jbd2_journal_release_jbd_inode(journal_t *journal, struct jbd2_inode *jinode);