ext4: fix warning in ext4_dio_write_end_io()

The syzbot has reported that it can hit the warning in
ext4_dio_write_end_io() because i_size < i_disksize. Indeed the
reproducer creates a race between DIO IO completion and truncate
expanding the file and thus ext4_dio_write_end_io() sees an inconsistent
inode state where i_disksize is already updated but i_size is not
updated yet. Since we are careful when setting up DIO write and consider
it extending (and thus performing the IO synchronously with i_rwsem held
exclusively) whenever it goes past either of i_size or i_disksize, we
can use the same test during IO completion without risking entering
ext4_handle_inode_extension() without i_rwsem held. This way we make it
obvious both i_size and i_disksize are large enough when we report DIO
completion without relying on unreliable WARN_ON.

Reported-by:  <syzbot+47479b71cdfc78f56d30@syzkaller.appspotmail.com>
Fixes: 91562895f803 ("ext4: properly sync file size update after O_SYNC direct IO")
Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Link: https://lore.kernel.org/r/20231130095653.22679-1-jack@suse.cz
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 0166bb9ca160bdb5196aa9731b2c2daebd3c7116..6aa15dafc67786559d3b68ebfefd8f90e119b3fc 100644 (file)
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -349,9 +349,10 @@ static void ext4_inode_extension_cleanup(struct inode *inode, ssize_t count)
                return;
        }
        /*
-        * If i_disksize got extended due to writeback of delalloc blocks while
-        * the DIO was running we could fail to cleanup the orphan list in
-        * ext4_handle_inode_extension(). Do it now.
+        * If i_disksize got extended either due to writeback of delalloc
+        * blocks or extending truncate while the DIO was running we could fail
+        * to cleanup the orphan list in ext4_handle_inode_extension(). Do it
+        * now.
         */
        if (!list_empty(&EXT4_I(inode)->i_orphan) && inode->i_nlink) {
                handle_t *handle = ext4_journal_start(inode, EXT4_HT_INODE, 2);
@@ -386,10 +387,11 @@ static int ext4_dio_write_end_io(struct kiocb *iocb, ssize_t size,
         * blocks. But the code in ext4_iomap_alloc() is careful to use
         * zeroed/unwritten extents if this is possible; thus we won't leave
         * uninitialized blocks in a file even if we didn't succeed in writing
-        * as much as we intended.
+        * as much as we intended. Also we can race with truncate or write
+        * expanding the file so we have to be a bit careful here.
         */
-       WARN_ON_ONCE(i_size_read(inode) < READ_ONCE(EXT4_I(inode)->i_disksize));
-       if (pos + size <= READ_ONCE(EXT4_I(inode)->i_disksize))
+       if (pos + size <= READ_ONCE(EXT4_I(inode)->i_disksize) &&
+           pos + size <= i_size_read(inode))
                return size;
        return ext4_handle_inode_extension(inode, pos, size);
 }