direct-io: Fix negative return from dio read beyond eof

Assume a filesystem with 4KB blocks. When a file has size 1000 bytes and
we issue direct IO read at offset 1024, blockdev_direct_IO() reads the
tail of the last block and the logic for handling short DIO reads in
dio_complete() results in a return value -24 (1000 - 1024) which
obviously confuses userspace.

Fix the problem by bailing out early once we sample i_size and can
reliably check that direct IO read starts beyond i_size.

Reported-by: Avi Kivity <avi@scylladb.com>
Fixes: 9fe55eea7e4b444bafc42fa0000cc2d1d2847275
CC: stable@vger.kernel.org
CC: Steven Whitehouse <swhiteho@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 9 insertions, 1 deletions

diff --git a/fs/direct-io.c b/fs/direct-io.c
index cb5337d8c273..1c75a3a07f8f 100644
--- a/fs/direct-io.c
+++ b/fs/direct-io.c
@@ -1169,6 +1169,15 @@ do_blockdev_direct_IO(struct kiocb *iocb, struct inode *inode,
                 }
         }
 
+        /* Once we sampled i_size check for reads beyond EOF */
+        dio->i_size = i_size_read(inode);
+        if (iov_iter_rw(iter) == READ && offset >= dio->i_size) {
+                if (dio->flags & DIO_LOCKING)
+                        mutex_unlock(&inode->i_mutex);
+                kmem_cache_free(dio_cache, dio);
+                goto out;
+        }
+
         /*
          * For file extending writes updating i_size before data writeouts
          * complete can expose uninitialized blocks in dumb filesystems.
@@ -1222,7 +1231,6 @@ do_blockdev_direct_IO(struct kiocb *iocb, struct inode *inode,
         sdio.next_block_for_io = -1;
 
         dio->iocb = iocb;
-        dio->i_size = i_size_read(inode);
 
         spin_lock_init(&dio->bio_lock);
         dio->refcount = 1;