fs: update atime before I/O in generic_file_read_iter

After the call to ->direct_IO the final reference to the file might have
been dropped by aio_complete already, and the call to file_accessed might
cause a use after free.

Instead update the access time before the I/O, similar to how we
update the time stamps before writes.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

mm/filemap.c

@@ -1910,16 +1910,18 @@ generic_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 	if (iocb->ki_flags & IOCB_DIRECT) {
 		struct address_space *mapping = file->f_mapping;
 		struct inode *inode = mapping->host;
+		struct iov_iter data = *iter;
 		loff_t size;
 
 		size = i_size_read(inode);
 		retval = filemap_write_and_wait_range(mapping, iocb->ki_pos,
 					iocb->ki_pos + count - 1);
-		if (!retval) {
-			struct iov_iter data = *iter;
-			retval = mapping->a_ops->direct_IO(iocb, &data);
-		}
+		if (retval < 0)
+			goto out;
 
+		file_accessed(file);
+
+		retval = mapping->a_ops->direct_IO(iocb, &data);
 		if (retval > 0) {
 			iocb->ki_pos += retval;
 			iov_iter_advance(iter, retval);
@@ -1935,10 +1937,8 @@ generic_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 		 * DAX files, so don't bother trying.
 		 */
 		if (retval < 0 || !iov_iter_count(iter) || iocb->ki_pos >= size ||
-		    IS_DAX(inode)) {
-			file_accessed(file);
+		    IS_DAX(inode))
 			goto out;
-		}
 	}
 
 	retval = do_generic_file_read(file, &iocb->ki_pos, iter, retval);