static journal_t *ext4_get_journal(struct super_block *sb,
				   unsigned int journal_inum)
{
	struct inode *journal_inode;
	journal_t *journal;

	BUG_ON(!ext4_has_feature_journal(sb));

	journal_inode = ext4_get_journal_inode(sb, journal_inum);
	if (!journal_inode)
		return NULL;

	journal = jbd2_journal_init_inode(journal_inode);
	if (!journal) {
		ext4_msg(sb, KERN_ERR, "Could not load journal inode");
		iput(journal_inode);
		return NULL;
	}
	journal->j_private = sb;
	ext4_init_journal_params(sb, journal);
	return journal;
}

The product reads data past the end, or before the beginning, of the intended buffer.