xfs_attr_shortform_bytesfit(xfs_inode_t *dp, int bytes)
{
	int offset;
	int minforkoff;	/* lower limit on valid forkoff locations */
	int maxforkoff;	/* upper limit on valid forkoff locations */
	int dsize;
	xfs_mount_t *mp = dp->i_mount;

	/* rounded down */
	offset = (XFS_LITINO(mp, dp->i_d.di_version) - bytes) >> 3;

	if (dp->i_d.di_format == XFS_DINODE_FMT_DEV) {
		minforkoff = roundup(sizeof(xfs_dev_t), 8) >> 3;
		return (offset >= minforkoff) ? minforkoff : 0;
	}

	/*
	 * If the requested numbers of bytes is smaller or equal to the
	 * current attribute fork size we can always proceed.
	 *
	 * Note that if_bytes in the data fork might actually be larger than
	 * the current data fork size is due to delalloc extents. In that
	 * case either the extent count will go down when they are converted
	 * to real extents, or the delalloc conversion will take care of the
	 * literal area rebalancing.
	 */
	if (bytes <= XFS_IFORK_ASIZE(dp))
		return dp->i_d.di_forkoff;

	/*
	 * For attr2 we can try to move the forkoff if there is space in the
	 * literal area, but for the old format we are done if there is no
	 * space in the fixed attribute fork.
	 */
	if (!(mp->m_flags & XFS_MOUNT_ATTR2))
		return 0;

	dsize = dp->i_df.if_bytes;

	switch (dp->i_d.di_format) {
	case XFS_DINODE_FMT_EXTENTS:
		/*
		 * If there is no attr fork and the data fork is extents, 
		 * determine if creating the default attr fork will result
		 * in the extents form migrating to btree. If so, the
		 * minimum offset only needs to be the space required for
		 * the btree root.
		 */
		if (!dp->i_d.di_forkoff && dp->i_df.if_bytes >
		    xfs_default_attroffset(dp))
			dsize = XFS_BMDR_SPACE_CALC(MINDBTPTRS);
		break;
	case XFS_DINODE_FMT_BTREE:
		/*
		 * If we have a data btree then keep forkoff if we have one,
		 * otherwise we are adding a new attr, so then we set
		 * minforkoff to where the btree root can finish so we have
		 * plenty of room for attrs
		 */
		if (dp->i_d.di_forkoff) {
			if (offset < dp->i_d.di_forkoff)
				return 0;
			return dp->i_d.di_forkoff;
		}
		dsize = XFS_BMAP_BROOT_SPACE(mp, dp->i_df.if_broot);
		break;
	}

	/*
	 * A data fork btree root must have space for at least
	 * MINDBTPTRS key/ptr pairs if the data fork is small or empty.
	 */
	minforkoff = max(dsize, XFS_BMDR_SPACE_CALC(MINDBTPTRS));
	minforkoff = roundup(minforkoff, 8) >> 3;

	/* attr fork btree root can have at least this many key/ptr pairs */
	maxforkoff = XFS_LITINO(mp, dp->i_d.di_version) -
			XFS_BMDR_SPACE_CALC(MINABTPTRS);
	maxforkoff = maxforkoff >> 3;	/* rounded down */

	if (offset >= maxforkoff)
		return maxforkoff;
	if (offset >= minforkoff)
		return offset;
	return 0;
}

The product dereferences a pointer that it expects to be valid but is NULL.