Below are two diffs for squashfs-4 (as found in 2.6.29
and 2.6.30 kernels) to kernel 2.6.27.

In order to build, grab fs/squashfs directory from 2.6.29 or
2.6.30 (or maybe later version), copy it to fs/squashfs in
2.6.27 tree, and apply 2 provided patches.

First (squashfs-2.6.29-to-.27-backport.diff) changes squashfs
itself to be compatible with kernel 2.6.27 API (namely, adding
magic number definition and implementing d_obtain_alias() which
is missing in 2.6.27.

And second changes 2.6.27 build infrastructure to actually
include squashfs filesystem -- init/do_mounts_rd.c, fs/Kconfig
and fs/Makefile.

JFYI, in case it will be useful to someone.

What I use it for is -- since 2.6.29+ does not work with version
3 of the filesystem layout, and 2.6.28- only recognizes v.3, and
since I'm tired updating kernels on all supported machines, I want
to stop for some time, relying on 2.6.27.y stable series.  But we
also use squashfs and some machines requires more modern kernel
features.  So I thought about updating squashfs to v4 on all.
So here it goes.

Thanks.

/mjt