BUG: unable to handle kernel NULL pointer dereference with unionfs and kernel 2.6.21.5

[Rene Mayrhofer <rene.mayrhofer@gibraltar.at>]

Hi lists(s),

[Please CC me in replies to LKML, I am not subscribed to this list.]
[Sorry for the cross-post, but I'm not sure that this is only a unionfs issue, 
so any hints are appreciated.]

I have a problem with unionfs 2.6.21.5-u3 more or less crashing my system upon 
the first write attempt (to a tmpfs file system mounted over a read-only tree 
from a squashfs image that is mounted loopback as the root filesystem). 
Attached is the "dmesg" output from bootup to the BUG (which happens 
immediately). After that, only a hard reboot will make anything happen 
anymore...

The kernel is based on a vanilla 2.6.21.5 from k.o with patches in the 
following order:
- pax-linux-2.6.21.5-test9.patch
- squashfs 3.2-r2: linux-2.6.20/squashfs3.2-patch
- linux-2.6.21-imq2.diff
- linux-2.6.21.5-u3.diff.gz (unionfs)
- netfilter-layer7-v2.9.tar.gz
- routes-2.6.21-15.diff
and a very few select pom-ng patches.

I don't think that the IMQ, routes, layer7, or pom-ng patches can have 
anything to do with it, and the squashfs patch has been working for ages on 
different kernel versions. So my only suspicion so far is a conflict between 
the PAX patch and unionfs. Btw, the same combination of patches worked nicely 
when based on 2.6.20.x with the respective older patch versions.

I'd be happy about any hints on solving this.

Thanks,
Rene

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