I just found that I can't mount ULTRIX filesystems anymore. There is
some consistency check which uses a variable which is not initialized at
this point in case the ffs is an old one:

fs->fs_cssize > fragroundup(fs, fs->fs_ncg * sizeof(struct csum))

The fragroundup() macro uses the unintialized fs->fs_qfmask.
The check went in with that patch:

[ffs_vfsops.c]
revision 1.46
date: 1998/12/04 11:00:40;  author: bouyer;  state: Exp;  lines: +11 -5
Sanity check a few values in the superblock, to avoid mallocing huge
memory area if we try to mount a corrupted filesystem. Fixes kern/3933.

So - what do do about that? Obviously we have 2 choices:
a) move the check down
b) move the ffs_oldfscompat() call up

b looks easier for me, or am I missing something?

best regards
Matthias