These are hacks against the macOS-modified versions of these FUSE filesystems.
This is not a general-purpose fork. This has ugly experimental hacks to allow loading an odd Plexus-specific SVR2 fs flavor (and to build on modern macOS). These hacks will break the ability to work with non-Plexus filesystems.
This is based primarily on this filesystem information from the <sys/filsys.h> header on the Plexus filesystem (note this is a version edited by me to note offsets and what's nonstandard... in particular the ADJUST to make sizeof filsys be 512 comment, which is original and not my edit, was not followed and that's partially why this suberblock format is non-standard... s_magic is in the wrong place and the order is also slightly wrong ):
/* SID @(#)filsys.h.1.1 */
/* @(#)filsys.h.6.1 */
/*
* Structure of the super-block
*/
struct filsys { // XXX: KI3V - //offset+size /* original comment */
ushort s_isize; //0+2 /* size in blocks of i-list */
daddr_t s_fsize; //2+4 /* size in blocks of entire volume */
short s_nfree; //6+2 /* number of addresses in s_free */
daddr_t s_free[NICFREE]; //8+4*50 /* free block list */
short s_ninode; //208+2 /* number of i-nodes in s_inode */
ino_t s_inode[NICINOD]; //210+2*100 /* free i-node list */
char s_flock; //410+1 /* lock during free list manipulation */
char s_ilock; //411+1 /* lock during i-list manipulation */
char s_fmod; //412+1 /* super block modified flag */
char s_ronly; //413+1 /* mounted read-only flag*/
time_t s_time; //414+4 /* last super block update */
short s_dinfo[4]; //418+2*4 /* device information */
daddr_t s_tfree; //426+4 /* total free blocks*/
ino_t s_tinode; //430+2 /* total free inodes */
char s_fname[6]; //432+6 /* file system name */
char s_fpack[6]; //438+6 /* file system pack name */
long s_fill[13]; //444+4*13 /* ADJUST to make sizeof filsys be 512 */
long s_magic; //496+4 /* magic number to indicate new file system */
// XXX: KI3V - WRONG! this should be at offset 504!
// XXX: looks like Plexus compiled with wrong struct packing
// XXX: or something?
long s_type; //500+4 /* type of new file system */
};
#define FsMAGI 0xfd187e20 /* s_magic number */
#define Fs1b 1 /* 512 byte block */
#define Fs2b 2 /* 1024 byte block */