I created this a while ago; it's a trivial change from
32vfs.c and you might want to (somehow) fold it into 32vfs.
I'm pretty sure I needed it to read a 4.1 file system and 32vfs
wouldn't do the job.

Notes:
Sun Feb 26 09:59:09 EST 2006 rsc
    i added a -b option for 32vfs


Reference: /n/sources/patch/applied/tapefs-41bsdfs
Date: Sun Feb 26 01:37:08 CET 2006
Reviewed-by: rsc

--- /sys/src/cmd/tapefs/41bsdfs.c	Thu Jan  1 00:00:00 1970
+++ /sys/src/cmd/tapefs/41bsdfs.c	Sun Feb 26 01:35:32 2006
@@ -0,0 +1,208 @@
+/*
+ * 4.1BSD VAX Unix filesystem (derived from 32vfs.c)
+ */
+#include <u.h>
+#include <libc.h>
+#include <auth.h>
+#include <fcall.h>
+#include "tapefs.h"
+
+/*
+ * v32 disk inode
+ */
+#define	VNADDR	13
+#define	VFMT	0160000
+#define	VIFREG	0100000
+#define	VIFDIR	0040000
+#define	VIFCHR	0120000
+#define	VIFBLK	0160000
+#define	VMODE	0777
+#define	VSUPERB	1
+#define	VROOT		2	/* root inode */
+#define	VNAMELEN	14
+#define	BLSIZE	1024		/* in theory could have been 512 or 2048 */
+#define	LINOPB	(BLSIZE/sizeof(struct v32dinode))
+#define	LNINDIR	(BLSIZE/sizeof(unsigned long))
+
+struct v32dinode {
+	unsigned char flags[2];
+	unsigned char nlinks[2];
+	unsigned char uid[2];
+	unsigned char gid[2];
+	unsigned char size[4];
+	unsigned char addr[40];
+	unsigned char atime[4];
+	unsigned char mtime[4];
+	unsigned char ctime[4];
+};
+
+struct	v32dir {
+	uchar	ino[2];
+	char	name[VNAMELEN];
+};
+
+int	tapefile;
+Fileinf	iget(int ino);
+long	bmap(Ram *r, long bno);
+void	getblk(Ram *r, long bno, char *buf);
+
+void
+populate(char *name)
+{
+	Fileinf f;
+
+	replete = 0;
+	tapefile = open(name, OREAD);
+	if (tapefile<0)
+		error("Can't open argument file");
+	f = iget(VROOT);
+	ram->perm = f.mode;
+	ram->mtime = f.mdate;
+	ram->addr = f.addr;
+	ram->data = f.data;
+	ram->ndata = f.size;
+}
+
+void
+popdir(Ram *r)
+{
+	int i, ino;
+	char *cp;
+	struct v32dir *dp;
+	Fileinf f;
+	char name[VNAMELEN+1];
+
+	cp = 0;
+	for (i=0; i<r->ndata; i+=sizeof(struct v32dir)) {
+		if (i%BLSIZE==0)
+			cp = doread(r, i, BLSIZE);
+		dp = (struct v32dir *)(cp+i%BLSIZE);
+		ino = g2byte(dp->ino);
+		if (strcmp(dp->name, ".")==0 || strcmp(dp->name, "..")==0)
+			continue;
+		if (ino==0)
+			continue;
+		f = iget(ino);
+		strncpy(name, dp->name, VNAMELEN);
+		name[VNAMELEN+1] = '\0';
+		f.name = name;
+		popfile(r, f);
+	}
+	r->replete = 1;
+}
+
+void
+dotrunc(Ram *r)
+{
+	USED(r);
+}
+
+void
+docreate(Ram *r)
+{
+	USED(r);
+}
+
+char *
+doread(Ram *r, vlong off, long cnt)
+{
+	static char buf[Maxbuf+BLSIZE];
+	int bno, i;
+
+	bno = off/BLSIZE;
+	off -= bno*BLSIZE;
+	if (cnt>Maxbuf)
+		error("count too large");
+	if (off)
+		cnt += off;
+	i = 0;
+	while (cnt>0) {
+		getblk(r, bno, &buf[i*BLSIZE]);
+		cnt -= BLSIZE;
+		bno++;
+		i++;
+	}
+	return buf;
+}
+
+void
+dowrite(Ram *r, char *buf, long off, long cnt)
+{
+	USED(r); USED(buf); USED(off); USED(cnt);
+}
+
+int
+dopermw(Ram *r)
+{
+	USED(r);
+	return 0;
+}
+
+/*
+ * fetch an i-node
+ * -- no sanity check for now
+ * -- magic inode-to-disk-block stuff here
+ */
+
+Fileinf
+iget(int ino)
+{
+	char buf[BLSIZE];
+	struct v32dinode *dp;
+	long flags, i;
+	Fileinf f;
+
+	seek(tapefile, BLSIZE*((ino-1)/LINOPB + VSUPERB + 1), 0);
+	if (read(tapefile, buf, BLSIZE) != BLSIZE)
+		error("Can't read inode");
+	dp = ((struct v32dinode *)buf) + ((ino-1)%LINOPB);
+	flags = g2byte(dp->flags);
+	f.size = g4byte(dp->size);
+	if ((flags&VFMT)==VIFCHR || (flags&VFMT)==VIFBLK)
+		f.size = 0;
+	f.data = emalloc(VNADDR*sizeof(long));
+	for (i = 0; i < VNADDR; i++)
+		((long*)f.data)[i] = g3byte(dp->addr+3*i);
+	f.mode = flags & VMODE;
+	if ((flags&VFMT)==VIFDIR)
+		f.mode |= DMDIR;
+	f.uid = g2byte(dp->uid);
+	f.gid = g2byte(dp->gid);	
+	f.mdate = g4byte(dp->mtime);
+	return f;
+}
+
+void
+getblk(Ram *r, long bno, char *buf)
+{
+	long dbno;
+
+	if ((dbno = bmap(r, bno)) == 0) {
+		memset(buf, 0, BLSIZE);
+		return;
+	}
+	seek(tapefile, dbno*BLSIZE, 0);
+	if (read(tapefile, buf, BLSIZE) != BLSIZE)
+		error("bad read");
+}
+
+/*
+ * logical to physical block
+ * only singly-indirect files for now
+ */
+
+long
+bmap(Ram *r, long bno)
+{
+	unsigned char indbuf[LNINDIR][sizeof(long)];
+
+	if (bno < VNADDR-3)
+		return ((long*)r->data)[bno];
+	if (bno < VNADDR*LNINDIR) {
+		seek(tapefile, ((long *)r->data)[(bno-(VNADDR-3))/LNINDIR]*BLSIZE, 0);
+		if (read(tapefile, (char *)indbuf, BLSIZE) != BLSIZE)
+			return 0;
+		return ((indbuf[bno%LNINDIR][1]<<8) + indbuf[bno%LNINDIR][0]);
+	}
+	return 0;
+}