1 de - A Minix Disk Editor
2
3 Terrence W. Holm, Jan. 1989
4
5
6 INTRODUCTION
7
8 The de(1) disk editor allows a system administrator to
9 look at and modify a Minix file system device. Commands
10 allow movement throughout a file system device, displaying
11 information in a couple of formats, writing blocks from
12 the device onto another file, and rewriting words on the
13 disk.
14
15 A few changes to the Minix file system aid recovering files.
16 I-node numbers are retained in directory entries now (they
17 get moved to the end). And all the i-node information is not
18 zeroed-out when a file is unlinked. So, after a file is
19 accidently rm(1)'ed, you can find the old i-node, and then
20 manually (or automatically) go to each of the freed blocks
21 and write them to a new file.
22
23
24 USES FOR THE DISK EDITOR
25
26 1) EDUCATION. Students can look at a file system in
27 a painless manner. For example you don't have to
28 use od(1) to look at the zone numbers in i-nodes.
29
30 A simple assignment is to change the size of an un-mounted
31 floppy disk file system from 360 to 300 blocks. (A more
32 difficult assignment is to explain why this works, even
33 though fsck(1) and df(1) do not report the correct number
34 of free blocks. :-)
35
36 2) ADMINISTRATION. You can visually check inconsistencies
37 reported by fsck(1) before letting fsck(1) fix them.
38 You can change any word on the disk, this greatly simplifies
39 editing file system information. For example, changing the
40 size of a block special device is actually fun, no more
41 "blind" writing to your partitions.
42
43 Bit maps can be displayed with 2048 "bits" per screen,
44 (on the IBM/PC console), see how your zones are allocated!
45
46 3) RECOVERING LOST FILES. You can search a disk for an ASCII
47 string, once found, the block can be written out to a file.
48
49 A one line change to fs/path.c allows users to get the i-node
50 number for a file after it has been removed from a directory.
51
52 Another couple lines changed in the file system keep the
53 i-node information available until the i-node is reused
54 (normally this information is zeroed out when an i-node is
55 released.) This allows a de(1) user to go to a released
56 i-node, get all the block numbers, go to these blocks and
57 write them back to a new file.
58
59 The whole recovery process is automated by running "de -r file".
60 So, IF a file is unlink(2)'ed (eg. "rm file"), AND IF no one
61 allocates a new i-node or block in the mean-time, THEN you
62 can recover the file.
63
64
65 RECOVERY SECURITY
66
67 Normally Minix hard disk partitions are r/w only by the super-user,
68 and floppy disks are r/w by anyone. This means that only "root"
69 can look at hard disk partitions, but others can use de(1) to play
70 with their floppy disks.
71
72 When recovering files ("de -r file"), a user requires access to
73 the major file system partitions. This can be done by:
74
75 (a) Give everyone access to the hard disks. DON'T DO THIS, it
76 defeats all the file system protection we already have.
77
78 (b) Make de(1) set-uid "root". This is the way to go, IF you
79 are running a Minix system that has NO ACCESS from the
80 outside. This allows anyone to execute "de -r file", but only
81 root to use "de /dev/hd3". De(1) does some checking when
82 retrieving lost blocks, eg. making sure they really are
83 free blocks and making sure the user owned the i-node.
84 BUT, file system information has been lost when the file
85 was unlink(2)'ed, so de(1) can not be 100% sure that a
86 recovered block really belonged to the user. THIS IS A
87 SECURITY HOLE. [Since the only access to my machine is from
88 observable terminals and their associated humans, I run
89 de(1) as set-uid root.]
90
91 (c) Keep the disks rw-------, and don't set-uid de(1). This
92 means that only the super-user can recover lost files.
93 So, if you accidently "rm", you must tell the system
94 administrator to "su" and recover your file, (be sure to
95 inform the other users to stop whatever they are doing
96 until the file is restored).
97
98
99 INSTALLATION
100
101 - Install de.1 in /usr/man/cat1.
102
103 - Install the files: Makefile, README, de.h, de.c, de_stdin.c,
104 de_stdout.c, de_diskio.c and de_recover.c in commands/de.
105 Add -F and -T. to the Makefile, if necessary.
106
107 - "make" de(1). If a header file is not found, don't worry:
108 You probably have it somewhere, just link it to what de(1)
109 is looking for. This program also requires the subroutine
110 tolower(3), see EFTH MINIX report #50, if you don't have it.
111
112 - Do you really want set-uid root on de?
113
114 - Patch the files fs/path.c, fs/link.c and fs/open.c. If
115 you don't patch the file system then the recover option
116 "-r" and associated commands ('x' and 'X') will not work,
117 but de(1) is still functional and useful.
118
119 - "make" a new fs, using -DRECOVER. Rebuild a boot diskette.
120
121
122 USING DE(1) FOR THE FIRST TIME
123
124 De(1) starts up in "word" mode at block 0 of the specified
125 device. Hit the PGDN (or space bar) a few times, observing
126 all the information on the screen. Each PGUP/PGDN moves to
127 the next 1024 byte block, (de(1) only knows about 1 block per
128 zone file systems). Note that "word" mode only displays 32
129 bytes at a time, so you are only observing the first 32 bytes
130 in the first few blocks when you skip using PGDN.
131
132 Now go back to block 3, (zone bit map), using "g 3 ENTER".
133 Change to "map" mode "v m", and then use the down arrow key
134 to check each 2 Megs in the zone bit map.
135
136 Now change to "block" mode using "v b". And go to some data
137 block, eg. "g 1000 ENTER". Use PGUP/PGDN to see what data
138 is in each nearby block.
139
140 Remember 'h' gives you a help page.
141
142 Try some more commands, for example: 'END', 'I', '/'.
143 (Note: searching through a whole disk under Minix takes a
144 long time: 30-60 seconds per megabyte, depending on your
145 machine, drive and controller, [Minix is embarrassingly slow].)
146
147 Don't worry about looking at a mounted device, you must specify
148 the "-w" option before the 's' command is operational, and
149 this command is the only one which will try to modify the
150 contents of the device.
151
152
153 MINIX-ST
154
155 Please contact me if you are interesting in attempting a port
156 to MINIX-ST.
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