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study/linux-travel/MINIX-1.5/1.5/Source/commands/de/README
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study/linux-travel/MINIX-1.5/1.5/Source/commands/de/README
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de - A Minix Disk Editor
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Terrence W. Holm, Jan. 1989
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INTRODUCTION
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The de(1) disk editor allows a system administrator to
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look at and modify a Minix file system device. Commands
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allow movement throughout a file system device, displaying
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information in a couple of formats, writing blocks from
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the device onto another file, and rewriting words on the
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disk.
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A few changes to the Minix file system aid recovering files.
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I-node numbers are retained in directory entries now (they
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get moved to the end). And all the i-node information is not
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zeroed-out when a file is unlinked. So, after a file is
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accidently rm(1)'ed, you can find the old i-node, and then
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manually (or automatically) go to each of the freed blocks
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and write them to a new file.
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USES FOR THE DISK EDITOR
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1) EDUCATION. Students can look at a file system in
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a painless manner. For example you don't have to
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use od(1) to look at the zone numbers in i-nodes.
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A simple assignment is to change the size of an un-mounted
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floppy disk file system from 360 to 300 blocks. (A more
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difficult assignment is to explain why this works, even
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though fsck(1) and df(1) do not report the correct number
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of free blocks. :-)
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2) ADMINISTRATION. You can visually check inconsistencies
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reported by fsck(1) before letting fsck(1) fix them.
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You can change any word on the disk, this greatly simplifies
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editing file system information. For example, changing the
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size of a block special device is actually fun, no more
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"blind" writing to your partitions.
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Bit maps can be displayed with 2048 "bits" per screen,
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(on the IBM/PC console), see how your zones are allocated!
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3) RECOVERING LOST FILES. You can search a disk for an ASCII
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string, once found, the block can be written out to a file.
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A one line change to fs/path.c allows users to get the i-node
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number for a file after it has been removed from a directory.
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Another couple lines changed in the file system keep the
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i-node information available until the i-node is reused
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(normally this information is zeroed out when an i-node is
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released.) This allows a de(1) user to go to a released
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i-node, get all the block numbers, go to these blocks and
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write them back to a new file.
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The whole recovery process is automated by running "de -r file".
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So, IF a file is unlink(2)'ed (eg. "rm file"), AND IF no one
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allocates a new i-node or block in the mean-time, THEN you
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can recover the file.
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RECOVERY SECURITY
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Normally Minix hard disk partitions are r/w only by the super-user,
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and floppy disks are r/w by anyone. This means that only "root"
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can look at hard disk partitions, but others can use de(1) to play
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with their floppy disks.
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When recovering files ("de -r file"), a user requires access to
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the major file system partitions. This can be done by:
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(a) Give everyone access to the hard disks. DON'T DO THIS, it
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defeats all the file system protection we already have.
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(b) Make de(1) set-uid "root". This is the way to go, IF you
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are running a Minix system that has NO ACCESS from the
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outside. This allows anyone to execute "de -r file", but only
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root to use "de /dev/hd3". De(1) does some checking when
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retrieving lost blocks, eg. making sure they really are
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free blocks and making sure the user owned the i-node.
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BUT, file system information has been lost when the file
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was unlink(2)'ed, so de(1) can not be 100% sure that a
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recovered block really belonged to the user. THIS IS A
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SECURITY HOLE. [Since the only access to my machine is from
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observable terminals and their associated humans, I run
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de(1) as set-uid root.]
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(c) Keep the disks rw-------, and don't set-uid de(1). This
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means that only the super-user can recover lost files.
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So, if you accidently "rm", you must tell the system
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administrator to "su" and recover your file, (be sure to
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inform the other users to stop whatever they are doing
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until the file is restored).
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INSTALLATION
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- Install de.1 in /usr/man/cat1.
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- Install the files: Makefile, README, de.h, de.c, de_stdin.c,
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de_stdout.c, de_diskio.c and de_recover.c in commands/de.
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Add -F and -T. to the Makefile, if necessary.
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- "make" de(1). If a header file is not found, don't worry:
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You probably have it somewhere, just link it to what de(1)
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is looking for. This program also requires the subroutine
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tolower(3), see EFTH MINIX report #50, if you don't have it.
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- Do you really want set-uid root on de?
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- Patch the files fs/path.c, fs/link.c and fs/open.c. If
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you don't patch the file system then the recover option
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"-r" and associated commands ('x' and 'X') will not work,
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but de(1) is still functional and useful.
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- "make" a new fs, using -DRECOVER. Rebuild a boot diskette.
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USING DE(1) FOR THE FIRST TIME
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De(1) starts up in "word" mode at block 0 of the specified
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device. Hit the PGDN (or space bar) a few times, observing
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all the information on the screen. Each PGUP/PGDN moves to
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the next 1024 byte block, (de(1) only knows about 1 block per
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zone file systems). Note that "word" mode only displays 32
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bytes at a time, so you are only observing the first 32 bytes
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in the first few blocks when you skip using PGDN.
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Now go back to block 3, (zone bit map), using "g 3 ENTER".
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Change to "map" mode "v m", and then use the down arrow key
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to check each 2 Megs in the zone bit map.
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Now change to "block" mode using "v b". And go to some data
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block, eg. "g 1000 ENTER". Use PGUP/PGDN to see what data
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is in each nearby block.
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Remember 'h' gives you a help page.
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Try some more commands, for example: 'END', 'I', '/'.
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(Note: searching through a whole disk under Minix takes a
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long time: 30-60 seconds per megabyte, depending on your
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machine, drive and controller, [Minix is embarrassingly slow].)
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Don't worry about looking at a mounted device, you must specify
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the "-w" option before the 's' command is operational, and
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this command is the only one which will try to modify the
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contents of the device.
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MINIX-ST
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Please contact me if you are interesting in attempting a port
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to MINIX-ST.
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