textfiles/computers/rm-rf

Then it must be version-dependent.  On this Sun, "cp /bin/rm foo"
followed by "./foo foo" does not leave a foo behind, and strings
shows that rm appears not to call rmdir (which makes sense, as it
can just use unlink()).

In any case, I'm reminded of the following article.  This is a classic
which, like the story of Mel, has been on the net several times;
it was in this newsgroup in January.  It was first posted in 1986.

--------------------------------------------------------------------

Have you ever left your terminal logged in, only to find when you came
back to it that a (supposed) friend had typed "rm -rf ~/*" and was
hovering over the keyboard with threats along the lines of "lend me
a fiver 'til Thursday, or I hit return"?  Undoubtedly the person in
question would not have had the nerve to inflict such a trauma upon
you, and was doing it in jest.  So you've probably never experienced
the worst of such disasters....

It was a quiet Wednesday afternoon.  Wednesday, 1st October, 15:15
BST, to be precise, when Peter, an office-mate of mine, leaned away
from his terminal and said to me, "Mario, I'm having a little trouble
sending mail."  Knowing that msg was capable of confusing even the
most capable of people, I sauntered over to his terminal to see what
was wrong.  A strange error message of the form (I forget the exact
details) "cannot access /foo/bar for userid 147" had been issued by
msg.  My first thought was "Who's userid 147?; the sender of the
message, the destination, or what?"  So I leant over to another
terminal, already logged in, and typed
        grep 147 /etc/passwd
only to receive the response
        /etc/passwd: No such file or directory.

Instantly, I guessed that something was amiss.  This was confirmed
when in response to
        ls /etc
I got
        ls: not found.

I suggested to Peter that it would be a good idea not to try anything
for a while, and went off to find our system manager.

When I arrived at his office, his door was ajar, and within ten
seconds I realised what the problem was.  James, our manager, was
sat down, head in hands, hands between knees, as one whose world has
just come to an end.  Our newly-appointed system programmer, Neil,
was beside him, gazing listlessly at the screen of his terminal.
And at the top of the screen I spied the following lines:
        # cd
        # rm -rf *

Oh, shit, I thought.  That would just about explain it.

I can't remember what happened in the succeeding minutes; my memory is
just a blur.  I do remember trying ls (again), ps, who and maybe a few
other commands beside, all to no avail.  The next thing I remember was
being at my terminal again (a multi-window graphics terminal), and
typing
        cd /
        echo *
I owe a debt of thanks to David Korn for making echo a built-in of
his shell; needless to say, /bin, together with /bin/echo, had been
deleted.  What transpired in the next few minutes was that /dev,
/etc and /lib had also gone in their entirety; fortunately Neil had
interrupted rm while it was somewhere down below /news, and /tmp,
/usr and /users were all untouched.

Meanwhile James had made for our tape cupboard and had retrieved
what claimed to be a dump tape of the root filesystem, taken four
weeks earlier.  The pressing question was, "How do we recover the
contents of the tape?".  Not only had we lost /etc/restore, but all
of the device entries for the tape deck had vanished.  And where
does mknod live?  You guessed it, /etc.  How about recovery across
Ethernet of any of this from another VAX?  Well, /bin/tar had gone,
and thoughtfully the Berkeley people had put rcp in /bin in the
4.3 distribution.  What's more, none of the Ether stuff wanted to
know without /etc/hosts at least.  We found a version of cpio in
/usr/local, but that was unlikely to do us any good without a tape
deck.

Alternatively, we could get the boot tape out and rebuild the root
filesystem, but neither James nor Neil had done that before, and we
weren't sure that the first thing to happen would be that the whole
disk would be re-formatted, losing all our user files.  (We take dumps
of the user files every Thursday; by Murphy's Law this had to happen
on a Wednesday).  Another solution might be to borrow a disk from
another VAX, boot off that, and tidy up later, but that would have
entailed calling the DEC engineer out, at the very least.  We had a
number of users in the final throes of writing up PhD theses and the
loss of a maybe a weeks' work (not to mention the machine down time)
was unthinkable.

So, what to do?  The next idea was to write a program to make a device
descriptor for the tape deck, but we all know where cc, as and ld
live.  Or maybe make skeletal entries for /etc/passwd, /etc/hosts
and so on, so that /usr/bin/ftp would work.  By sheer luck, I had a
gnuemacs still running in one of my windows, which we could use to
create passwd, etc., but the first step was to create a directory to
put them in.  Of course /bin/mkdir had gone, and so had /bin/mv, so we
couldn't rename /tmp to /etc.  However, this looked like a reasonable
line of attack.

By now we had been joined by Alasdair, our resident UNIX guru, and
as luck would have it, someone who knows VAX assembler.  So our plan
became this: write a program in assembler which would either rename
/tmp to /etc, or make /etc, assemble it on another VAX, uuencode it,
type in the uuencoded file using my gnu, uudecode it (some bright
spark had thought to put uudecode in /usr/bin), run it, and hey
presto, it would all be plain sailing from there.  By yet another
miracle of good fortune, the terminal from which the damage had been
done was still su'd to root (su is in /bin, remember?), so at least
we stood a chance of all this working.

Off we set on our merry way, and within only an hour we had managed
to concoct the dozen or so lines of assembler to create /etc.  The
stripped binary was only 76 bytes long, so we converted it to hex
(slightly more readable than the output of uuencode), and typed it in
using my editor.  If any of you ever have the same problem, here's the
hex for future reference:
        070100002c000000000000000000000000000000000000000000000000000000
        0000dd8fff010000dd8f27000000fb02ef07000000fb01ef070000000000bc8f
        8800040000bc012f65746300

I had a handy program around (doesn't everybody?) for converting
ASCII hex to binary, and the output of /usr/bin/sum tallied with our
original binary.  But hang on---how do you set execute permission
without /bin/chmod?  A few seconds thought (which as usual, lasted a
couple of minutes) suggested that we write the binary on top of an
already existing binary, owned by me...problem solved.

So along we trotted to the terminal with the root login, carefully
remembered to set the umask to 0 (so that I could create files in it
using my gnu), and ran the binary.  So now we had a /etc, writable
by all.  From there it was but a few easy steps to creating passwd,
hosts, services, protocols, (etc), and then ftp was willing to play
ball.  Then we recovered the contents of /bin across the ether (it's
amazing how much you come to miss ls after just a few, short hours),
and selected files from /etc.  The key file was /etc/rrestore, with
which we recovered /dev from the dump tape, and the rest is history.

Now, you're asking yourself (as I am), what's the moral of this story?
Well, for one thing, you must always remember the immortal words,
DON'T PANIC.  Our initial reaction was to reboot the machine and try
everything as single user, but it's unlikely it would have come up
without /etc/init and /bin/sh.  Rational thought saved us from this
one.

The next thing to remember is that UNIX tools really can be put to
unusual purposes.  Even without my gnuemacs, we could have survived
by using, say, /usr/bin/grep as a substitute for /bin/cat.

And the final thing is, it's amazing how much of the system you can
delete without it falling apart completely.  Apart from the fact that
nobody could login (/bin/login?), and most of the useful commands
had gone, everything else seemed normal.  Of course, some things can't
stand life without say /etc/termcap, or /dev/kmem, or /etc/utmp, but
by and large it all hangs together.

I shall leave you with this question: if you were placed in the
same situation, and had the presence of mind that always comes with
hindsight, could you have got out of it in a simpler or easier way?
Answers on a postage stamp to:

Mario Wolczko
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Dept. of Computer Science       ARPA:   miw%uk.ac.man.cs.ux@cs.ucl.ac.uk
The University                  USENET: mcvax!ukc!man.cs.ux!miw
Manchester M13 9PL              JANET:  miw@uk.ac.man.cs.ux
U.K.                            061-273 7121 x 5699
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