chmod -x chmod José Castro
<cog@cpan.org> August 2010

During 2009 we posed this
problem to several of our candidates

You’re in a Data Center

With absolutely no contact with
the outside world

There’s a machine you must
not reboot

And someone had the brilliant
idea of performing a `chmod -x chmod`

Solve the problem

The following is a list
of possible solutions proposed by my co-workers

If the package is in
cache, reinstall it

On Debian: sudo apt-get install
--reinstall coreutils

Use a language that implements
chmod

Perl perl -e ‘chmod 0755,
“chmod”’

Python python -c "import os;os.chmod('/bin/chmod',
0777)"

d un teste Node.js require("fs").chmodSync("/bin/chmod",
0755);

Use existing executables or create
your own

$ cat - > chmod.c

$ cat - > chmod.c
int main () { } ^D

$ cat - > chmod.c
int main () { } ^D $ cc chmod.c

$ cat - > chmod.c
int main () { } ^D $ cc chmod.c $ cat /bin/chmod > a.out

$ cp cat new_chmod $
cat chmod > new_chmod

$ cat - > restore_chmod.c

$ cat - > restore_chmod.c
#include <sys/types.h> #include <sys/stat.h> int main () { chmod( "/bin/chmod", 0000777 ); } ^D

$ cat - > restore_chmod.c
#include <sys/types.h> #include <sys/stat.h> int main () { chmod( "/bin/chmod", 0000777 ); } ^D $ cc restore_chmod.c

$ cat - > restore_chmod.c
#include <sys/types.h> #include <sys/stat.h> int main () { chmod( "/bin/chmod", 0000777 ); } ^D $ cc restore_chmod.c $ ./a.out

launch BusyBox (it has a
chmod inside)

GNU tar

$ tar --mode 0777 -cf
chmod.tar /bin/chmod $ tar xvf chmod.tar

tar --mode 555 -cvf -
chmod | tar xvf -

d un teste $ tar
-cvf chmod.tar chmod edit the archive and alter the permissions

“You said I couldn’t go
to the internet...

“You said I couldn’t go
to the internet... but you said nothing about the other machines on the data center...”

Open a socket to another
machine and do a: $ tar --preserve-permissions -cf chmod.tar chmod

Open a socket to another
machine and do a: $ tar --preserve-permissions -cf chmod.tar chmod Get this tar to your machine and: $ tar xvf chmod.tar

cpio

cpio lets you copy files
to and from archives

bytes 19 to 24 are
the file mode (http://4bxf.sl.pt)

echo chmod | cpio -o
| perl -pe 's/^(.{21}).../${1}755/' | cpio -i -u

Hardcore

alias chmod='/lib/ld-2.11.1.so ./chmod'

d un teste • attrib
or ls -@ • force the inode into cache • check kcore for the VFS structures • use sed to alter the execution bit without the kernel realizing it • run chmod +x chmod

Text editors sometimes need to
overwrite a file

Thus, some of them have
something resembling chmod

Emacs

Ctrl+x b > *scratch* (set-file-modes
"/bin/chmod" (string-to-number "0755" 8)) Ctrl+j

There seem to be countless
solutions

But one of the best
answers I’ve seen...

Was from a guy who
replied to my “Solve the problem” with...

“What problem? Isn’t the machine
still running?”

The End
(for now)