chmod -x chmod

José Castro <cog@cpan.org>August 2010

During 2009we posed this problem

to several of our candidates

You’re in a Data Center

With absolutelyno contact

with the outside world

There’s a machineyou must not reboot

And someone hadthe brilliant ideaof performing a

`chmod -x chmod`

Solve the problem

The following is a listof 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 languagethat implements chmod

perl -e ‘chmod 0755, “chmod”’

Perl

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

Python

Node.js

require("fs").chmodSync("/bin/chmod", 0755);

untested

Use existing executablesor 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 -

$ tar -cvf chmod.tar chmod

edit the archive and alter the permissions

untested

“You said I couldn’tgo to the internet...

“You said I couldn’tgo to the internet...

but you said nothing aboutthe 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 youcopy files

to and from archives

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

echo chmod | -o |

cpio -i -u

cpio 21 ... 755perl -pe 's/^(.{ }) /${1} /' |

Hardcore

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

• 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

untested

Text editorssometimes need

to overwrite a file

Thus, some of themhave 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)