chmod -x chmod

José Castro
José CastroTechnology Evangelist by Day, Evil Magician by Night at Codacy
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)
1 of 49

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chmod -x chmod