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

2. During 2009
we posed this problem
to several of our candidates

3. You’re in a Data Center

4. With absolutely
no contact
with the outside world

5. There’s a machine
you must not reboot

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

7. Solve the problem

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

9. If the package is in
cache, reinstall it

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

11. Use a language
that implements chmod

12. Perl
perl -e ‘chmod 0755, “chmod”’

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

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

15. Use existing executables
or create your own

16. $ cat - > chmod.c

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

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

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

20. $ cp cat new_chmod
$ cat chmod > new_chmod

21. $ cat - > restore_chmod.c

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

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

24. $ 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

25. launch BusyBox
(it has a chmod inside)

26. GNU tar

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

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

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

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

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

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

33. 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

34. cpio

35. cpio lets you
copy files
to and from archives

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

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

38. Hardcore

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

40. 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

41. Text editors
sometimes need
to overwrite a file

42. Thus, some of them
have something
resembling chmod

43. Emacs

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

45. There seem to be
countless solutions

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

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

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

49. The End
(for now)