Comment 7 for bug 201673

If you don't have a root password, a solution that I used to downgrade the package which involves a reboot follows:

- If you have upgraded, but didn't reboot yet, get the older version from your favorite mirror and save it to somewhere you know. An example link is http://us.archive.ubuntu.com/ubuntu/pool/main/g/glibc/libc6_2.7-5ubuntu2_i386.deb
- If you can't launch a browser for whatever reason, use alt+f2, type this: uxterm -e /bin/dash and use links to download.
- reboot.
- on the grub, select your favorite kernel, hit the 'e' key, select the line starting with 'kernel', change the 'ro' word on the line to 'rw' and add 'init=/bin/dash' to the end.
- wait for 10 seconds, nothing will be displayed when the system is booted into a root shell. try 'ls' to see whether it's done.
- cd /to/the/path/you/downloaded/the/package and use dpkg to downgrade: 'dpkg -i libc6_2.7-5ubuntu2_i386.deb'
- type 'sync' to make sure that everything gets written for sure
- hit ctrl+alt+del to reboot. you should be set now.

-HTH