Workaround: dpkg goes Segmentation Fault

So, your apt-get [dist-]upgrade stops when a post/pre-installation script goes in Segmentation Fault. The strange thing is that the same script, extracted from your favourite deb package, works if launched standalone. That's probably a debconf or dpkg bug but, frankly, I didn't even google to find if it's really a known bug: I get this problem quite often on my Debian testing (especially while upgrading libc6, tzdatam console_common), so I chose to spend my time looking for a workaround. And, finally, I found a very simple one.

That's, more or less, the error I get (note: my system is localized in italian):

(Lettura del database ... 184524 file e directory attualmente installati.)
Mi preparo a sostituire libc6 2.7-3 (con libc6 2.7-5) ...
Spacchetto il sostituto di libc6 ...
Configuro libc6 (2.7-5) ...
dpkg: errore processando libc6 (--install):
il sottoprocesso post-installation script è stato terminato dal segnale (Segmentation fault)
Sono occorsi degli errori processando:

In a nutshell, my solution is:
  • Unpack the .deb package causing the problem and grab the meta-files
  • Clear the pre/post-installation script (depending on which one causes the segmentation fault)
  • Pack a new .deb package with these fake scripts and install it
  • Run manually the scripts you modified (do this before the installation if it was a preinst or pre/post-rm)
  • Done: you can go on with your dist-upgrade
That's easy to do within a bash shell if you know the right commands. And they are:

# Make a copy of crashing package
cp /var/cache/apt/archives/crashing_pkg_x.y.z.deb ./crashing_package.deb

# Extracts package data
dpkg-deb -x crashing_package.deb ./temp_dir

# Extracts package meta-files
dpkg-deb -e crashing_package.deb ./temp_dir/DEBIAN

# Make fake post/pre installation scripts
echo "echo Fake" > ./temp_dir/DEBIAN/postinst

# Pack a new modified package
dpkg-deb -b ./temp_dir/ mod_package.deb

# Install the hacked package (should not go in segfault)
dpkg -i mod_package.deb

# Manually run postinst script

# Clean
rm -rf ./temp_dir
rm crashing_package.deb mod_package.deb
Dirty and functional, as usual. But pay attention to the manual execution of scripts: they may crash, or refuse to go on, or they may contain debconf commands that bash can't execute; so my advice is: take a look inside and try to manually do, step by step, what the script was supposed to do (uninstalling packages, stopping processes, restarting daemons, upgrading configurations, and so on).

I guess you're experienced enough to change them to suit your needs (and, above all, I'm in a hurry now!). So, good luck and may the GNU-force be with you =)


Anonymous said...

Very helpful - worked like a charm. Thanks :)

Narcolessico said...

I'm glad it was helpful!

Did you have the same problem in Debian Testing (like me) or in another distro?

Unknown said...

libc6 update just failed on unstable, for some reason I had not thought of going berserk like that. Worked like a charm, thanks!

Post a Comment