Bug#831823: cowbuilder: doesn't copy on write when using eatmydata

[Emilio Pozuelo Monfort]

Package: cowbuilder
Version: 0.80
Severity: grave
Tags: security

I enabled eatmydata by adding EATMYDATA=eatmydata to my ~/.pbuilderrc,
and the result is that copy-on-write no longer works, making any modifications
persistent:

eatmydata enabled

emilio at tatooine:~$ sudo cowbuilder --login 
root at tatooine:/# echo asdf > /etc/apt/sources.list
root at tatooine:/# logout
emilio at tatooine:~$ sudo cowbuilder --login 
root at tatooine:/# cat /etc/apt/sources.list
asdf

Note how it has overwritten sources.list even though --save-after-login
wasn't passed. It also overwrites files during a package build, which
is a big problem.

Now, with eatmydata disabled:

First, restore sources.list:

emilio at tatooine:~$ sudo cowbuilder --login --save-after-login
root at tatooine:/# cat /etc/apt/sources.list
asdf
root at tatooine:/# echo "deb http://ftp.es.debian.org/debian/ unstable main" > /etc/apt/sources.list
root at tatooine:/# logout

It is restored. Now, let's try to overwrite it without --save-after-login:

emilio at tatooine:~$ sudo cowbuilder --login 
root at tatooine:/# cat /etc/apt/sources.list
deb http://ftp.es.debian.org/debian/ unstable main
root at tatooine:/# echo asdf > /etc/apt/sources.list
root at tatooine:/# logout
emilio at tatooine:~$ sudo cowbuilder --login
root at tatooine:/# cat /etc/apt/sources.list
deb http://ftp.es.debian.org/debian/ unstable main
root at tatooine:/# 

It isn't overwritten.

Cheers,
Emilio

-- System Information:
Debian Release: stretch/sid
  APT prefers unstable
  APT policy: (800, 'unstable'), (700, 'experimental'), (650, 'testing'), (500, 'unstable-debug')
Architecture: amd64 (x86_64)
Foreign Architectures: i386, armhf

Kernel: Linux 4.6.0-1-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_GB.utf8, LC_CTYPE=en_GB.utf8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages cowbuilder depends on:
ii  cowdancer  0.80
ii  libc6      2.23-1
ii  pbuilder   0.225.1

cowbuilder recommends no packages.

cowbuilder suggests no packages.

-- no debconf information