Bug#494001: debian-installer: /etc/mtab must be a symlink to /proc/mounts with linux >= 2.6.26

[Roger Leigh <rleigh@debian.org>]

Package: debian-installer
Severity: important

/etc/mtab can be either a regular file updated by mount/umount, or a symlink 
to /proc/mounts.  Currently, it is a regular file, though the user can change 
this by hand.

With linux < 2.6.26, /proc/mounts lacks information present in /etc/mtab such 
as additional mount options.  Thus a symlink breaks things like discquotas 
which rely on parsing the additional mount options.  As a result, we are 
mostly all still using it as a plain file.

With linux >= 2.6.26, /proc/mounts contains all of the information in 
/etc/mtab, plus more.  The mount system call can now pass all of the mount 
options to the kernel, so no information is missing in /proc/mounts.  This 
has obviously useful benefits such as read-only root, and the state in 
/etc/mtab never gets out of sync with reality (there are a number of open 
bugs against mount where this occurs).

Additionally, with the addition of per-process namespaces with CLONE_NEWNS to 
clone(2), each process has its own set of mounts, and as such a system-wide 
/etc/mtab is useless: it's only valid in one of the potentially many 
namespaces and can quickly get into a horrible mess.  At this point, 
/etc/mtab *must* be a symlink to avoid breakage.  Note that /proc/mounts is 
now a symlink to /proc/self/mounts for this reason: each process has
potentially different mounts.


After discussion on #debian-devel, we came up with these points:
- we could detect the kernel version on boot, and set up a file or a symlink 
  as needed.  However, this breaks read-only root.
- we could change on upgrades rather than boot, but because it's dependent 
  upon the kernel version, breakage could result if an older kernel is
  booted.
- doing it at install time if a kernel >= 2.6.26 is installed is the most 
  robust solution.


I had a look at a number of sources (debootstrap, base-files, 
debian-installer), but I have yet to find the code that initially creates 
/etc/mtab (if any).  Some of the init scripts will re-create it if not 
present (and these could do with updating too for Lenny+1), but ideally if 
d-i installs a kernel >= 2.6.26, it should just ensure that /etc/mtab is a 
symlink during the install.

If we could get this change made for Lenny, that would be great.


Thanks for your time,
Roger

-- System Information:
Debian Release: lenny/sid
  APT prefers testing
  APT policy: (500, 'testing')
Architecture: i386 (i686)

Kernel: Linux 2.6.20.3-bytemark-uml-2
Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/bash