Bug#494001: debian-installer: /etc/mtab must be a symlink to /proc/mounts with linux >= 2.6.26
/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
- doing it at install time if a kernel >= 2.6.26 is installed is the most
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,
-- System Information:
Debian Release: lenny/sid
APT prefers testing
APT policy: (500, 'testing')
Architecture: i386 (i686)
Kernel: Linux 22.214.171.124-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