RE: coreutils-7.6: "df" does not show filesystem mounted via mount(2)

[Gilles Espinasse]

Selon Stuart Kemp <address@hidden>:

>
> It is mount(8) and umount(8) that maintain /etc/mtab. I don't think this is
> intended to be a definitive list (on Linux) of all mounted areas. As it
> states in the mount(8) man-page 'It is possible that /etc/mtab and
> /proc/mounts don't match. The first file is based only on the mount command
> options..."
>
/proc/mounts is to my understanding a linux only feature and /etc/mtab usage is
more general than linux.

> Yes, the app I was playing with does not call addmntent() or anything else to
> explicitly manipulate /etc/mtab. But it should not have to, since all of this
> mount-data is already available from the kernel (via /proc/mounts); as such
> it seems redundant to even have /etc/mtab. The problem could also arise using
> the "-n" option of mount(8). It would seem logical to use a definitive set of
> information, given that it is readily available i.e. get the mounted
> filesystems from /proc/mounts, rather than the arbitrary data that may (or
> may not) be in /etc/mtab. And this addresses a limitation in the current
> implementation.
>
There is sometime less informations in /proc/mounts than in /etc/mtab so linking
/etc/mtab to /proc/mounts may drive to issues.
This could be demonstrated by
mkdir /tmp/test1 /tmp/test2
mount -bind /tmp/test1 /tmp/test2

With /etc/mtab as symlink, you can't do umount /tmp/test1 and could only
umount /tmp/test2
With /etc/mtab as a real file, umount /tmp/test1 work too.

As booting from initramfs is now very frequent, /proc /sys /dev are mounted
inside initramfs script. So the boot script on the root file system has to
mount -f /proc /sys /dev again to update the rootfs /etc/mtab (mount -f only
update /etc/mtab and mount nothing)

> And of course, using mount(8) with no arguments also has a "problem" in that
> it lists the contents of /etc/mtab, rather than what is actually mounted!
>
> -Stuart Kemp
>
that could be considered as a feature when you are inside a chroot and don't
want to be distracted by what was done outside.

Gilles