Discrepancies between real and symlinked mtab

[Matt Burgess <matthew@linuxfromscratch.org>]

Hi,

Consider trying to answer the question "What partition is mounted
on /" (it happens to be /dev/sda3 in this example), when /etc/mtab is a
symlink pointing to /proc/mounts:

1) mount(8)

/dev/root on / type ext3 (rw,relatime,data=ordered)

Nope, that's not /dev/sda3.  And what is /dev/root anyway? There's no
such node under /dev here.

2) df(1)

Filesystem     1K-blocks    Used Available Use% Mounted on
rootfs          20642428 1576788  18017064   9% /
/dev/root       20642428 1576788  18017064   9% /

Uh-oh - now I appear to have 2 filesystems mounted at '/' (yes, I know
you can tell by the same stats that it is in fact the same FS).  What is
this rootfs entry?

3) lsblk(8)

NAME FSTYPE LABEL UUID                                 MOUNTPOINT
sda3 ext3   /lfs  aaf09e8b-d50e-429e-ba8e-11fa07ab5b75 

Well, that's odd.  I know that /dev/sda3 is mounted at /, but apparently
lsblk(8) doesn't!

If /etc/mtab is a real file, then the output of all 3 commands is
correct (and by that I mean it shows /dev/sda3).

As all 3 utils show the same issue, I assume this is something to do
with libblkid?

Thanks,

Matt.