GNU bug report logs - #28670
df: detect over-mounted filesystems

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From: Alan Jenkins <alan.christopher.jenkins <at> gmail.com>
To: 28670 <at> debbugs.gnu.org
Subject: bug#28670: df: detection of overmounted filesystems is not quite complete
Date: Mon, 2 Oct 2017 13:28:14 +0100

$ rpm -q --whatprovides `which df`
coreutils-8.27-6.fc26.x86_64

(Fedora Linux 26)


On 02/10/17 13:27, Alan Jenkins wrote:
> Scenario 1 - lovely diagnostic
>
> $ df -h /dev/sda1
> df: cannot access '/dev/sda1': over-mounted by another device
>
> $ mount | grep boot/efi
> /dev/sda6 on /boot type ext4 (rw,relatime,seclabel,data=ordered)
> /dev/sda1 on /boot/efi type vfat 
> (rw,relatime,fmask=0077,dmask=0077,codepage=437,iocharset=ascii,shortname=winnt,errors=remount-ro)
> none on /boot/efi type tmpfs (rw,relatime,seclabel)
>
>
> Scenario 2 - breaks down
>
> $ df -h |grep /dev/sda1
> $ df -ah |grep /dev/sda1
> /dev/sda1                             -     -     -    - /boot/efi
>
> above is consistent (same output as in scenario 1), but below is bad:
>
> $ df -h /dev/sda1  # or same output  from `/boot/efi`
> Filesystem      Size  Used Avail Use% Mounted on
> /dev/sda1       3.9G     0  3.9G   0% /boot/efi
>
> Case 2 situation is as follows:
>
> $ mount | grep boot
> /dev/sda6 on /boot type ext4 (rw,relatime,seclabel,data=ordered)
> /dev/sda1 on /boot/efi type vfat 
> (rw,relatime,fmask=0077,dmask=0077,codepage=437,iocharset=ascii,shortname=winnt,errors=remount-ro)
> none on /boot type tmpfs (rw,relatime,seclabel)
>
> i.e. the tmpfs on /boot is the most recent mount, masking the 
> /boot/efi mount (and I created a dummy `/boot/efi` directory in the 
> tmpfs).
>
> The practical use-case is that people get very confused when their 
> distribution sets up pam_ecryptfs.  If `df` was more reliable about 
> detecting overmounts, then it would provide better diagnostics for 
> when pam_ecryptfs overmounts a filesystem which was mounted by fstab.
>
> https://unix.stackexchange.com/questions/370043/mounting-a-subdirectory-of-home-onto-an-own-partition-confuses-disk-usage-report/395630?noredirect=1#comment706009_395630 
>

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