Re: df: use mountinfo from /proc on Linux

[Fridolin Pokorny]

On Wednesday 20 August 2014 16:21:27 Pádraig Brady wrote:
> On 08/20/2014 03:53 PM, Fridolin Pokorny wrote:
> > Hi,
> > 
> > I am about to implement /proc/self/mountinfo support for df. This feature
> > is marked as TODO in sources. I have found a suitable parser provided by
> > libmount from util-linux [1]. This could be the best option here in my
> > opinion.
> > 
> > The change might also reflect gnulib. Items in mount entries (struct
> > mount_entry from mountlist.h) should be propagated from libmount.
> > 
> > I want to know if such change will be accepted. Do you have any additional
> > notes to discuss?
> > 
> > Have a nice day,
> > Fridolin Pokorny.
> > 
> > [1] http://git.kernel.org/cgit/utils/util-linux/util-linux.git
> 
> Note the kernel processing to generate /proc/self/mountinfo is significant,
> so ideally gnulib's read_file_system_list() would read that once (falling
> back to /etc/mtab if not available).

Agreed.
 
> There is more info in /proc/$$/mountinfo wrt bind mounts etc.
> and I've yet to look into if any of that is useful for df.

I went through kernel sources and I cannot find any difference in generated
output. Accessing /proc/self is treated as accessing /proc/$$. I saw that 
there are used exclusive locks when accessing /proc/self, which will be good 
to omit by using /proc/$$.

There can be found info about shared subtrees and parent mount IDs among 
others in /proc/$$/mountinfo. I don't know if either of this could be useful 
for df.

> I'd come up with a concrete set of advantages of using libmount before
> proceeding. Avoid stat()s may not warrant the change.

The biggest advantage for me is that mount ID could be propagated from /proc
instead of using stat()s:

# mkdir /chroot/home
# mount /dev/sda1 /chroot/home
# mount /dev/sda2 /home
# df
...
/dev/sda1 - - -  -% /chroot/home
/dev/sda2 - - -  -% /home
...
# mount -t proc proc /chroot/proc/
# chroot /chroot
chroot# df
...
/dev/sda2 - - -  -% /home
...

There should be /dev/sda1.

Filesystems mounted outside of chroot are not displayed in /proc/$$/mountinfo. 
Now df prints errors complaining ENOENT when filesystems are not accessible 
inside chroot.

Another bogus output for me could be simulated by creating directory with same
name as mountpoint outside of chroot.

chroot# ls /run
ls: cannot access /run: No such file or directory
chroot# df
...
df: '/run': No such file or directory
...
chroot# mkdir /run
chroot# df -a
...
tmpfs - - - -% /run
...

In all cases above, mountpoints are in mount namespace of the current process, 
but they are not accessible due to chroot. All of this can be improved by 
using /proc/$$/mountinfo.