On Wed, Aug 12, 2026 at 05:05:03PM +0200, Sylvain Saboua wrote:
> Here are the two pages I apply the instructions of to that end:
> https://dataswamp.org/~solene/2018-05-08-mfs-tmp.html
The method described there to mount mfs on /tmp with 1777 permissions is very
naive, and yet people continue to keep doing it.
The problem is that if your mfs fails to mount, or somehow becomes unmounted,
you're left with a 1777 world writable directory on the root volume.
On a multi-user system, this can easily be abused to create hard links to
files in places such as /etc/ where normal users shouldn't be able to create
them.
This in turn can open your system up to exploits.
So _do not do this_, and please do not propogate the above suggestion.
There are various more sensible ways to achieve a ram-based filesystem on
/tmp.
Ideally, mount_mfs would allow the users to specify a mode for it's root in a
similar way to mount_tmpfs. I posted patches some time ago to implement this,
but despite some initial interest they never got into cvs and I got busy with
other things.
Of course, if you're happy using tmpfs, (or my patches), that is one answer.
Alternatively, and more practically, you can use the -P option to mount_mfs to
populate /tmp from an empty directory elsewhere that has 1777 permissions.
Obviously, putting _that_ directory somewhere where it's world writable
properties cannot be abused is also a good idea.
However, the simplest way to get an mfs on /tmp with 1777 is to add two lines
to /etc/rc, just after 'mount -a -t nonfs,vnd' :
# Adjust permissions for /tmp if it is on an mfs filesystem
/bin/df -t mfs /tmp/ > /dev/null 2> /dev/null && { echo "/tmp on mfs, setting
permissons to 1777" ; chmod 1777 /tmp/ ; }
Now you can leave the permissions for /tmp on the real root filesystem alone,
and have a mfs mount in /etc/fstab:
swap /tmp mfs rw,nodev,nosuid,-s=512m 0 0
