zram-init prevents option X-mount.mkdir from being used

[fabianoengler]

X-mount.mkdir is an option for mount where mount creates the target directory if it doesn't already exists (see man mount).

If one passes this option to zram-init it won't even call mount to give it a change to create the target directory, it will fail first:

root@localhost:~ # zram-init -t ext2 -o x-mount.mkdir=775 -d 3 512 /var/tmp/portage
zram-init: 
${dir:-(empty)} is not a directory

If one calls zramctl and mount directly, it works as expected:

root@localhost:~ # zramctl -f -s 512M
/dev/zram3

root@localhost:~ # mkfs.ext2 /dev/zram3 -q

root@localhost:~ # mount /dev/zram3 /var/tmp/portage
mount: /var/tmp/portage: mount point does not exist.

root@localhost:~ # mount /dev/zram3 /var/tmp/portage -o x-mount.mkdir=775

root@localhost:~ # ls -ld /var/tmp/portage
drwxr-xr-x 3 root root 4096 Mar 17 19:41 /var/tmp/portage

---

Q: What's is the use case for this? Why not simply do an mkdir before running zram-init?

A: On my setup, both /var/tmp and /var/tmp/portage are mounted by zram-init, with different options each. Among the differences there is noexec for /var/tmp (for security reasons) but exec for /var/tmp/portage (required).

Since both mount points are mounted by zram-init triggered by the distribution init scripts (openrc), /var/tmp will necessarily be empty after it's mounted and /var/tmp/portage is then mounted immediately after, leaving no opportunity for me to insert a mkdir command between the two mount operations.

For reference, my actual setup from /etc/conf.d/zram-init:

type2=/var/tmp
flag2=ext4
size2=16384
mlim2=
back2=
opts2="noatime,nosuid,nodev,noexec"
mode2=1777
owgr2=
notr2=
maxs2=1
algo2=zstd
labl2=var_tmp_dir
uuid2=
args2=

type3=/var/tmp/portage
flag3=ext4
size3=16384
mlim3=
back3=
opts3="noatime,nosuid,nodev,x-mount.mkdir=775"
mode3=
owgr3=portage:portage
notr3=
maxs3=1
algo3=zstd
labl3=tmp_portage_dir
uuid3=
args3=

---

Solution

I did simple check for X-mount.mkdir where zram-init checks if the directory exists, if it's there it won't trigger the error. I'm not sure if it's the best or even appropriate solution but it was what I came up with and it worked for me:

root@localhost:~ # ls -ld /var/tmp/portage
ls: cannot access '/var/tmp/portage': No such file or directory

root@localhost:~ # sh zram-init.sh -t ext2 -o noatime,nosuid,nodev,x-mount.mkdir=775 -c portage:portage -d 3 -D 4 -s 1 -a zstd -L tmp_portage_dir -- 16384 /var/tmp/portage
mke2fs 1.45.5 (07-Jan-2020)

root@localhost:~ # # ls -ld /var/tmp/portage
drwxr-xr-x 3 portage portage 4096 Mar 17 19:47 /var/tmp/portage

I'll be opening a PR next with my patch for discussion.