Created attachment 7456 [details]
umount-fstype.patch

Hello,

I find weird behaviour of umount applet when it is calles with "-t fstype" flag. It causes additional unmount of last entry in mtab with this filesystem type:

# mount -t tmpfs test /test1
# mount -t tmpfs test /test2
# mount -t tmpfs test /test3
# cat /proc/mounts | grep test
test /test1 tmpfs rw,relatime 0 0
test /test2 tmpfs rw,relatime 0 0
test /test3 tmpfs rw,relatime 0 0
# cat /proc/mounts | tail -n 3
test /test1 tmpfs rw,relatime 0 0
test /test2 tmpfs rw,relatime 0 0
test /test3 tmpfs rw,relatime 0 0
# busybox umount -t tmpfs /test2
# cat /proc/mounts | grep test
test /test1 tmpfs rw,relatime 0 0
# busybox umount -t tmpfs /test1
umount: can't unmount /test1: Invalid argument
# cat /proc/mounts | grep test
# 

After first unmount run for /test2 - we can see that /test3 has been umounted too. And when we unmount /test1 which is the last tmpfs in mtab, it tries to umount it twice (you can see it in strace) and the second call fails.

As I see in the code:
https://git.busybox.net/busybox/tree/util-linux/umount.c

It reads entries from mtab that matches the filesystem type into a linked list "m" and should forget about it in the case when we do not want to unmount all. But for whatever reason it checks that "-t" option is not specifed too and so do not forgets the list. I think this causes it to umount the top of the list (last enty in the mtab with that fstype) and then move towards to required path.

I think attached patch should fix this behaviour.

PS. Also it states in the code that "-t" option is ignored, but it is not so. And it is not documented.