"zfs mount -a" adds incorrect device/dataset entry to /etc/mtab when current directory is "/"

[fajarnugraha]

Currently running your latest zpl branch. I found this issue when writing an init script that will run "zfs mount -a" during boot process. Here's a simple test: http://pastebin.com/rHfuzzSa

In that example, there's a pool called testpool. When the fs is unmounted (zfs umount -a), and then you run "zfs mount -a" when current directory is "/", /etc/mtab will record the wrong device/dataset with leading "/" ("/testpool" instead of "testpool").

This effectively prevents "zfs umount -a" from running correctly.

Workaround: change working directory somewhere else (anywhere but "/") before running "zfs mount -a"

[behlendorf]

The problem here is actually with the mount(8) command. Because I needed to ensure that the /etc/mtab file is always locked properly, the 'zfs (u)mount' command actually calls mount(8) to do the heavy lifting. The mount(8) command then helpfully tries to resolve the 'device' which is passed to it based on some heuristics. One of these heuristics is to prepend the cwd to the passed 'device', if the resulting path exists this new device name is passed to mount(2). In this case when your cwd is root it prepends '/' to say the dataset named 'tank'. Since the directory '/tank' exists it calls mount(2) with this device. The mount then fails because the kernel doesn't understand the dataset named '/tank', it expects 'tank'. You can cause this same failure by running to mount command in any directory with a file/dir named the same as your dataset.

The long term fix for this will be to push a patch upstream to util-linux so I doesn't mess around with the 'device' names for zfs type datasets. In the short term I'll update the mount.zfs helpers to undo the damage done by mount(8) before it calls mount(2).

[behlendorf]

Fix mount helper

Several issues related to strange mount/umount behavior were reported
and this commit should address most of them. The original idea was
to put in place a zfs mount helper (mount.zfs). This helper is used
to enforce 'legacy' mount behavior, and perform any extra mount argument
processing (selinux, zfsutil, etc). This helper wasn't ready for the
0.6.0-rc1 release but with this change it's functional but needs to
extensively tested.

This change addresses the following open issues.
Closed by 6adf458
Closed by 6adf458
Closed by 6adf458
Closed by 6adf458
Closed by 6adf458