dracut: support mountpoint=legacy for root dataset

[rkitover]

Support mountpoint=legacy for the root dataset in the dracut zfs support scripts.

mountpoint=/ or mountpoint=/sysroot also works.

Change zfs-env-bootfs.service to add zfsutil to BOOTFSFLAGS only for non-legacy root datasets, using the support function determine_rootflags() added to zfs-lib.sh.

Motivation and Context

This allows using the dracut kernel commandline option root=ZFS=pool/dataset for datasets that have mountpoint=legacy set.

This is documented as already working, but it looks like once the root mount was switched to use the generator stuff it broke.

How Has This Been Tested?

I made the changes in my /usr/lib/dracut/modules.d/90zfs and reran dracut on my system, testing booting the root dataset with mountpoint being /, /sysroot and legacy.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

[rkitover]

@nabijaczleweli Hi, could you take a look please?

[nabijaczleweli]

yep, must've missed this use-case when converting. functionally looks sane, but see comment

[nabijaczleweli]

If you use 3eaacdf which has a diff -w of

diff --git a/contrib/dracut/90zfs/zfs-env-bootfs.service.in b/contrib/dracut/90zfs/zfs-env-bootfs.service.in
index 7ebab4c1a..fe362b930 100644
--- a/contrib/dracut/90zfs/zfs-env-bootfs.service.in
+++ b/contrib/dracut/90zfs/zfs-env-bootfs.service.in
@@ -12,6 +12,7 @@ ExecStart=/bin/sh -c '
     decode_root_args || exit 0;                                                                \
     [ "$root" = "zfs:AUTO" ] && root="$(@sbindir@/zpool list -H -o bootfs | grep -m1 -vFx -)"; \
     rootflags="$(getarg rootflags=)";                                                          \
+    [ "$(@sbindir@/zfs get -H -o value mountpoint "$root")" = legacy ] ||                      \
         case ",$rootflags," in                                                                 \
             *,zfsutil,*) ;;                                                                    \
             ,,) rootflags=zfsutil ;;                                                           \

(untested, but no reason it shouldn't work) then this is g2g imo.

[rkitover]

Thanks for the quick response. I will test this and update the PR.

[rkitover]

@nabijaczleweli tested and updated to your commit, thank you.

By the way, for future reference, how do I install just the dracut modules from the zfs tree?

[nabijaczleweli]

idk, i just apply the diffs. but if you make install DESTDIR=/whatever you can pick out the dracut stuff from under /whatever (which may also be wrong if you otherwise have a distribution zfs installed)