bectl: document the -r flag to bectl

Independent of all of the commands, bectl itself takes an `-r` flag that
specifies the BE root to use.  This was originally added to facilitate
testing, but it was later discovered to be incredibly useful in other
scenarios; e.g., trying to recover some boot environments in rescue
media.

The "BE root" described here is the parent dataset that holds boot
environments, but I've no idea if that's an accepted definition for that
dataset.

Reviewed by:	gallatin, imp, Pau Amma
MFC after:	1 week
Differential Review:	https://reviews.freebsd.org/D39710

2 files changed, 50 insertions, 19 deletions

diff --git a/sbin/bectl/bectl.8 b/sbin/bectl/bectl.8
index 79bdec751a86..974c6ece9d12 100644
--- a/sbin/bectl/bectl.8
+++ b/sbin/bectl/bectl.8
@@ -17,7 +17,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd March 31, 2022
+.Dd April 26, 2023
 .Dt BECTL 8
 .Os
 .Sh NAME
@@ -25,54 +25,67 @@
 .Nd Utility to manage boot environments on ZFS
 .Sh SYNOPSIS
 .Nm
+.Op Fl r Ar beroot
 .Cm activate
 .Op Fl t | Fl T
 .Ar beName
 .Nm
+.Op Fl r Ar beroot
 .Cm check
 .Nm
+.Op Fl r Ar beroot
 .Cm create
 .Op Fl r
 .Op Fl e Brq Ar nonActiveBe | Ar beName Ns Cm @ Ns Ar snapshot
 .Ar newBeName
 .Nm
+.Op Fl r Ar beroot
 .Cm create
 .Op Fl r
 .Ar beName@snapshot
 .Nm
+.Op Fl r Ar beroot
 .Cm destroy
 .Op Fl \&Fo
 .Ar beName Ns Op Cm @ Ns Ar snapshot
 .Nm
+.Op Fl r Ar beroot
 .Cm export
 .Ar sourceBe
 .Nm
+.Op Fl r Ar beroot
 .Cm import
 .Ar targetBe
 .Nm
+.Op Fl r Ar beroot
 .Cm jail
 .Op Fl bU
 .Oo Bro Fl o Ar key Ns Cm = Ns Ar value | Fl u Ar key Brc Oc Ns ...
 .Ar beName
 .Op Ar utility Op Ar argument ...
 .Nm
+.Op Fl r Ar beroot
 .Cm list
 .Op Fl aDHs
 .Op Fl c Ar property
 .Op Fl C Ar property
 .Oo Bro Fl c Ar property | Fl C Ar property Brc Oc
 .Nm
+.Op Fl r Ar beroot
 .Cm mount
 .Ar beName
 .Op Ar mountpoint
 .Nm
+.Op Fl r Ar beroot
 .Cm rename
 .Ar origBeName
 .Ar newBeName
 .Nm
+.Op Fl r Ar beroot
 .Brq Cm ujail | unjail
 .Brq Ar jailId | jailName | beName
 .Nm
+.Op Fl r Ar beroot
 .Brq Cm umount | unmount
 .Op Fl f
 .Ar beName
@@ -89,6 +102,23 @@ Boot environments
 allow the system to be upgraded, while preserving the old system environment in
 a separate ZFS dataset.
 .Pp
+.Nm
+itself accepts an
+.Fl r
+flag specified before the command to indicate the
+.Ar beroot
+that should be used as the boot environment root, or the dataset whose children
+are all boot environments.
+Normally this information is derived from the bootfs property of the pool that
+is mounted at
+.Pa / ,
+but it is useful when the system has not been booted into a ZFS root or a
+different pool should be operated on.
+For instance, booting into the recovery media and manually importing a pool from
+one of the system's resident disks will require the
+.Fl r
+flag to work.
+.Pp
 The following commands are supported by
 .Nm :
 .Bl -tag -width activate
@@ -141,11 +171,13 @@ flag is specified, the new environment will be cloned from the given
 .Ar nonActiveBe
 or
 .Ar beName Ns Cm @ Ns Ar snapshot .
-Otherwise, the new environment will be created from the currently booted environment.
+Otherwise, the new environment will be created from the currently booted
+environment.
 .Pp
 If
 .Nm
-is creating from another boot environment, a snapshot of that boot environment will be created to clone from.
+is creating from another boot environment, a snapshot of that boot environment
+will be created to clone from.
 .It Xo
 .Cm create
 .Op Fl r
@@ -276,7 +308,6 @@ is used on next boot once
 .Pq Em \&T ;
 or combination of
 .Pq Em \&NRT .
-.Pp
 .Bl -tag -width indent
 .It Fl a
 Display all datasets.
diff --git a/sbin/bectl/bectl.c b/sbin/bectl/bectl.c
index bc414d6baf41..9e3833ba834e 100644
--- a/sbin/bectl/bectl.c
+++ b/sbin/bectl/bectl.c
@@ -69,23 +69,23 @@ usage(bool explicit)
 	fprintf(fp, "%s",
 	    "Usage:\tbectl {-h | -? | subcommand [args...]}\n"
 #if SOON
-	    "\tbectl add (path)*\n"
+	    "\tbectl [-r beroot] add (path)*\n"
 #endif
-	    "\tbectl activate [-t] beName\n"
-	    "\tbectl activate [-T]\n"
-	    "\tbectl check\n"
-	    "\tbectl create [-r] [-e {nonActiveBe | beName@snapshot}] beName\n"
-	    "\tbectl create [-r] beName@snapshot\n"
-	    "\tbectl destroy [-Fo] {beName | beName@snapshot}\n"
-	    "\tbectl export sourceBe\n"
-	    "\tbectl import targetBe\n"
-	    "\tbectl jail [-bU] [{-o key=value | -u key}]... beName\n"
+	    "\tbectl [-r beroot] activate [-t] beName\n"
+	    "\tbectl [-r beroot] activate [-T]\n"
+	    "\tbectl [-r beroot] check\n"
+	    "\tbectl [-r beroot] create [-r] [-e {nonActiveBe | beName@snapshot}] beName\n"
+	    "\tbectl [-r beroot] create [-r] beName@snapshot\n"
+	    "\tbectl [-r beroot] destroy [-Fo] {beName | beName@snapshot}\n"
+	    "\tbectl [-r beroot] export sourceBe\n"
+	    "\tbectl [-r beroot] import targetBe\n"
+	    "\tbectl [-r beroot] jail [-bU] [{-o key=value | -u key}]... beName\n"
 	    "\t      [utility [argument ...]]\n"
-	    "\tbectl list [-aDHs] [{-c property | -C property}]\n"
-	    "\tbectl mount beName [mountpoint]\n"
-	    "\tbectl rename origBeName newBeName\n"
-	    "\tbectl {ujail | unjail} {jailID | jailName | beName}\n"
-	    "\tbectl {umount | unmount} [-f] beName\n");
+	    "\tbectl [-r beroot] list [-aDHs] [{-c property | -C property}]\n"
+	    "\tbectl [-r beroot] mount beName [mountpoint]\n"
+	    "\tbectl [-r beroot] rename origBeName newBeName\n"
+	    "\tbectl [-r beroot] {ujail | unjail} {jailID | jailName | beName}\n"
+	    "\tbectl [-r beroot] {umount | unmount} [-f] beName\n");
 
 	return (explicit ? 0 : EX_USAGE);
 }