#!/bin/bash
#
# Privilege Escalation Helper (PE Helper) example - Fileset manipulation
#
# The purpose of this script is to aid in creating and manipulating filesets by using elevated privileges (root) in a
# controlled way. It works around the problem that - on Linux - only root may manipulate the global filesystem
# namespace. That means that only root may mount and umount filesystems, even if a less privileged user has been
# granted permission to mount using "zfs allow".
#
#############
# This script is an example, and by no means complete or guaranteed to be secure. Use at your own risk!
# There *are* race conditions, always make sure to only pass clean arguments to the script and run only one instance.
#############
#
# The theory of operation for this script is that it allows the caller to create and delete filesets with mountpoints
# and to manipulate the mountpoint property of existing filesets within certain restrictions:
# * The fileset be a (grand...)child of a configured parent dataset, i.e. the caller can only manipulate filesets in
#   a subtree of the hierarchy.
# * The mountpoint must be below a certain filesystem node, i.e. the caller can mount or unmount filesets below a
#   configured parent directory.
#
# Note that it is not currently possible to use non-ascii characters in the path or fileset name
#
####################
# Calling convention
#
# The first argument specifies the action such as "create" for creating a fileset. Depending on the action, one or
# more arguments are required, most take two:
#
# * "create" has 2 args:         fileset name and mountpoint
# * "destroy" has 1 arg:         fileset name
# * "set_mountpoint" has 2 args: fileset name and new mountpoint
#
# The exit code is the primary way to communicate back if something happened. Additionally, messages are written to
# stdout or stderr to provide more insight.
#
# * 0: Everything went well
# * 1: Parameter or general error (such as commands not found)
# * 2: Parent directory does not exist or is not a directory
# * 3: Parent dataset does not exist
# * 4: Target fileset is not a (grand)child of parent dataset or parent does not exist
# * 5: Mountpoint is not inside the parent directory or otherwise invalid
# * 6: Calling the zfs utility failed
#
###############
# Configuration
#
# The section following this comment block is for configuration. The following can be configured:
#
# * "PARENT_DATASET" is a string pointing to the dataset below which the caller can create and destroy filesets.
# * "PARENT_DIRECTORY" is a string pointing to the directory below which filesets can be mounted/unmounted by the
#   caller.
# * "ZFS_BIN": full path to the zfs(8) utility.
# * "USE_SUDO": Whether to use sudo for the manipulation commands. The lookups will be done without sudo regardless of
#   this setting.
#
# It is not required for the parent dataset to be mounted at or below the parent directory, but it must exist prior to
# calling the helper. The parent directory must also exist.
#

PARENT_DATASET=rpool/test
PARENT_DIRECTORY=/data
ZFS_BIN=/usr/bin/zfs
USE_SUDO=false

######################################################################################################################
# End of configuration section - Do NOT modify anything below this line
######################################################################################################################

if [ ! -x $ZFS_BIN ]
then
    echo "'zfs' binary can't be executed"
    exit 1
fi

# check the parameters
if [ $# -lt 1 ]
then
    echo "not enough parameters"
    exit 1
fi

case $1 in
    create|set_mountpoint)
        if [ $# -ne 3 ]
        then
            echo "incorrect number of arguments, expected 3: ${1} <fileset> <mountpoint>"
            exit 1
        fi
        ;;
    destroy)
        if [ $# -ne 2 ]
        then
            echo "incorrect number of arguments, expected 2: ${1} <fileset>"
            exit 1
        fi
        ;;
    *)
        echo "unknown command, chose from: create | destroy | set_mountpoint"
        exit 1
        ;;
esac

# Execute the given parameters with ZFS, optionally using sudo if USE_SUDO is true.
function exec_zfs()
{
    params=$*
    # Simple and not at all sufficient check to limit the commands that can be run
    # It also limits the characters allowed as names to the ASCII set, so unicode directory names are not possible
    if [[ $params =~ [a-zA-Z0-9=_\ /-] ]]
    then
        if [[ $USE_SUDO == true ]]
        then
            echo "Issuing sudo command: sudo ${ZFS_BIN} ${params}"
            # shellcheck disable=SC2086
            sudo $ZFS_BIN $params
        else
            echo "Issuing command: ${ZFS_BIN} ${params}"
            # shellcheck disable=SC2086
            $ZFS_BIN $params
        fi
    else
        echo "Command string contains disallowed characters"
        exit 1
    fi
}

# Test parent directory
######
# test that the parent directory exists
if ! readlink -qe ${PARENT_DIRECTORY} > /dev/null
then
    echo "Parent directory ${PARENT_DIRECTORY} does not exist."
    exit 2
fi

# and is a directory
if [ ! -d ${PARENT_DIRECTORY} ]
then
    echo "Parent directory ${PARENT_DIRECTORY} is not a directory."
    exit 2
fi

# Test parent dataset
######
# Test that the parent dataset exists
if ! $ZFS_BIN list -d 2 "${PARENT_DATASET}" -o name -H > /dev/null
then
    echo "Parent dataset ${PARENT_DATASET} does not exist."
    exit 3
fi

fileset=$2

# Check that the fileset is below the PARENT_DATASET. It does not need to exist.
pds_tmp="${PARENT_DATASET}/"
if [[ $fileset != $pds_tmp* ]]
then
    echo "Fileset does not appear to be a child of parent dataset ${PARENT_DATASET}"
    exit 4
fi

# Branch off for the different actions. At this point, we know that the parent dataset exists and the fileset argument
# points to a path inside the parent dataset.
if [[ $1 == 'create' ]]
then
    echo create
    mountpoint=${3%/}

    # make sure the direct parent of our soon-to-be fileset exists
    new_fileset_parent=$(dirname "$fileset")
    if ! $ZFS_BIN list -d 2 "$new_fileset_parent" -o name -H > /dev/null
    then
        echo "Parent dataset of new fileset does not exist"
        exit 4
    fi

    # resolve the mountpoint through symlinks, then try to make sure it is below PARENT_DIRECTORY
    if ! mp_dir=$(readlink -qf "$mountpoint")
    then
        echo "Parent of mountpoint does not exist."
        exit 5
    fi

    if [[ $mp_dir != $PARENT_DIRECTORY* ]]
    then
        echo "Mountpoint does not appear to be located somewhere below ${PARENT_DIRECTORY}"
        exit 5
    fi

    # create the fileset
    exec_zfs create -o "mountpoint=${mountpoint}" "$fileset"

elif [[ $1 == 'set_mountpoint' ]]
then
    echo set_mountpoint

    # check that the fileset exists
    if ! $ZFS_BIN list -d 2 "$fileset" -o name -H > /dev/null
    then
        echo "Fileset does not exist"
        exit 4
    fi

    # get the type and bail out if it is not a "filesystem"
    if [[ $($ZFS_BIN get -H -o value type "$fileset") != 'filesystem' ]]
    then
        echo "Fileset parameter does not point to a filesystem"
        exit 5
    fi

    # get the current mountpoint
    if ! mp=$($ZFS_BIN get -H -o value mountpoint "$fileset")
    then
        echo "The old value of the mountpoint property can't be retrieved"
        exit 5
    fi

    # check that the mountpoint resides below the PARENT_DIRECTORY
    if ! mp_dir=$(readlink -qe "$mp")
    then
        echo "The filesets mountpoint does not exist"
        exit 5
    fi

    if [[ $mp_dir != $PARENT_DIRECTORY* ]]
    then
        echo "The current mountpoint is outside of ${PARENT_DIRECTORY}"
        exit 5
    fi

    # check that the parent directory of the new mountpoint exists
    if ! mp_dir_new=$(readlink -qf "$mountpoint")
    then
        echo "The parent of the new mountpoint does not exist"
        exit 5
    fi

    # check that the new mountpoint is below the PARENT_DIRECTORY, too
    if [[ $mp_dir_new != $PARENT_DIRECTORY* ]]
    then
        echo "The new mountpoint is outside of $PARENT_DIRECTORY"
        exit 5
    fi

    # change the mountpoint property
    exec_zfs set "mountpoint=${mountpoint}" "$fileset"

elif [[ $1 == 'destroy' ]]
then
    echo destroy

    if ! $ZFS_BIN list -d 2 "$fileset" -o name -H > /dev/null
    then
        echo "Fileset does not exist"
        exit 4
    fi

    # get the mountpoint value
    mp=$($ZFS_BIN get -H -o value mountpoint "$fileset")

    # check that the mountpoint resides below the PARENT_DIRECTORY
    if ! mp_dir=$(readlink -qe "$mp")
    then
        echo "Mountpoint does not exist"
        exit 5
    fi

    if [[ $mp_dir != $PARENT_DIRECTORY* ]]
    then
        echo "Mountpoint appears to be outside of $PARENT_DIRECTORY"
        exit 5
    fi

    # remove the fileset
    exec_zfs destroy "$fileset"

else
    echo "Unknown action, this should not be reachable"
    exit 1
fi

exit 0