safe.tcl –

This file provide a safe loading/sourcing mechanism for safe interpreters.

It implements a virtual path mecanism to hide the real pathnames from the

slave. It runs in a master interpreter and sets up data structure and

aliases that will be invoked when used from a slave interpreter.

See the safe.n man page for details.

Copyright (c) 1996-1997 Sun Microsystems, Inc.

See the file “license.terms” for information on usage and redistribution

of this file, and for a DISCLAIMER OF ALL WARRANTIES.

RCS: @(#) $Id: safe.tcl,v 1.9.2.3 2005/07/22 21:59:41 dgp Exp $

The implementation is based on namespaces. These naming conventions

are followed:

Private procs starts with uppercase.

Public procs are exported and starts with lowercase

Needed utilities package

package require opt 0.4.1;

Create the safe namespace

namespace eval ::safe {

# Exported API:
namespace export interpCreate interpInit interpConfigure interpDelete \
    interpAddToAccessPath interpFindInAccessPath setLogCmd

####
#
# Setup the arguments parsing
#
####

# Make sure that our temporary variable is local to this
# namespace.  [Bug 981733]
variable temp

# Share the descriptions
set temp [::tcl::OptKeyRegister {
{-accessPath -list {} "access path for the slave"}
{-noStatics "prevent loading of statically linked pkgs"}
{-statics true "loading of statically linked pkgs"}
{-nestedLoadOk "allow nested loading"}
{-nested false "nested loading"}
{-deleteHook -script {} "delete hook"}
}]

# create case (slave is optional)
::tcl::OptKeyRegister {
{?slave? -name {} "name of the slave (optional)"}
} ::safe::interpCreate
# adding the flags sub programs to the command program
# (relying on Opt's internal implementation details)
lappend ::tcl::OptDesc(::safe::interpCreate) $::tcl::OptDesc($temp)

# init and configure (slave is needed)
::tcl::OptKeyRegister {
{slave -name {} "name of the slave"}
} ::safe::interpIC
# adding the flags sub programs to the command program
# (relying on Opt's internal implementation details)
lappend ::tcl::OptDesc(::safe::interpIC) $::tcl::OptDesc($temp)
# temp not needed anymore
::tcl::OptKeyDelete $temp


# Helper function to resolve the dual way of specifying staticsok
# (either by -noStatics or -statics 0)
proc InterpStatics {} {
foreach v {Args statics noStatics} {
    upvar $v $v
}
set flag [::tcl::OptProcArgGiven -noStatics];
if {$flag && (!$noStatics == !$statics) 
          && ([::tcl::OptProcArgGiven -statics])} {
    return -code error\
	    "conflicting values given for -statics and -noStatics"
}
if {$flag} {
    return [expr {!$noStatics}]
} else {
    return $statics
}
}

# Helper function to resolve the dual way of specifying nested loading
# (either by -nestedLoadOk or -nested 1)
proc InterpNested {} {
foreach v {Args nested nestedLoadOk} {
    upvar $v $v
}
set flag [::tcl::OptProcArgGiven -nestedLoadOk];
# note that the test here is the opposite of the "InterpStatics"
# one (it is not -noNested... because of the wanted default value)
if {$flag && (!$nestedLoadOk != !$nested) 
          && ([::tcl::OptProcArgGiven -nested])} {
    return -code error\
	    "conflicting values given for -nested and -nestedLoadOk"
}
if {$flag} {
    # another difference with "InterpStatics"
    return $nestedLoadOk
} else {
    return $nested
}
}

####
#
#  API entry points that needs argument parsing :
#
####


# Interface/entry point function and front end for "Create"
proc interpCreate {args} {
set Args [::tcl::OptKeyParse ::safe::interpCreate $args]
InterpCreate $slave $accessPath \
	[InterpStatics] [InterpNested] $deleteHook
}

proc interpInit {args} {
set Args [::tcl::OptKeyParse ::safe::interpIC $args]
if {![::interp exists $slave]} {
    return -code error "\"$slave\" is not an interpreter"
}
InterpInit $slave $accessPath \
	[InterpStatics] [InterpNested] $deleteHook;
}

proc CheckInterp {slave} {
if {![IsInterp $slave]} {
    return -code error \
	    "\"$slave\" is not an interpreter managed by ::safe::"
}
}

# Interface/entry point function and front end for "Configure"
# This code is awfully pedestrian because it would need
# more coupling and support between the way we store the
# configuration values in safe::interp's and the Opt package
# Obviously we would like an OptConfigure
# to avoid duplicating all this code everywhere. -> TODO
# (the app should share or access easily the program/value
#  stored by opt)
# This is even more complicated by the boolean flags with no values
# that we had the bad idea to support for the sake of user simplicity
# in create/init but which makes life hard in configure...
# So this will be hopefully written and some integrated with opt1.0
# (hopefully for tcl8.1 ?)
proc interpConfigure {args} {
switch [llength $args] {
    1 {
	# If we have exactly 1 argument
	# the semantic is to return all the current configuration
	# We still call OptKeyParse though we know that "slave"
	# is our given argument because it also checks
	# for the "-help" option.
	set Args [::tcl::OptKeyParse ::safe::interpIC $args]
	CheckInterp $slave
	set res {}
	lappend res [list -accessPath [Set [PathListName $slave]]]
	lappend res [list -statics    [Set [StaticsOkName $slave]]]
	lappend res [list -nested     [Set [NestedOkName $slave]]]
	lappend res [list -deleteHook [Set [DeleteHookName $slave]]]
	join $res
    }
    2 {
	# If we have exactly 2 arguments
	# the semantic is a "configure get"
	::tcl::Lassign $args slave arg
	# get the flag sub program (we 'know' about Opt's internal
	# representation of data)
	set desc [lindex [::tcl::OptKeyGetDesc ::safe::interpIC] 2]
	set hits [::tcl::OptHits desc $arg]
            if {$hits > 1} {
                return -code error [::tcl::OptAmbigous $desc $arg]
            } elseif {$hits == 0} {
                return -code error [::tcl::OptFlagUsage $desc $arg]
            }
	CheckInterp $slave
	set item [::tcl::OptCurDesc $desc]
	set name [::tcl::OptName $item]
	switch -exact -- $name {
	    -accessPath {
		return [list -accessPath [Set [PathListName $slave]]]
	    }
	    -statics {
		return [list -statics    [Set [StaticsOkName $slave]]]
	    }
	    -nested {
		return [list -nested     [Set [NestedOkName $slave]]]
	    }
	    -deleteHook {
		return [list -deleteHook [Set [DeleteHookName $slave]]]
	    }
	    -noStatics {
		# it is most probably a set in fact
		# but we would need then to jump to the set part
		# and it is not *sure* that it is a set action
		# that the user want, so force it to use the
		# unambigous -statics ?value? instead:
		return -code error\
			"ambigous query (get or set -noStatics ?)\
			use -statics instead"
	    }
	    -nestedLoadOk {
		return -code error\
			"ambigous query (get or set -nestedLoadOk ?)\
			use -nested instead"
	    }
	    default {
		return -code error "unknown flag $name (bug)"
	    }
	}
    }
    default {
	# Otherwise we want to parse the arguments like init and create
	# did
	set Args [::tcl::OptKeyParse ::safe::interpIC $args]
	CheckInterp $slave
	# Get the current (and not the default) values of
	# whatever has not been given:
	if {![::tcl::OptProcArgGiven -accessPath]} {
	    set doreset 1
	    set accessPath [Set [PathListName $slave]]
	} else {
	    set doreset 0
	}
	if {(![::tcl::OptProcArgGiven -statics]) \
		&& (![::tcl::OptProcArgGiven -noStatics]) } {
	    set statics    [Set [StaticsOkName $slave]]
	} else {
	    set statics    [InterpStatics]
	}
	if {([::tcl::OptProcArgGiven -nested]) \
		|| ([::tcl::OptProcArgGiven -nestedLoadOk]) } {
	    set nested     [InterpNested]
	} else {
	    set nested     [Set [NestedOkName $slave]]
	}
	if {![::tcl::OptProcArgGiven -deleteHook]} {
	    set deleteHook [Set [DeleteHookName $slave]]
	}
	# we can now reconfigure :
	InterpSetConfig $slave $accessPath $statics $nested $deleteHook
	# auto_reset the slave (to completly synch the new access_path)
	if {$doreset} {
	    if {[catch {::interp eval $slave {auto_reset}} msg]} {
		Log $slave "auto_reset failed: $msg"
	    } else {
		Log $slave "successful auto_reset" NOTICE
	    }
	}
    }
}
}


####
#
#  Functions that actually implements the exported APIs
#
####


#
# safe::InterpCreate : doing the real job
#
# This procedure creates a safe slave and initializes it with the
# safe base aliases.
# NB: slave name must be simple alphanumeric string, no spaces,
# no (), no {},...  {because the state array is stored as part of the name}
#
# Returns the slave name.
#
# Optional Arguments : 
# + slave name : if empty, generated name will be used
# + access_path: path list controlling where load/source can occur,
#                if empty: the master auto_path will be used.
# + staticsok  : flag, if 0 :no static package can be loaded (load {} Xxx)
#                      if 1 :static packages are ok.
# + nestedok: flag, if 0 :no loading to sub-sub interps (load xx xx sub)
#                      if 1 : multiple levels are ok.

# use the full name and no indent so auto_mkIndex can find us
proc ::safe::InterpCreate {
slave 
access_path
staticsok
nestedok
deletehook
} {
# Create the slave.
if {$slave ne ""} {
    ::interp create -safe $slave
} else {
    # empty argument: generate slave name
    set slave [::interp create -safe]
}
Log $slave "Created" NOTICE

# Initialize it. (returns slave name)
InterpInit $slave $access_path $staticsok $nestedok $deletehook
}


#
# InterpSetConfig (was setAccessPath) :
#    Sets up slave virtual auto_path and corresponding structure
#    within the master. Also sets the tcl_library in the slave
#    to be the first directory in the path.
#    Nb: If you change the path after the slave has been initialized
#    you probably need to call "auto_reset" in the slave in order that it
#    gets the right auto_index() array values.

proc ::safe::InterpSetConfig {slave access_path staticsok\
    nestedok deletehook} {

# determine and store the access path if empty
if {$access_path eq ""} {
    set access_path [uplevel \#0 set auto_path]
    # Make sure that tcl_library is in auto_path
    # and at the first position (needed by setAccessPath)
    set where [lsearch -exact $access_path [info library]]
    if {$where == -1} {
	# not found, add it.
	set access_path [concat [list [info library]] $access_path]
	Log $slave "tcl_library was not in auto_path,\
		added it to slave's access_path" NOTICE
    } elseif {$where != 0} {
	# not first, move it first
	set access_path [concat [list [info library]]\
		[lreplace $access_path $where $where]]
	Log $slave "tcl_libray was not in first in auto_path,\
		moved it to front of slave's access_path" NOTICE
    
    }

    # Add 1st level sub dirs (will searched by auto loading from tcl
    # code in the slave using glob and thus fail, so we add them
    # here so by default it works the same).
    set access_path [AddSubDirs $access_path]
}

Log $slave "Setting accessPath=($access_path) staticsok=$staticsok\
	nestedok=$nestedok deletehook=($deletehook)" NOTICE

# clear old autopath if it existed
set nname [PathNumberName $slave]
if {[Exists $nname]} {
    set n [Set $nname]
    for {set i 0} {$i<$n} {incr i} {
	Unset [PathToken $i $slave]
    }
}

# build new one
set slave_auto_path {}
set i 0
foreach dir $access_path {
    Set [PathToken $i $slave] $dir
    lappend slave_auto_path "\$[PathToken $i]"
    incr i
}
Set $nname $i
Set [PathListName $slave] $access_path
Set [VirtualPathListName $slave] $slave_auto_path

Set [StaticsOkName $slave] $staticsok
Set [NestedOkName $slave] $nestedok
Set [DeleteHookName $slave] $deletehook

SyncAccessPath $slave
}

#
#
# FindInAccessPath:
#    Search for a real directory and returns its virtual Id
#    (including the "$")

proc ::safe::interpFindInAccessPath {slave path} {
set access_path [GetAccessPath $slave]
set where [lsearch -exact $access_path path] if {where == -1} {
return -code error “$path not found in access path $access_path”
}
return “$[PathToken $where]”
}

#
# addToAccessPath:
#    add (if needed) a real directory to access path
#    and return its virtual token (including the "$").

proc ::safe::interpAddToAccessPath {slave path} {
# first check if the directory is already in there
if {![catch {interpFindInAccessPath $slave $path} res]} {
return $res
}
# new one, add it:
set nname [PathNumberName $slave]
set n [Set $nname]
Set [PathToken $n $slave] $path

set token "\$[PathToken $n]"

Lappend [VirtualPathListName $slave] $token
Lappend [PathListName $slave] $path
Set $nname [expr {$n+1}]

SyncAccessPath $slave

return $token
}

# This procedure applies the initializations to an already existing
# interpreter. It is useful when you want to install the safe base
# aliases into a preexisting safe interpreter.
proc ::safe::InterpInit {
slave 
access_path
staticsok
nestedok
deletehook
} {

# Configure will generate an access_path when access_path is
# empty.
InterpSetConfig $slave $access_path $staticsok $nestedok $deletehook

# These aliases let the slave load files to define new commands

# NB we need to add [namespace current], aliases are always
# absolute paths.
::interp alias $slave source {} [namespace current]::AliasSource $slave
::interp alias $slave load {} [namespace current]::AliasLoad $slave

# This alias lets the slave use the encoding names, convertfrom,
# convertto, and system, but not "encoding system <name>" to set
# the system encoding.

::interp alias $slave encoding {} [namespace current]::AliasEncoding \
	$slave

# This alias lets the slave have access to a subset of the 'file'
# command functionality.

AliasSubset $slave file file dir.* join root.* ext.* tail \
	path.* split

# This alias interposes on the 'exit' command and cleanly terminates
# the slave.

::interp alias $slave exit {} [namespace current]::interpDelete $slave

# The allowed slave variables already have been set
# by Tcl_MakeSafe(3)


# Source init.tcl into the slave, to get auto_load and other
# procedures defined:

# We don't try to use the -rsrc on the mac because it would get
# confusing if you would want to customize init.tcl
# for a given set of safe slaves, on all the platforms
# you just need to give a specific access_path and
# the mac should be no exception. As there is no
# obvious full "safe ressources" design nor implementation
# for the mac, safe interps there will just don't
# have that ability. (A specific app can still reenable
# that using custom aliases if they want to).
# It would also make the security analysis and the Safe Tcl security
# model platform dependant and thus more error prone.

if {[catch {::interp eval $slave\
	{source [file join $tcl_library init.tcl]}} msg]} {
    Log $slave "can't source init.tcl ($msg)"
    error "can't source init.tcl into slave $slave ($msg)"
}

return $slave
}


# Add (only if needed, avoid duplicates) 1 level of
# sub directories to an existing path list.
# Also removes non directories from the returned list.
proc AddSubDirs {pathList} {
set res {}
foreach dir $pathList {
    if {[file isdirectory $dir]} {
	# check that we don't have it yet as a children
	# of a previous dir
	if {[lsearch -exact $res $dir]<0} {
	    lappend res $dir
	}
	foreach sub [glob -directory $dir -nocomplain *] {
	    if {([file isdirectory $sub]) \
		    && ([lsearch -exact $res $sub]<0) } {
		# new sub dir, add it !
                lappend res $sub
            }
	}
    }
}
return $res
}

# This procedure deletes a safe slave managed by Safe Tcl and
# cleans up associated state:

proc ::safe::interpDelete {slave} {

    Log $slave "About to delete" NOTICE

# If the slave has a cleanup hook registered, call it.
# check the existance because we might be called to delete an interp
# which has not been registered with us at all
set hookname [DeleteHookName $slave]
if {[Exists $hookname]} {
    set hook [Set $hookname]
    if {![::tcl::Lempty $hook]} {
	# remove the hook now, otherwise if the hook
	# calls us somehow, we'll loop
	Unset $hookname
	if {[catch {eval $hook [list $slave]} err]} {
	    Log $slave "Delete hook error ($err)"
	}
    }
}

# Discard the global array of state associated with the slave, and
# delete the interpreter.

set statename [InterpStateName $slave]
if {[Exists $statename]} {
    Unset $statename
}

# if we have been called twice, the interp might have been deleted
# already
if {[::interp exists $slave]} {
    ::interp delete $slave
    Log $slave "Deleted" NOTICE
}

return
}

# Set (or get) the loging mecanism 

proc ::safe::setLogCmd {args} {
variable Log
if {[llength $args] == 0} {
return $Log
} else {
if {[llength $args] == 1} {
set Log [lindex $args 0]
} else {
set Log $args
}
}
}

# internal variable
variable Log {}

# ------------------- END OF PUBLIC METHODS ------------


#
# sets the slave auto_path to the master recorded value.
# also sets tcl_library to the first token of the virtual path.
#
proc SyncAccessPath {slave} {
set slave_auto_path [Set [VirtualPathListName $slave]]
::interp eval $slave [list set auto_path $slave_auto_path]
Log $slave "auto_path in $slave has been set to $slave_auto_path"\
	NOTICE
::interp eval $slave [list set tcl_library [lindex $slave_auto_path 0]]
}

# base name for storing all the slave states
# the array variable name for slave foo is thus "Sfoo"
# and for sub slave {foo bar} "Sfoo bar" (spaces are handled
# ok everywhere (or should))
# We add the S prefix to avoid that a slave interp called "Log"
# would smash our "Log" variable.
proc InterpStateName {slave} {
return "S$slave"
}

# Check that the given slave is "one of us"
proc IsInterp {slave} {
expr {[Exists [InterpStateName $slave]] && [::interp exists $slave]}
}

# returns the virtual token for directory number N
# if the slave argument is given, 
# it will return the corresponding master global variable name
proc PathToken {n {slave ""}} {
if {$slave ne ""} {
    return "[InterpStateName $slave](access_path,$n)"
} else {
    # We need to have a ":" in the token string so
    # [file join] on the mac won't turn it into a relative
    # path.
    return "p(:$n:)"
}
}
# returns the variable name of the complete path list
proc PathListName {slave} {
return "[InterpStateName $slave](access_path)"
}
# returns the variable name of the complete path list
proc VirtualPathListName {slave} {
return "[InterpStateName $slave](access_path_slave)"
}
# returns the variable name of the number of items
proc PathNumberName {slave} {
return "[InterpStateName $slave](access_path,n)"
}
# returns the staticsok flag var name
proc StaticsOkName {slave} {
return "[InterpStateName $slave](staticsok)"
}
# returns the nestedok flag var name
proc NestedOkName {slave} {
return "[InterpStateName $slave](nestedok)"
}
# Run some code at the namespace toplevel
proc Toplevel {args} {
namespace eval [namespace current] $args
}
# set/get values
proc Set {args} {
eval [linsert $args 0 Toplevel set]
}
# lappend on toplevel vars
proc Lappend {args} {
eval [linsert $args 0 Toplevel lappend]
}
# unset a var/token (currently just an global level eval)
proc Unset {args} {
eval [linsert $args 0 Toplevel unset]
}
# test existance 
proc Exists {varname} {
Toplevel info exists $varname
}
# short cut for access path getting
proc GetAccessPath {slave} {
Set [PathListName $slave]
}
# short cut for statics ok flag getting
proc StaticsOk {slave} {
Set [StaticsOkName $slave]
}
# short cut for getting the multiples interps sub loading ok flag
proc NestedOk {slave} {
Set [NestedOkName $slave]
}
# interp deletion storing hook name
proc DeleteHookName {slave} {
return [InterpStateName $slave](cleanupHook)
}

#
# translate virtual path into real path
#
proc TranslatePath {slave path} {
# somehow strip the namespaces 'functionality' out (the danger
# is that we would strip valid macintosh "../" queries... :
if {[regexp {(::)|(\.\.)} $path]} {
    error "invalid characters in path $path"
}
set n [expr {[Set [PathNumberName $slave]]-1}]
for {} {$n>=0} {incr n -1} {
    # fill the token virtual names with their real value
    set [PathToken $n] [Set [PathToken $n $slave]]
}
# replaces the token by their value
subst -nobackslashes -nocommands $path
}


# Log eventually log an error
# to enable error logging, set Log to {puts stderr} for instance
proc Log {slave msg {type ERROR}} {
variable Log
if {[info exists Log] && [llength $Log]} {
    eval $Log [list "$type for slave $slave : $msg"]
}
}


# file name control (limit access to files/ressources that should be
# a valid tcl source file)
proc CheckFileName {slave file} {
# This used to limit what can be sourced to ".tcl" and forbid files
# with more than 1 dot and longer than 14 chars, but I changed that
# for 8.4 as a safe interp has enough internal protection already
# to allow sourcing anything. - hobbs

if {![file exists $file]} {
    # don't tell the file path
    error "no such file or directory"
}

if {![file readable $file]} {
    # don't tell the file path
    error "not readable"
}
}


# AliasSource is the target of the "source" alias in safe interpreters.

proc AliasSource {slave args} {

set argc [llength $args]
# Allow only "source filename"
# (and not mac specific -rsrc for instance - see comment in ::init
# for current rationale)
if {$argc != 1} {
    set msg "wrong # args: should be \"source fileName\""
    Log $slave "$msg ($args)"
    return -code error $msg
}
set file [lindex $args 0]

# get the real path from the virtual one.
if {[catch {set file [TranslatePath $slave $file]} msg]} {
    Log $slave $msg
    return -code error "permission denied"
}

# check that the path is in the access path of that slave
if {[catch {FileInAccessPath $slave $file} msg]} {
    Log $slave $msg
    return -code error "permission denied"
}

# do the checks on the filename :
if {[catch {CheckFileName $slave $file} msg]} {
    Log $slave "$file:$msg"
    return -code error $msg
}

# passed all the tests , lets source it:
if {[catch {::interp invokehidden $slave source $file} msg]} {
    Log $slave $msg
    return -code error "script error"
}
return $msg
}

# AliasLoad is the target of the "load" alias in safe interpreters.

proc AliasLoad {slave file args} {

set argc [llength $args]
if {$argc > 2} {
    set msg "load error: too many arguments"
    Log $slave "$msg ($argc) {$file $args}"
    return -code error $msg
}

# package name (can be empty if file is not).
set package [lindex $args 0]

# Determine where to load. load use a relative interp path
# and {} means self, so we can directly and safely use passed arg.
set target [lindex $args 1]
if {$target ne ""} {
    # we will try to load into a sub sub interp
    # check that we want to authorize that.
    if {![NestedOk $slave]} {
	Log $slave "loading to a sub interp (nestedok)\
		disabled (trying to load $package to $target)"
	return -code error "permission denied (nested load)"
    }
    
}

# Determine what kind of load is requested
if {$file eq ""} {
    # static package loading
    if {$package eq ""} {
	set msg "load error: empty filename and no package name"
	Log $slave $msg
	return -code error $msg
    }
    if {![StaticsOk $slave]} {
	Log $slave "static packages loading disabled\
		(trying to load $package to $target)"
	return -code error "permission denied (static package)"
    }
} else {
    # file loading

    # get the real path from the virtual one.
    if {[catch {set file [TranslatePath $slave $file]} msg]} {
	Log $slave $msg
	return -code error "permission denied"
    }

    # check the translated path
    if {[catch {FileInAccessPath $slave $file} msg]} {
	Log $slave $msg
	return -code error "permission denied (path)"
    }
}

if {[catch {::interp invokehidden\
	$slave load $file $package $target} msg]} {
    Log $slave $msg
    return -code error $msg
}

return $msg
}

# FileInAccessPath raises an error if the file is not found in
# the list of directories contained in the (master side recorded) slave's
# access path.

# the security here relies on "file dirname" answering the proper
# result.... needs checking ?
proc FileInAccessPath {slave file} {

set access_path [GetAccessPath $slave]

if {[file isdirectory $file]} {
    error "\"$file\": is a directory"
}
set parent [file dirname $file]

# Normalize paths for comparison since lsearch knows nothing of
# potential pathname anomalies.
set norm_parent [file normalize $parent]
foreach path $access_path {
    lappend norm_access_path [file normalize $path]
}

if {[lsearch -exact $norm_access_path $norm_parent] == -1} {
    error "\"$file\": not in access_path"
}
}

# This procedure enables access from a safe interpreter to only a subset of
# the subcommands of a command:

proc Subset {slave command okpat args} {
set subcommand [lindex $args 0]
if {[regexp $okpat $subcommand]} {
    return [eval [linsert $args 0 $command]]
}
set msg "not allowed to invoke subcommand $subcommand of $command"
Log $slave $msg
error $msg
}

# This procedure installs an alias in a slave that invokes "safesubset"
# in the master to execute allowed subcommands. It precomputes the pattern
# of allowed subcommands; you can use wildcards in the pattern if you wish
# to allow subcommand abbreviation.
#
# Syntax is: AliasSubset slave alias target subcommand1 subcommand2...

proc AliasSubset {slave alias target args} {
set pat ^(; set sep ""
foreach sub $args {
    append pat $sep$sub
    set sep |
}
append pat )\$
::interp alias $slave $alias {}\
	[namespace current]::Subset $slave $target $pat
}

# AliasEncoding is the target of the "encoding" alias in safe interpreters.

proc AliasEncoding {slave args} {

set argc [llength $args]

set okpat "^(name.*|convert.*)\$"
set subcommand [lindex $args 0]

if {[regexp $okpat $subcommand]} {
    return [eval [linsert $args 0 \
	    ::interp invokehidden $slave encoding]]
}

if {[string first $subcommand system] == 0} {
    if {$argc == 1} {
	# passed all the tests , lets source it:
	if {[catch {::interp invokehidden \
		$slave encoding system} msg]} {
	    Log $slave $msg
	    return -code error "script error"
	}
    } else {
	set msg "wrong # args: should be \"encoding system\""
	Log $slave $msg
	error $msg
    }
} else {
    set msg "wrong # args: should be \"encoding option ?arg ...?\""
    Log $slave $msg
    error $msg
}

return $msg
}

}