source: data/Media/tcl8.4/safe.tcl @ 5135

# 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
    }

}