NAME
       Safe Base - A mechanism for creating and manipulating safe
       interpreters.

SYNOPSIS
       ::safe::interpCreate ?slave? ?options...?

       ::safe::interpInit slave ?options...?

       ::safe::interpConfigure slave ?options...?

       ::safe::interpDelete slave

       ::safe::interpAddToAccessPath slave directory

       ::safe::interpFindInAccessPath slave directory

       ::safe::setLogCmd ?cmd arg...?

OPTIONS
       ?-accessPath pathList?   ?-statics  boolean?  ?-noStatics?
       ?-nested boolean? ?-nestedLoadOk?  ?-deleteHook script?


DESCRIPTION
       Safe  Tcl  is  a  mechanism  for  executing  untrusted Tcl
       scripts safely and for providing mediated access  by  such
       scripts to potentially dangerous functionality.

       The  Safe  Base  ensures that untrusted Tcl scripts cannot
       harm the hosting  application.   The  Safe  Base  prevents
       integrity  and  privacy attacks. Untrusted Tcl scripts are
       prevented from corrupting the state of the hosting  appli-
       cation  or  computer. Untrusted scripts are also prevented
       from disclosing information stored on the hosting computer
       or in the hosting application to any party.

       The  Safe Base allows a master interpreter to create safe,
       restricted interpreters that contain a set  of  predefined
       aliases  for  the source, load, file and exit commands and
       are able to use the auto-loading and package mechanisms.

       No knowledge of the file system structure is leaked to the
       safe interpreter, because it has access only to a virtual-
       ized path containing tokens.  When  the  safe  interpreter
       requests  to  source a file, it uses the token in the vir-
       tual path as part of the file name to source;  the  master
       interpreter transparently translates the token into a real
       directory name and executes the requested  operation  (see
       the section SECURITY below for details).  Different levels
       of security can be selected by using the optional flags of
       the commands described below.

       All  commands  provided  in  the master interpreter by the
       Safe Base reside in the safe namespace:


COMMANDS
       The following commands are provided in the  master  inter-
       preter:

       ::safe::interpCreate ?slave? ?options...?
              Creates  a  safe  interpreter, installs the aliases
              described in the section  ALIASES  and  initializes
              the auto-loading and package mechanism as specified
              by the supplied options.  See the  OPTIONS  section
              below  for a description of the optional arguments.
              If the slave argument is omitted, a  name  will  be
              generated.  ::safe::interpCreate always returns the
              interpreter name.

       ::safe::interpInit slave ?options...?
              This command is similar to interpCreate  except  it
              that  does  not  create the safe interpreter. slave
              must have been created by some  other  means,  like
              interp create -safe.

       ::safe::interpConfigure slave ?options...?
              If  no  options are given, returns the settings for
              all options for the named  safe  interpreter  as  a
              list  of  options and their current values for that
              slave.  If a single  additional  argument  is  pro-
              vided, it will return a list of 2 elements name and
              value where name is the full name  of  that  option
              and value the current value for that option and the
              slave.  If more than two additional  arguments  are
              provided,  it will reconfigure the safe interpreter
              and change each and only the provided options.  See
              the  section  on OPTIONS below for options descrip-
              tion.  Example of use:
                     # Create a new interp with the same configuration as "$i0" :
                     set i1 [eval safe::interpCreate [safe::interpConfigure $i0]]
                     # Get the current deleteHook
                     set dh [safe::interpConfigure $i0  -del]
                     # Change (only) the statics loading ok attribute of an interp
                     # and its deleteHook (leaving the rest unchanged) :
                     safe::interpConfigure $i0  -delete {foo bar} -statics 0 ;

       ::safe::interpDelete slave
              Deletes the safe interpreter and cleans up the cor-
              responding  master interpreter data structures.  If
              a deleteHook script was specified for  this  inter-
              preter  it  is  evaluated before the interpreter is
              deleted, with the name of  the  interpreter  as  an
              additional argument.

       ::safe::interpFindInAccessPath slave directory
              This  command  finds  and returns the token for the
              real directory directory in the safe  interpreter's
              current virtual access path.  It generates an error
              if the directory is not found.  Example of use:
                     $slave eval [list set tk_library [::safe::interpFindInAccessPath $name $tk_library]]

       ::safe::interpAddToAccessPath slave directory
              This command adds directory  to  the  virtual  path
              maintained  for the safe interpreter in the master,
              and returns the token that can be used in the  safe
              interpreter  to  obtain  access  to  files  in that
              directory.  If the directory is already in the vir-
              tual path, it only returns the token without adding
              the directory to the virtual path  again.   Example
              of use:
                     $slave eval [list set tk_library [::safe::interpAddToAccessPath $name $tk_library]]

       ::safe::setLogCmd ?cmd arg...?
              This  command installs a script that will be called
              when interesting life cycle events occur for a safe
              interpreter.   When  called  with  no arguments, it
              returns  the  currently  installed  script.    When
              called with one argument, an empty string, the cur-
              rently installed script is removed and  logging  is
              turned  off.   The  script will be invoked with one
              additional argument, a string describing the  event
              of interest.  The main purpose is to help in debug-
              ging safe interpreters.  Using  this  facility  you
              can  get  complete  error  messages  while the safe
              interpreter gets only generic error messages.  This
              prevents  a  safe  interpreter from seeing messages
              about failures and other events that might  contain
              sensitive information such as real directory names.
              Example of use:
                     ::safe::setLogCmd puts stderr
              Below is the output of a sample session in which  a
              safe  interpreter  attempted  to  source a file not
              found in its virtual access path.   Note  that  the
              safe  interpreter  only  received  an error message
              saying that the file was not found:
                     NOTICE for slave interp10 : Created
                     NOTICE for slave interp10 : Setting accessPath=(/foo/bar) staticsok=1 nestedok=0 deletehook=()
                     NOTICE for slave interp10 : auto_path in interp10 has been set to {$p(:0:)}
                     ERROR for slave interp10 : /foo/bar/init.tcl: no such file or directory


OPTIONS
       The following options are common to  ::safe::interpCreate,
       ::safe::interpInit,   and   ::safe::interpConfigure.   Any
       option name can be abbreviated to its minimal  non-ambigu-
       ous name.  Option names are not case sensitive.

       -accessPath directoryList
              This option sets the list of directories from which
              the safe interpreter can source and load files.  If
              this  option is not specified, or if it is given as
              the empty list, the safe interpreter will  use  the
              same  directories  as  its master for auto-loading.
              See the section  SECURITY  below  for  more  detail
              about virtual paths, tokens and access control.

       -statics boolean
              This  option specifies if the safe interpreter will
              be allowed to load statically linked packages (like
              load  {}  Tk).   The  default  value is true : safe
              interpreters are allowed to load statically  linked
              packages.

       -noStatics
              This  option is a convenience shortcut for -statics
              false and thus specifies that the safe  interpreter
              will not be allowed to load statically linked pack-
              ages.

       -nested boolean
              This option specifies if the safe interpreter  will
              be allowed to load packages into its own sub-inter-
              preters.  The default value is false : safe  inter-
              preters are not allowed to load packages into their
              own sub-interpreters.

       -nestedLoadOk
              This option is a convenience shortcut  for  -nested
              true  and  thus specifies the safe interpreter will
              be allowed to load packages into its own sub-inter-
              preters.

       -deleteHook script
              When  this  option is given an non empty script, it
              will be evaluated in the master with  the  name  of
              the safe interpreter as an additional argument just
              before  actually  deleting  the  safe  interpreter.
              Giving   an   empty  value  removes  any  currently
              installed deletion hook script for that safe inter-
              preter.   The default value ({}) is not to have any
              deletion call back.

ALIASES
       The following aliases are provided in a safe interpreter:

       source fileName
              The requested file, a Tcl source file,  is  sourced
              into  the  safe  interpreter  if  it is found.  The
              source alias can only source files from directories
              in  the  virtual path for the safe interpreter. The
              source alias requires the safe interpreter  to  use
              one  of  the  token  names  in  its virtual path to
              denote the  directory  in  which  the  file  to  be
              sourced  can be found.  See the section on SECURITY
              for more discussion of restrictions on valid  file-
              names.

       load fileName
              The requested file, a shared object file, is dynam-
              ically loaded into the safe interpreter  if  it  is
              found.  The filename must contain a token name men-
              tioned in the virtual path for the safe interpreter
              for it to be found successfully.  Additionally, the
              shared object file must contain a safe entry point;
              see  the  manual page for the load command for more
              details.

       file ?subCmd args...?
              The file alias provides access to a safe subset  of
              the subcommands of the file command; it allows only
              dirname, join, extension, root, tail, pathname  and
              split  subcommands.  For more details on what these
              subcommands do see the manual  page  for  the  file
              command.

       exit   The calling interpreter is deleted and its computa-
              tion is stopped, but the Tcl process in which  this
              interpreter exists is not terminated.


SECURITY
       The  Safe  Base  does  not  attempt  to completely prevent
       annoyance and denial of service attacks.  These  forms  of
       attack  prevent  the  application or user from temporarily
       using the computer to perform useful work, for example  by
       consuming  all  available CPU time or all available screen
       real estate.  These attacks, while aggravating, are deemed
       to  be  of lesser importance in general than integrity and
       privacy attacks that the Safe Base is to prevent.

       The commands available in a safe interpreter, in  addition
       to  the  safe  set  as  defined in interp manual page, are
       mediated aliases for source, load, exit, and a safe subset
       of file.  The safe interpreter can also auto-load code and
       it can request that packages be loaded.

       Because some of these commands access the local file  sys-
       tem,  there  is  a potential for information leakage about
       its directory structure.  To prevent this,  commands  that
       take  file  names  as  arguments in a safe interpreter use
       tokens instead of the real directory names.  These  tokens
       are  translated to the real directory name while a request
       to, e.g., source a file is mediated by the  master  inter-
       preter.   This  virtual  path  system is maintained in the
       master interpreter for each safe  interpreter  created  by
       ::safe::interpCreate  or initialized by ::safe::interpInit
       and the path maps tokens accessible  in  the  safe  inter-
       preter  into real path names on the local file system thus
       preventing safe interpreters from gaining knowledge  about
       the  structure of the file system of the host on which the
       interpreter is executing.  The only valid file names argu-
       ments  for  the  source  and  load aliases provided to the
       slave are path in the form of [file join  token  filename]
       (ie,  when using the native file path formats: token/file-
       name on Unix, token\filename on Windows,  and  token:file-
       name  on  the Mac), where token is representing one of the
       directories of the accessPath list  and  filename  is  one
       file  in  that  directory  (no  sub directories access are
       allowed).

       When a token is used in a safe interpreter in a request to
       source or load a file, the token is checked and translated
       to a real path name and the file to be sourced  or  loaded
       is located on the file system.  The safe interpreter never
       gains knowledge of the actual path name  under  which  the
       file is stored on the file system.

       To further prevent potential information leakage from sen-
       sitive files that are accidentally included in the set  of
       files  that  can  be  sourced  by  a safe interpreter, the
       source alias restricts access to files meeting the follow-
       ing constraints: the file name must fourteen characters or
       shorter, must not contain more than one  dot  ("."),  must
       end up with the extension .tcl or be called tclIndex.

       Each  element  of  the  initial  access  path list will be
       assigned a token that will be set in the  slave  auto_path
       and  the  first  element  of  that list will be set as the
       tcl_library for that slave.

       If the access path argument is not given or is  the  empty
       list,  the default behavior is to let the slave access the
       same packages as the master has access to (Or to  be  more
       precise: only packages written in Tcl (which by definition
       can't be dangerous as they run in the  slave  interpreter)
       and  C  extensions that provides a Safe_Init entry point).
       For that purpose, the master's auto_path will be  used  to
       construct  the slave access path.  In order that the slave
       successfully loads the Tcl library files  (which  includes
       the auto-loading mechanism itself) the tcl_library will be
       added or moved to the first position if necessary, in  the
       slave  access  path,  so the slave tcl_library will be the
       same as the master's (its real path will still be  invisi-
       ble  to  the  slave  though).   In order that auto-loading
       works the same for the slave and the  master  in  this  by
       default  case,  the  first-level  sub  directories of each
       directory in the master auto_path will also be  added  (if
       not  already  included) to the slave access path.  You can
       always specify a  more  restrictive  path  for  which  sub
       directories  will never be searched by explicitly specify-
       ing your directory list with the -accessPath flag  instead
       of relying on this default mechanism.
       When the accessPath is changed after the first creation or
       initialization  (ie  through  interpConfigure  -accessPath
       list),  an  auto_reset  is  automatically evaluated in the
       safe interpreter to synchronize its  auto_index  with  the
       new token list.


SEE ALSO
       interp(n),  library(n),  load(n),  package(n),  source(n),
       unknown(n)


KEYWORDS
       alias, auto-loading,  auto_mkindex,  load,  master  inter-
       preter, safe interpreter, slave interpreter, source
