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       safe - Creating and manipulating safe interpreters


       ::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...?

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


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

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

       Safe  Tcl allows a master interpreter to create safe, restricted interpreters that contain
       a set of predefined aliases for the source, load, file, encoding, 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  virtualized  path  containing  tokens.  When  the  safe  interpreter
       requests  to source a file, it uses the token in the virtual 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 Safe Tcl reside in the safe namespace.


       The following commands are provided in the master interpreter:

       ::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::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 provided, 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 description.  Example of

                     # Create new interp with the same configuration as "$i0":
                     set i1 [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  corresponding  master  interpreter
              data  structures.   If a deleteHook script was specified for this interpreter 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
              virtual 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
              currently 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 debugging 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

       The  following  options  are  common  to  ::safe::interpCreate,  ::safe::interpInit,   and
       ::safe::interpConfigure.   Any option name can be abbreviated to its minimal non-ambiguous
       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.

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

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

              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-interpreters.

       -deleteHook script
              When this option is given a 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 interpreter.  The default
              value ({}) is not to have any deletion call back.


       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 filenames.

       load fileName
              The  requested  file,  a  shared  object  file, is dynamically loaded into the safe
              interpreter if it is found.  The filename must contain a token  name  mentioned  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.

       encoding ?subCmd args...?
              The encoding alias provides access to a safe  subset  of  the  subcommands  of  the
              encoding  command;   it  disallows  setting  of the system encoding, but allows all
              other subcommands including system to check the current encoding.

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


       Safe  Tcl  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 Safe Tcl 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 safe subsets of  file
       and  encoding.  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 system,  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  interpreter.   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  interpreter  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 arguments
       for the source and load aliases provided to the slave are path in the form of  [file  join
       token  filename] (i.e. when using the native file path formats: token/filename on Unix and
       token\filename on Windows), 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

       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  sensitive  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 following 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 cannot be dangerous as they run in the
       slave interpreter) and C extensions that provides  a  _SafeInit  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 invisible 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 specifying 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  (i.e.  through
       interpConfigure  -accessPath  list),  an auto_reset is automatically evaluated in the safe
       interpreter to synchronize its auto_index with the new token list.


       interp(3tcl), library(3tcl), load(3tcl), package(3tcl), source(3tcl), unknown(3tcl)


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