This command makes it possible to create one or more new Tcl
interpreters that co-exist with the creating interpreter in the
same application. The creating interpreter is called the ``master''
and the new interpreter is called a ``slave''.
A master can create any number of slaves, and each slave can
itself create additional slaves for which it is master, resulting
in a hierarchy of interpreters.
Each interpreter is independent from the others: it has its own name
space for commands, procedures, and global variables.
A master interpreter may create connections between its slaves and
itself using a mechanism called an alias. An alias is
a command in a slave interpreter which, when invoked, causes a
command to be invoked in its master interpreter or in another slave
interpreter. The only other connections between interpreters are
through environment variables (the env variable), which are
normally shared among all interpreters in the application. Note that the
name space for files (such as the names returned by the open command)
is no longer shared between interpreters. Explicit commands are provided to
share files and to transfer references to open files from one interpreter
to another.
The interp command also provides support for safe
interpreters. A safe interpreter is a slave whose functions have
been greatly restricted, so that it is safe to execute untrusted
scripts without fear of them damaging other interpreters or the
application's environment. For example, all IO channel creation
commands and subprocess creation commands are removed from safe interpreters.
See "Safe interpreters" below for more information on what features
are present in a safe interpreter. The alias mechanism can be
used for protected communication (analogous to a kernel call)
between a slave interpreter and its master.
A qualified interpreter name is a proper Tcl lists containing a subset of its
ancestors in the interpreter hierarchy, terminated by the string naming the
interpreter in its immediate master. Interpreter names are relative to the
interpreter in which they are used. For example, if a is a slave of
the current interpreter and it has a slave a1, which in turn has a
slave a11, the qualified name of a11 in a is the list
{a1 a11}.
The interp command, described below, accepts qualified interpreter
names as arguments; the interpreter in which the command is being evaluated
can always be referred to as {} (the empty list or string). Note that
it is impossible to refer to a master (ancestor) interpreter by name in a
slave interpreter except through aliases. Also, there is no global name by
which one can refer to the first interpreter created in an application.
Both restrictions are motivated by safety concerns.
The interp command is used to create, delete, and manipulate
slave interpreters, and to share or transfer
channels between interpreters. It can have any of several forms, depending
on the option argument:
interpaliassrcPathsrcCmd
Returns a Tcl list whose elements are the targetCmd and
args associated with the alias named srcCmd
(all of these are the values specified when the alias was
created; it is possible that the actual source command in the
slave is different from srcCmd if it was renamed).
interpaliassrcPathsrcCmd{}
Deletes the alias for srcCmd in the slave interpreter identified by
srcPath.
srcCmd refers to the name under which the alias
was created; if the source command has been renamed, the renamed
command will be deleted.
This command creates an alias between one slave and another (see the
alias slave command below for creating aliases between a slave
and its master). In this command, either of the slave interpreters
may be anywhere in the hierarchy of interpreters under the interpreter
invoking the command.
srcPath and srcCmd identify the source of the alias.
srcPath is a Tcl list whose elements select a particular
interpreter. For example, ``a b'' identifies an interpreter
b, which is a slave of interpreter a, which is a slave
of the invoking interpreter. An empty list specifies the interpreter
invoking the command. srcCmd gives the name of a new
command, which will be created in the source interpreter.
targetPath and targetCmd specify a target interpreter
and command, and the arg arguments, if any, specify additional
arguments to targetCmd which are prepended to any arguments specified
in the invocation of srcCmd.
targetCmd may be undefined at the time of this call, or it may
already exist; it is not created by this command.
The alias arranges for the given target command to be invoked
in the target interpreter whenever the given source command is
invoked in the source interpreter. See "Alias invocation" below for
more details.
interpaliases [path]
This command returns a Tcl list of the names of all the source commands for
aliases defined in the interpreter identified by path.
interpcreate [-safe? [--? [path]
Creates a slave interpreter identified by path and a new command,
called a ``slave command''. The name of the slave command is the last
component of path. The new slave interpreter and the slave command
are created in the interpreter identified by the path obtained by removing
the last component from path. For example, if path
is ``a b c'' then a new slave interpreter and slave command
named ``c'' are created in the interpreter identified by the path
``a b''.
The slave command may be used to manipulate the new interpreter as
described below. If path is omitted, Tcl creates a unique name of the
form interpx, where x is an integer, and uses it for the
interpreter and the slave command. If the -safe switch is specified
(or if the master interpreter is a safe interpreter), the new slave
interpreter will be created as a safe interpreter with limited
functionality; otherwise the slave will include the full set of Tcl
built-in commands and variables. The -- switch can be used to
mark the end of switches; it may be needed if path is an unusual
value such as -safe. The result of the command is the name of the
new interpreter. The name of a slave interpreter must be unique among all
the slaves for its master; an error occurs if a slave interpreter by the
given name already exists in this master.
interpdelete [path ...]
Deletes zero or more interpreters given by the optional path
arguments, and for each interpreter, it also deletes its slaves. The
command also deletes the slave command for each interpreter deleted.
For each path argument, if no interpreter by that name
exists, the command raises an error.
interpevalpath arg [arg ...]
This command concatenates all of the arg arguments in the same
fashion as the concat command, then evaluates the resulting string as
a Tcl script in the slave interpreter identified by path. The result
of this evaluation (including error information such as the errorInfo
and errorCode variables, if an error occurs) is returned to the
invoking interpreter.
interpexistspath
Returns 1 if a slave interpreter by the specified path
exists in this master, 0 otherwise. If path is omitted, the
invoking interpreter is used.
interpissafe [path]
Returns 1 if the interpreter identified by the specified path
is safe, 0 otherwise.
interpsharesrcPathchannelId destPath
Causes the IO channel identified by channelId to become shared
between the interpreter identified by srcPath and the interpreter
identified by destPath. Both interpreters have the same permissions
on the IO channel.
Both interpreters must close it to close the underlying IO channel; IO
channels accessible in an interpreter are automatically closed when an
interpreter is destroyed.
interpslaves [path]
Returns a Tcl list of the names of all the slave interpreters associated
with the interpreter identified by path. If path is omitted,
the invoking interpreter is used.
interptargetpath alias
Returns a Tcl list describing the target interpreter for an alias. The
alias is specified with an interpreter path and source command name, just
as in interp alias above. The name of the target interpreter is
returned as an interpreter path, relative to the invoking interpreter.
If the target interpreter for the alias is the invoking interpreter then an
empty list is returned. If the target interpreter for the alias is not the
invoking interpreter or one of its descendants then an error is generated.
The target command does not have to be defined at the time of this invocation.
interptransfersrcPathchannelId destPath
Causes the IO channel identified by channelId to become available in
the interpreter identified by destPath and unavailable in the
interpreter identified by srcPath.
Slave command
For each slave interpreter created with the interp command, a
new Tcl command is created in the master interpreter with the same
name as the new interpreter. This command may be used to invoke
various operations on the interpreter. It has the following
general form:
slave command [arg arg ...]
slave is the name of the interpreter, and command
and the args determine the exact behavior of the command.
The valid forms of this command are:
slavealiases
Returns a Tcl list whose elements are the names of all the
aliases in slave. The names returned are the srcCmd
values used when the aliases were created (which may not be the same
as the current names of the commands, if they have been
renamed).
slavealiassrcCmd
Returns a Tcl list whose elements are the targetCmd and
args associated with the alias named srcCmd
(all of these are the values specified when the alias was
created; it is possible that the actual source command in the
slave is different from srcCmd if it was renamed).
slavealiassrcCmd{}
Deletes the alias for srcCmd in the slave interpreter.
srcCmd refers to the name under which the alias
was created; if the source command has been renamed, the renamed
command will be deleted.
slavealiassrcCmd targetCmd[arg ...]
Creates an alias such that whenever srcCmd is invoked
in slave, targetCmd is invoked in the master.
The arg arguments will be passed to targetCmd as additional
arguments, prepended before any arguments passed in the invocation of
srcCmd.
See "Alias invocation" below for details.
slaveevalarg [arg ...]
This command concatenates all of the arg arguments in
the same fashion as the concat command, then evaluates
the resulting string as a Tcl script in slave.
The result of this evaluation (including error information
such as the errorInfo and errorCode variables, if an
error occurs) is returned to the invoking interpreter.
slaveissafe
Returns 1 if the slave interpreter is safe, 0 otherwise.
Alias invocation
The alias mechanism has been carefully designed so that it can
be used safely when an untrusted script is executing
in a safe slave and the target of the alias is a trusted
master. The most important thing in guaranteeing safety is to
ensure that information passed from the slave to the master is
never evaluated or substituted in the master; if this were to
occur, it would enable an evil script in the slave to invoke
arbitrary functions in the master, which would compromise security.
When the source for an alias is invoked in the slave interpreter, the
usual Tcl substitutions are performed when parsing that command.
These substitutions are carried out in the source interpreter just
as they would be for any other command invoked in that interpreter.
The command procedure for the source command takes its arguments
and merges them with the targetCmd and args for the
alias to create a new array of arguments. If the words
of srcCmd were ``srcCmd arg1 arg2 ... argN'',
the new set of words will be
``targetCmd arg arg ... arg arg1 arg2 ... argN'',
where targetCmd and args are the values supplied when the
alias was created. targetCmd is then used to locate a command
procedure in the target interpreter, and that command procedure
is invoked with the new set of arguments. An error occurs if
there is no command named targetCmd in the target interpreter.
No additional substitutions are performed on the words: the
target command procedure is invoked directly, without
going through the normal Tcl evaluation mechanism.
Substitutions are thus performed on each word exactly once:
targetCmd and args were substituted when parsing the command
that created the alias, and arg1 - argN are substituted when
the alias's source command is parsed in the source interpreter.
When writing the targetCmd for aliases in safe interpreters,
it is very important that the arguments to that command never be
evaluated or substituted, since this would provide an escape
mechanism whereby the slave interpreter could execute arbitrary
code in the master. This in turn would compromise the security
of the system.
Safe interpreters
A safe interpreter is one with restricted functionality, so that
is safe to execute an arbitrary script from your worst enemy without
fear of that script damaging the enclosing application or the rest
of your computing environment. In order to make an interpreter
safe, certain commands and variables are removed from the interpreter.
For example, commands to create files on disk are removed, and the
exec command is removed, since it could be used to cause damage
through subprocesses.
Limited access to these facilities can be provided, by creating
aliases to the master interpreter which check their arguments carefully
and provide restricted access to a safe subset of facilities.
For example, file creation might be allowed in a particular subdirectory
and subprocess invocation might be allowed for a carefully selected and
fixed set of programs.
A safe interpreter is created by specifying the -safe switch
to the interp create command. Furthermore, any slave created
by a safe interpreter will also be safe.
A safe interpreter is created with exactly the following set of
built-in commands:
after
append
array
break
case
catch
clock
close
concat
continue
eof
error
eval
expr
fblocked
fileevent
flush
for
foreach
format
gets
global
history
if
incr
info
interp
join
lappend
lindex
linsert
list
llength
lower
lrange
lreplace
lsearch
lsort
package
pid
proc
puts
read
rename
return
scan
seek
set
split
string
subst
switch
tell
trace
unset
update
uplevel
upvar
vwait
while
All commands not on this list are removed by interp create when it
creates a safe interpreter.
These commands can be recreated later as Tcl procedures or aliases.
In addition, the env variable is not present in a safe interpreter,
so it cannot share environment variables with other interpreters. The
env variable poses a security risk, because users can store
sensitive information in an environment variable. For example, the PGP
manual recommends storing the PGP private key protection password in
the environment variable PGPPASS. Making this variable available
to untrusted code executing in a safe interpreter would incur a
security risk.
If extensions are loaded into a safe interpreter, they may also restrict
their own functionality to eliminate unsafe commands. For a discussion of
management of extensions for safety see the manual entries for the
package and load Tcl commands.
Credits
This mechanism is based on the Safe-Tcl prototype implemented
by Nathaniel Borenstein and Marshall Rose.