NAME

interp - Create and manipulate Tcl interpreters

SYNOPSIS

interp option ?arg arg ...?

DESCRIPTION

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. It can have any of several forms, depending on the option argument:

interp alias srcPath srcCmd
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).
interp alias srcPath srcCmd {}
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.
interp alias srcPath srcCmd targetPath targetCmd ?arg arg ...?
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.
interp aliases ?path?
This command returns a Tcl list of the names of all the source commands for aliases defined in the interpreter identified by path.
interp create ?-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.
interp delete ?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.
interp eval path 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.
interp exists path
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.
interp issafe ?path?
Returns 1 if the interpreter identified by the specified path is safe, 0 otherwise.
interp share srcPath channelId 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.
interp slaves ?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.
interp target path 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.
interp transfer srcPath channelId 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:

slave aliases
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).
slave alias srcCmd
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).
slave alias srcCmd {}
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.
slave alias srcCmd 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.
slave eval 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 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.
slave issafe
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 targetCmds 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: 

append	array	break	case
catch	clock	close	concat
continue	eof	error	eval
expr	fblocked	flush	for
foreach	format	gets	global
history	if	incr	info
interp	join	lappend	lindex
list	llength	lrange	lreplace
pid	proc	puts	read
regexp	regsub	rename	return
scan	set	seek	split
string	switch	tell	trace
All commands not on this list are removed from the interpreter by the interp create command. Of course, the missing 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. The management of extensions for safety will be explained in 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.

SEE ALSO

load(n), package(n) Tcl_CreateSlave(3)

KEYWORDS

alias, master interpreter, safe interpreter, slave interpreter 
Copyright © 1995-1996 Sun Microsystems, Inc.
Copyright © 1995, 1996 Roger E. Critchlow Jr.