interp.n

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as the current names of the commands, if they have been
renamed).
.TP
\fIslave \fBalias \fIsrcCmd\fR
Returns a Tcl list whose elements are the \fItargetCmd\fR and
\fIarg\fRs associated with the alias named \fIsrcCmd\fR
(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 \fIsrcCmd\fR if it was renamed).
.TP
\fIslave \fBalias \fIsrcCmd \fB{}\fR
Deletes the alias for \fIsrcCmd\fR in the slave interpreter.
\fIsrcCmd\fR refers to the name under which the alias
was created;  if the source command has been renamed, the renamed
command will be deleted.
.TP
\fIslave \fBalias \fIsrcCmd targetCmd \fR?\fIarg ..\fR?
Creates an alias such that whenever \fIsrcCmd\fR is invoked
in \fIslave\fR, \fItargetCmd\fR is invoked in the master.
The \fIarg\fR arguments will be passed to \fItargetCmd\fR as additional
arguments, prepended before any arguments passed in the invocation of
\fIsrcCmd\fR.
See ALIAS INVOCATION below for details.
.TP
\fIslave \fBeval \fIarg \fR?\fIarg ..\fR?
This command concatenates all of the \fIarg\fR arguments in
the same fashion as the \fBconcat\fR command, then evaluates
the resulting string as a Tcl script in \fIslave\fR.
The result of this evaluation (including error information
such as the \fBerrorInfo\fR and \fBerrorCode\fR variables, if an
error occurs) is returned to the invoking interpreter.
.VS "" BR
.TP
\fIslave \fBexpose \fIhiddenName \fR?\fIexposedCmdName\fR?
This command exposes the hidden command \fIhiddenName\fR, eventually bringing
it back under a new \fIexposedCmdName\fR name (this name is currently
accepted only if it is a valid global name space name without any ::),
in \fIslave\fR.
If an exposed command with the targetted name already exists, this command
fails.
For more details on hidden commands, see HIDDEN COMMANDS, below.
.TP
\fIslave \fBhide \fIexposedCmdName\fR ?\fIhiddenCmdName\fR?
This command hides the exposed command \fIexposedCmdName\fR, renaming it to 
the hidden command \fIhiddenCmdName\fR, or keeping the same name if the
the argument is not given, in the \fIslave\fR interpreter.
If a hidden command with the targetted name already exists, this command
fails.
Currently both \fIexposedCmdName\fR and \fIhiddenCmdName\fR can 
not contain namespace qualifiers, or an error is raised.
Commands to be hidden are looked up in the global
namespace even if the current namespace is not the global one. This
prevents slaves from fooling a master interpreter into hiding the wrong
command, by making the current namespace be different from the global one.
For more details on hidden commands, see HIDDEN COMMANDS, below.
.TP
\fIslave \fBhidden\fR
Returns a list of the names of all hidden commands in \fIslave\fR.
.TP
\fIslave \fBinvokehidden\fR ?\fB-global\fR \fIhiddenName \fR?\fIarg ..\fR?
This command invokes the hidden command \fIhiddenName\fR with the
supplied arguments, in \fIslave\fR. No substitutions or evaluations are
applied to the arguments.
If the \fB-global\fR flag is given, the command is invoked at the global
level in the slave; otherwise it is invoked at the current call frame and
can access local variables in that or outer call frames.
For more details on hidden commands, see HIDDEN
COMMANDS, below.
.VE
.TP
\fIslave \fBissafe\fR
Returns  \fB1\fR if the slave interpreter is safe, \fB0\fR otherwise.
.VS "" BR
.TP
\fIslave \fBmarktrusted\fR
Marks the slave interpreter as trusted. Can only be invoked by a
trusted interpreter. This command does not expose any hidden
commands in the slave interpreter. The command has no effect if the slave
is already trusted.
.VE

.SH "SAFE INTERPRETERS"
.PP
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
\fBexec\fR 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.
.PP
A safe interpreter is created by specifying the \fB\-safe\fR switch
to the \fBinterp create\fR command.  Furthermore, any slave created
by a safe interpreter will also be safe.
.PP
A safe interpreter is created with exactly the following set of
built-in commands:
.DS
.ta 1.2i 2.4i 3.6i
\fBafter	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\fR
.DE
.VS ""  BR
The following commands are hidden by \fBinterp create\fR when it
creates a safe interpreter:
.DS
.ta 1.2i 2.4i 3.6i
\fBcd	exec	exit	fconfigure
file	glob	load	open
pwd	socket	source	vwait\fR
.DE
These commands can be recreated later as Tcl procedures or aliases, or
re-exposed by \fBinterp expose\fR.
.VE
.PP
In addition, the \fBenv\fR variable is not present in a safe interpreter,
so it cannot share environment variables with other interpreters. The
\fBenv\fR 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 \fIPGPPASS\fR. Making this variable available
to untrusted code executing in a safe interpreter would incur a
security risk.
.PP
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
\fBSafe\-Tcl\fR and the \fBload\fR Tcl command.

.SH "ALIAS INVOCATION"
.PP
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.
.PP
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 \fItargetCmd\fR and \fIarg\fRs for the
alias to create a new array of arguments.  If the words
of \fIsrcCmd\fR were ``\fIsrcCmd arg1 arg2 ... argN\fR'',
the new set of words will be
``\fItargetCmd arg arg ... arg arg1 arg2 ... argN\fR'',
where \fItargetCmd\fR and \fIarg\fRs are the values supplied when the
alias was created.  \fITargetCmd\fR 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 \fItargetCmd\fR 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:
\fItargetCmd\fR and \fIargs\fR were substituted when parsing the command
that created the alias, and \fIarg1 - argN\fR are substituted when
the alias's source command is parsed in the source interpreter.
.PP
When writing the \fItargetCmd\fRs 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.

.VS
.SH "HIDDEN COMMANDS"
.PP
Safe interpreters greatly restrict the functionality available to Tcl
programs executing within them.
Allowing the untrusted Tcl program to have direct access to this
functionality is unsafe, because it can be used for a variety of
attacks on the environment.
However, there are times when there is a legitimate need to use the
dangerous functionality in the context of the safe interpreter. For
example, sometimes a program must be \fBsource\fRd into the interpreter.
Another example is Tk, where windows are bound to the hierarchy of windows
for a specific interpreter; some potentially dangerous functions, e.g.
window management, must be performed on these windows within the
interpreter context.
.PP
The \fBinterp\fR command provides a solution to this problem in the form of
\fIhidden commands\fR. Instead of removing the dangerous commands entirely
from a safe interpreter, these commands are hidden so they become
unavailable to Tcl scripts executing in the interpreter. However, such
hidden commands can be invoked by any trusted ancestor of the safe
interpreter, in the context of the safe interpreter, using \fBinterp
invoke\fR. Hidden commands and exposed commands reside in separate name
spaces. It is possible to define a hidden command and an exposed command by
the same name within one interpreter.
.PP
Hidden commands in a slave interpreter can be invoked in the body of
procedures called in the master during alias invocation. For example, an
alias for \fBsource\fR could be created in a slave interpreter. When it is
invoked in the slave interpreter, a procedure is called in the master
interpreter to check that the operation is allowable (e.g. it asks to
source a file that the slave interpreter is allowed to access). The
procedure then it invokes the hidden \fBsource\fR command in the slave
interpreter to actually source in the contents of the file. Note that two
commands named \fBsource\fR exist in the slave interpreter: the alias, and
the hidden command.
.PP
Because a master interpreter may invoke a hidden command as part of
handling an alias invocation, great care must be taken to avoid evaluating
any arguments passed in through the alias invocation.
Otherwise, malicious slave interpreters could cause a trusted master
interpreter to execute dangerous commands on their behalf. See the section
on ALIAS INVOCATION for a more complete discussion of this topic.
To help avoid this problem, no substitutions or evaluations are
applied to arguments of \fBinterp invokehidden\fR.
.PP
Safe interpreters are not allowed to invoke hidden commands in themselves
or in their descendants. This prevents safe slaves from gaining access to
hidden functionality in themselves or their descendants.
.PP
The set of hidden commands in an interpreter can be manipulated by a trusted
interpreter using \fBinterp expose\fR and \fBinterp hide\fR. The \fBinterp
expose\fR command moves a hidden command to the
set of exposed commands in the interpreter identified by \fIpath\fR,
potentially renaming the command in the process. If an exposed command by
the targetted name already exists, the operation fails. Similarly,
\fBinterp hide\fR moves an exposed command to the set of hidden commands in
that interpreter. Safe interpreters are not allowed to move commands
between the set of hidden and exposed commands, in either themselves or
their descendants.
.PP
Currently, the names of hidden commands cannot contain namespace
qualifiers, and you must first rename a command in a namespace to the
global namespace before you can hide it.
Commands to be hidden by \fBinterp hide\fR are looked up in the global
namespace even if the current namespace is not the global one. This
prevents slaves from fooling a master interpreter into hiding the wrong
command, by making the current namespace be different from the global one.
.VE
.SH CREDITS
.PP
This mechanism is based on the Safe-Tcl prototype implemented
by Nathaniel Borenstein and Marshall Rose.

.SH "SEE ALSO"
load(n), safe(n), Tcl_CreateSlave(3)

.SH KEYWORDS
alias, master interpreter, safe interpreter, slave interpreter