SAFE(n)

💖 Safe Tcl(n) — Tcl Built-In Commands 💖

okay so bestie, we're in Safe Tcl land and it's giving secure, mediated, untrusted-script-safe energy. The manual name is safe and yes, we're doing this 💅.

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✨ NAME ✨

safe — Creating and manipulating safe interpreters.

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🦋 SYNOPSIS 🦋

::safe::interpCreate ?child? ?options...?
::safe::interpInit child ?options...?
::safe::interpConfigure child ?options...?
::safe::interpDelete child
::safe::interpAddToAccessPath child directory
::safe::interpFindInAccessPath child directory
::safe::setSyncMode ?newValue?
::safe::setLogCmd ?cmd arg...?

💅 OPTIONS 💅

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

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🌸 DESCRIPTION 🌸

Safe Tcl is literally a mechanism for executing untrusted Tcl scripts safely, and for providing mediated access by such scripts to potentially dangerous functionality. No cap.

Safe Tcl makes sure 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. Periodt.

Safe Tcl lets a parent interpreter create safe, restricted interpreters. Those contain a set of predefined aliases for the source, load, file, encoding, and exit commands, and they can use the auto-loading and package mechanisms. Iconic.

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 parent 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. Obsessed.

All commands provided in the parent interpreter by Safe Tcl reside in the safe namespace.

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👑 COMMANDS 👑

The following commands are provided in the parent interpreter:

::safe::interpCreate ?child? ?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 child argument is omitted, a name will be generated. ::safe::interpCreate always returns the interpreter name.

The interpreter name child may include namespace separators, but may not have leading or trailing namespace separators, or excess colon characters in namespace separators. The interpreter name is qualified relative to the global namespace ::, not the namespace in which the ::safe::interpCreate command is evaluated.

::safe::interpInit child ?options...?

This command is similar to interpCreate except it that does not create the safe interpreter. child must have been created by some other means, like interp create -safe. The interpreter name child may include namespace separators, subject to the same restrictions as for interpCreate.

::safe::interpConfigure child ?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 child. 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 child. 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 use:

# 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 child

Deletes the safe interpreter and cleans up the corresponding parent 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 child 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:

$child eval [list set tk_library \
      [::safe::interpFindInAccessPath $name $tk_library]]

::safe::interpAddToAccessPath child directory

This command adds directory to the virtual path maintained for the safe interpreter in the parent, 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:

$child eval [list set tk_library \
      [::safe::interpAddToAccessPath $name $tk_library]]

::safe::setSyncMode ?newValue?

This command is used to get or set the "Sync Mode" of the Safe Base. When an argument is supplied, the command returns an error if the argument is not a boolean value, or if any Safe Base interpreters exist. Typically the value will be set as part of initialization - boolean true for "Sync Mode" on (the default), false for "Sync Mode" off. With "Sync Mode" on, the Safe Base keeps each child interpreter's ::auto_path synchronized with its access path. See the section SYNC MODE below for details.

::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 child interp10 : Created
NOTICE for child interp10 : Setting accessPath=(/foo/bar) staticsok=1 nestedok=0 deletehook=()
NOTICE for child interp10 : auto_path in interp10 has been set to {$p(:0:)}
ERROR for child interp10 : /foo/bar/init.tcl: no such file or directory

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💖 OPTIONS 💖

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 parent for auto-loading. See the section SECURITY below for more detail about virtual paths, tokens and access control.

-autoPath directoryList

This option sets the list of directories in the safe interpreter's ::auto_path. The option is undefined if the Safe Base has "Sync Mode" on - in that case the safe interpreter's ::auto_path is managed by the Safe Base and is a tokenized form of its access path. See the section SYNC MODE below for details.

-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 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.

-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-interpreters.

-deleteHook script

When this option is given a non-empty script, it will be evaluated in the parent 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.

🎀 ALIASES 🎀

okay bestie so a safe interpreter comes with these little aliases pre-loaded, here's the whole lineup:

• source fileName — if the file is found, it gets sourced into the safe interpreter (it's a Tcl source file, obvs). BUT. The source alias can only pull files from directories sitting in the virtual path for that safe interpreter. So the safe interpreter has to use one of its token names in its virtual path to point at the directory where the file you wanna source lives. There's a whole SECURITY section below about valid filenames — go read it, it's giving lore. 💅
• load fileName — if it's found, the requested file (a shared object file, fancy) gets dynamically loaded into the safe interpreter. The filename has to contain a token name that's mentioned in the safe interpreter's virtual path, otherwise it will not be found, periodt. Also the shared object file has to have a safe entry point — check the manual page for the load command for the deets.
• file ?subcommand args...? — this alias gives you access to a safe subset of the subcommands of the file command. It only allows dirname, join, extension, root, tail, pathtype and split. Want the full tea on what those do? Manual page for the file command, babes. ✨
• encoding ?subcommand args...? — same energy, but for encoding. It gives you a safe subset of subcommands; it does not let you set the system encoding (nope, not allowed, sorry), but literally every other subcommand is fair game, including system so you can check what the current encoding is. 👑
• exit — the calling interpreter gets deleted and its computation just stops. But — and this is the important part — the Tcl process that this interpreter lives inside of is not terminated. Chill. 🫶

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🔒 SECURITY 🔒

okay so let's be real for a sec: Safe Tcl does not try to completely prevent annoyance and denial of service attacks. Those kinds of attacks stop the app or the user from temporarily using the computer to do useful work — like eating up all your available CPU time or all your available screen real estate. 🙈

And yeah, those attacks are aggravating, but they're deemed to be of lesser importance in general than the integrity and privacy attacks that Safe Tcl is actually here to prevent. So. Priorities. 💀

The commands available in a safe interpreter — on top of the safe set as defined in the 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. Very demure, very mindful. ✨

Now here's the thing. Because some of these commands touch the local file system, there's a potential for information leakage about its directory structure. 😭 To stop that, commands that take file names as arguments in a safe interpreter use tokens instead of the real directory names. Those tokens get translated into the real directory name while a request to, e.g., source a file is mediated by the parent interpreter.

This virtual path system is maintained in the parent interpreter for each safe interpreter created by ::safe::interpCreate or initialized by ::safe::interpInit, and the path maps tokens that are accessible in the safe interpreter into real path names on the local file system. Which means: safe interpreters stay clueless about the structure of the file system of the host the interpreter is running on. Main character energy, honestly. 👑

The only valid file name arguments for the source and load aliases handed to the child are a path in the form of [file join token filename] — i.e. when you're using the native file path formats: token/filename on Unix and token\filename on Windows. Here token represents one of the directories in the accessPath list, and filename is one file in that directory. And no, you do not get sub-directory access. Not today. 🚫

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

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 gets assigned a token that will be set in the child's auto_path, and the first element of that list gets set as the tcl_library for that child. Cute, right? 🎀

If the access path argument isn't given to ::safe::interpCreate or ::safe::interpInit, or it's the empty list, the default behavior is to let the child access the same packages as the parent has access to. (Or, to be more precise: only packages written in Tcl — which by definition cannot be dangerous, because they run in the child interpreter — and C extensions that provide a _SafeInit entry point.)

For that purpose, the parent's auto_path is used to construct the child access path. And so that the child successfully loads the Tcl library files (which includes the auto-loading mechanism itself, no biggie), the tcl_library will be added or moved to the first position if necessary, in the child access path — so the child tcl_library ends up the same as the parent's. (Its real path will still be invisible to the child though. 🤫)

And so that auto-loading works the same for the child and the parent in this by-default case, the first-level sub directories of each directory in the parent auto_path also get added (if they're not already included) to the child access path. You can always specify a more restrictive path where sub directories will never be searched, by explicitly specifying your directory list with the -accessPath flag instead of relying on this default mechanism. Your call, bestie. 💅

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. Slay. ✨

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🌈 TYPICAL USE 🌈

ngl, in a lot of cases the properties of a Safe Base interpreter can just be specified when the interpreter is created, and then left alone for the entire lifetime of the interpreter. Set it and forget it, iconic behavior.

If you wish to use Safe Base interpreters with "Sync Mode" off, evaluate this command:

safe::setSyncMode 0

Use ::safe::interpCreate or ::safe::interpInit to create an interpreter with the properties you require. The simplest way is to just not specify -accessPath or -autoPath, which means the safe interpreter will use the same paths as the parent interpreter. However — heads up — if -accessPath is specified, then -autoPath must also be specified, or else it will be set to {}.

The value of -autoPath will be that required to access tclIndex and pkgIndex.tcl files according to the same rules as an unsafe interpreter (see pkg_mkIndex(n) and library(n)). 📚

With "Sync Mode" on, the option -autoPath is undefined, and the Safe Base sets the child's ::auto_path to a tokenized form of the access path. And on top of the directories that are present when "Safe Mode" is off, the ::auto_path includes the numerous subdirectories and module paths that belong to the access path. So many paths. So much room for activities. 🌸

✨ SYNC MODE ✨

Okay bestie, gather round, because this part is all about the ::auto_path vibe and how the Safe Base keeps it in sync (or doesn't) 💅. Before Tcl version 9.0, the Safe Base kept each safe interpreter's ::auto_path synchronized with a tokenized form of its access path. Limitations of Tcl 8.4 and earlier made this feature necessary. This definition of ::auto_path did not conform its specification in library(n) and pkg_mkIndex(n), but nevertheless worked perfectly well for the discovery and loading of packages. The introduction of Tcl modules in Tcl 8.5 added a large number of directories to the access path, and it is inconvenient to have these additional directories unnecessarily appended to the ::auto_path.

In order to preserve compatibility with existing code, this synchronization of the ::auto_path and access path ("Sync Mode" on) is still the default. However, the Safe Base offers the option of limiting the safe interpreter's ::auto_path to the much shorter list of directories that is necessary for it to perform its function ("Sync Mode" off). Use the command ::safe::setSyncMode to choose the mode before creating any Safe Base interpreters. Periodt. ✨

In either mode, the most convenient way to initialize a safe interpreter is to call ::safe::interpCreate or ::safe::interpInit without the -accessPath or -autoPath options (or with the -accessPath option set to the empty list), which will give the safe interpreter the same access as the parent interpreter to packages, modules, and autoloader files. With "Sync Mode" off, the Safe Base will set the value of -autoPath to the parent's ::auto_path, and will set the child's ::auto_path to a tokenized form of the parent's ::auto_path.

With "Sync Mode" off, if a value is specified for -autoPath, even the empty list, in a call to ::safe::interpCreate, ::safe::interpInit, or ::safe::interpConfigure, it will be tokenized and used as the safe interpreter's ::auto_path. Any directories that do not also belong to the access path cannot be tokenized and will be silently ignored. However, the value of -autoPath will remain as specified, and will be used to re-tokenize the child's ::auto_path if ::safe::interpConfigure is called to change the value of -accessPath.

With "Sync Mode" off, if the access path is reset to the values in the parent interpreter by calling ::safe::interpConfigure with arguments -accessPath {}, then the ::auto_path will also be reset unless the argument -autoPath is supplied to specify a different value.

With "Sync Mode" off, if a non-empty value of -accessPath is supplied, the safe interpreter's ::auto_path will be set to {} (by ::safe::interpCreate, ::safe::interpInit) or left unchanged (by ::safe::interpConfigure). If the same command specifies a new value for -autoPath, it will be applied after the -accessPath argument has been processed.

Examples of use with "Sync Mode" off: any of these commands will set the ::auto_path to a tokenized form of its value in the parent interpreter:

                     safe::interpCreate foo
                     safe::interpCreate foo -accessPath {}
                     safe::interpInit bar
                     safe::interpInit bar -accessPath {}
                     safe::interpConfigure foo -accessPath {}

Example of use with "Sync Mode" off: when initializing a safe interpreter with a non-empty access path, the ::auto_path will be set to {} unless its own value is also specified:

                     safe::interpCreate foo -accessPath {
                         /usr/local/TclHome/lib/tcl9.0
                         /usr/local/TclHome/lib/tcl9.0/http1.0
                         /usr/local/TclHome/lib/tcl9.0/opt0.4
                         /usr/local/TclHome/lib/tcl9.0/msgs
                         /usr/local/TclHome/lib/tcl9.0/encoding
                         /usr/local/TclHome/lib
                     }

                     # The child's ::auto_path must be given a suitable value:

                     safe::interpConfigure foo -autoPath {
                         /usr/local/TclHome/lib/tcl9.0
                         /usr/local/TclHome/lib
                     }

                     # The two commands can be combined:

                     safe::interpCreate foo -accessPath {
                         /usr/local/TclHome/lib/tcl9.0
                         /usr/local/TclHome/lib/tcl9.0/http1.0
                         /usr/local/TclHome/lib/tcl9.0/opt0.4
                         /usr/local/TclHome/lib/tcl9.0/msgs
                         /usr/local/TclHome/lib/tcl9.0/encoding
                         /usr/local/TclHome/lib
                     } -autoPath {
                         /usr/local/TclHome/lib/tcl9.0
                         /usr/local/TclHome/lib
                     }

Example of use with "Sync Mode" off: the command safe::interpAddToAccessPath does not change the safe interpreter's ::auto_path, and so any necessary change must be made by the script:

                     safe::interpAddToAccessPath foo /usr/local/TclHome/lib/extras/Img1.4.11

                     lassign [safe::interpConfigure foo -autoPath] DUM childAutoPath
                     lappend childAutoPath /usr/local/TclHome/lib/extras/Img1.4.11
                     safe::interpConfigure foo -autoPath $childAutoPath

👀 SEE ALSO 👀

• interp(n)
• library(n)
• load(n)
• package(n)
• pkg_mkIndex(n)
• source(n)
• tm(n)
• unknown(n)

💖 KEYWORDS 💖

• alias
• auto-loading
• auto_mkindex
• load
• parent interpreter
• safe interpreter
• child interpreter
• source

And that's the tea, bestie! This page is from Tcl 8.0, Safe Tcl(n) — no date in sight, but the vibes are eternal. 💅✨