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Practical Programming in Tcl & Tk, Third Edition
By Brent B. Welch

Table of Contents
Part II.  Advanced Tcl

Chapter 19. Multiple Interpreters and Safe-Tcl

This chapter describes how to create more than one Tcl interpreter in your application. A child interpreter can be made safe so that it can execute untrusted scripts without compromising your application or your computer. Command aliases, hidden commands, and shared I/O channels enable communication among interpreters. Tcl command described is: interp.

Safe-Tcl was invented by Nathaniel Borenstein and Marshall Rose so that they could send Tcl scripts via e-mail and have the recipient safely execute the script without worry of viruses or other attacks. Safe-Tcl works by removing dangerous commands like exec and open that would let an untrusted script damage the host computer. You can think of this restricted interpreter as a "padded cell" in which it is safe to execute untrusted scripts. To continue the analogy, if the untrusted code wants to do anything potentially unsafe, it must ask permission. This works by adding additional commands, or aliases, that are implemented by a different Tcl interpreter. For example, a safeopen command could be implemented by limiting file space to a temporary directory that is deleted when the untrusted code terminates.

The key concept of Safe-Tcl is that there are two Tcl interpreters in the application, a trusted one and an untrusted (or "safe") one. The trusted interpreter can do anything, and it is used for the main application (e.g., the Web browser or e-mail user interface). When the main application receives a message containing an untrusted script, it evaluates that script in the context of the untrusted interpreter. The restricted nature of the untrusted interpreter means that the application is safe from attack. This model is much like user mode and kernel mode in a multiuser operating system like UNIX or Windows/NT. In these systems, applications run in user mode and trap into the kernel to access resources like files and the network. The kernel implements access controls so that users cannot read and write each other's files, or hijack network services. In Safe-Tcl the application implements access controls for untrusted scripts.

The dual interpreter model of Safe-Tcl has been generalized in Tcl 7.5 and made accessible to Tcl scripts. A Tcl script can create other interpreters, destroy them, create command aliases among them, share I/O channels among them, and evaluate scripts in them.

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