16.2.7 Syntax Taints

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► | 16 | Macros

16.1 | Pattern-Based Macros
16.2 | General Macro Transformers
16.3 | Module Instantiations and Visits

▼ | 16.2 | General Macro Transformers

16.2.1 | Syntax Objects
16.2.2 | Macro Transformer Procedures
16.2.3 | Mixing Patterns and Expressions: syntax-case
16.2.4 | with-syntax and generate-temporaries
16.2.5 | Compile and Run-Time Phases
16.2.6 | General Phase Levels
16.2.7 | Syntax Taints

► | 16.2.7 | Syntax Taints

16.2.7.1 | Tainting Modes
16.2.7.2 | Taints and Code Inspectors
16.2.7.3 | Protected Exports

On this page:

16.2.7.1`` Tainting Modes
16.2.7.2`` Taints and Code Inspectors
16.2.7.3`` Protected Exports

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##### 16.2.7`` Syntax Taints

A use of a macro can expand into a use of an identifier that is not exported from the module that binds the macro. In general, such an identifier must not be extracted from the expanded expression and used in a different context, because using the identifier in a different context may break invariants of the macro’s module.

For example, the following module exports a macro go that expands to a use of unchecked-go:

> > "m.rkt"

> > > #lang racket
> > > (provide go)
> > > (define (unchecked-go n x)
> > > ; to avoid disaster, n must be a number
> > > (+ n 17))
> > > (define-syntax (go stx)
> > > (syntax-case stx ()
> > > [(_ x)
> > > #'(unchecked-go 8 x)]))

If the reference to unchecked-go is extracted from the expansion of (go 'a), then it might be inserted into a new expression, (unchecked-go #f 'a), leading to disaster. The datum->syntax procedure can be used similarly to construct references to an unexported identifier, even when no macro expansion includes a reference to the identifier.

To prevent such abuses of unexported identifiers, the go macro must explicitly protect its expansion by using syntax-protect:

> (define-syntax (go stx)
> (syntax-case stx ()
> [(_ x)
> (syntax-protect #'(unchecked-go 8 x))]))

The syntax-protect function causes any syntax object that is extracted from the result of go to be tainted. The macro expander rejects tainted identifiers, so attempting to extract unchecked-go from the expansion of (go 'a) produces an identifier that cannot be used to construct a new expression (or, at least, not one that the macro expander will accept). The syntax-rules, syntax-id-rule, and define-syntax-rule forms automatically protect their expansion results.

More precisely, syntax-protect arms a syntax object with a dye pack. When a syntax object is armed, then syntax-e taints any syntax object in its result. Similarly, datum->syntax taints its result when its first argument is armed. Finally, if any part of a quoted syntax object is armed, then the corresponding part is tainted in the resulting syntax constant.

Of course, the macro expander itself must be able to disarm a taint on a syntax object, so that it can further expand an expression or its sub-expressions. When a syntax object is armed with a dye pack, the dye pack has an associated inspector that can be used to disarm the dye pack. A (syntax-protect stx) function call is actually a shorthand for (syntax-arm stx #f #t), which arms stx using a suitable inspector. The expander uses syntax-disarm and with its inspector on every expression before trying to expand or compile it.

In much the same way that the macro expander copies properties from a syntax transformer’s input to its output (see Syntax Object Properties), the expander copies dye packs from a transformer’s input to its output. Building on the previous example,

> > "n.rkt"

> > > #lang racket
> > > (require "m.rkt")
> > > (provide go-more)
> > > (define y 'hello)
> > > (define-syntax (go-more stx)
> > > (syntax-protect #'(go y)))

the expansion of (go-more) introduces a reference to the unexported y in (go y), and the expansion result is armed so that y cannot be extracted from the expansion. Even if go did not use syntax-protect for its result (perhaps because it does not need to protect unchecked-go after all), the dye pack on (go y) is propagated to the final expansion (unchecked-go 8 y). The macro expander uses syntax-rearm to propagate dye packs from a transformer’s input to its output.

##### 16.2.7.1`` Tainting Modes

In some cases, a macro implementor intends to allow limited destructuring of a macro result without tainting the result. For example, given the following define-like-y macro,

> > "q.rkt"

> > > #lang racket
> > > (provide define-like-y)
> > > (define y 'hello)
> > > (define-syntax (define-like-y stx)
> > > (syntax-case stx ()
> > > [(_ id) (syntax-protect #'(define-values (id) y))]))

someone may use the macro in an internal definition:

> (let ()
> (define-like-y x)
> x)

The implementor of the "q.rkt" module most likely intended to allow such uses of define-like-y. To convert an internal definition into a letrec binding, however, the define form produced by define-like-y must be deconstructed, which would normally taint both the binding x and the reference to y.

Instead, the internal use of define-like-y is allowed, because syntax-protect treats specially a syntax list that begins with define-values. In that case, instead of arming the overall expression, each individual element of the syntax list is armed, pushing dye packs further into the second element of the list so that they are attached to the defined identifiers. Thus, define-values, x, and y in the expansion result (define-values (x) y) are individually armed, and the definition can be deconstructed for conversion to letrec.

Just like syntax-protect, the expander rearms a transformer result that starts with define-values, by pushing dye packs into the list elements. As a result, define-like-y could have been implemented to produce (define id y), which uses define instead of define-values. In that case, the entire define form is at first armed with a dye pack, but as the define form is expanded to define-values, the dye pack is moved to the parts.

The macro expander treats syntax-list results starting with define-syntaxes in the same way that it treats results starting with define-values. Syntax-list results starting with begin are treated similarly, except that the second element of the syntax list is treated like all the other elements (i.e., the immediate element is armed, instead of its content). Furthermore, the macro expander applies this special handling recursively, in case a macro produces a begin form that contains nested define-values forms.

The default application of dye packs can be overridden by attaching a 'taint-mode property (see Syntax Object Properties) to the resulting syntax object of a macro transformer. If the property value is 'opaque, then the syntax object is armed and not its parts. If the property value is 'transparent, then the syntax object’s parts are armed. If the property value is 'transparent-binding, then the syntax object’s parts and the sub-parts of the second part (as for define-values and define-syntaxes) are armed. The 'transparent and 'transparent-binding modes trigger recursive property checking at the parts, so that armings can be pushed arbitrarily deeply into a transformer’s result.

##### 16.2.7.2`` Taints and Code Inspectors

Tools that are intended to be privileged (such as a debugging transformer) must disarm dye packs in expanded programs. Privilege is granted through code inspectors. Each dye pack records an inspector, and a syntax object can be disarmed using a sufficiently powerful inspector.

When a module is declared, the declaration captures the current value of the current-code-inspector parameter. The captured inspector is used when syntax-protect is applied by a macro transformer that is defined within the module. A tool can disarm the resulting syntax object by supplying syntax-disarm with an inspector that is the same or a super-inspector of the module’s inspector. Untrusted code is ultimately run after setting current-code-inspector to a less powerful inspector (after trusted code, such as debugging tools, have been loaded).

With this arrangement, macro-generating macros require some care, since the generating macro may embed syntax objects in the generated macro that need to have the generating module’s protection level, rather than the protection level of the module that contains the generated macro. To avoid this problem, use the module’s declaration-time inspector, which is accessible as (variable-reference->module-declaration-inspector (#%variable-reference)), and use it to define a variant of syntax-protect.

For example, suppose that the go macro is implemented through a macro:

> #lang racket
> (provide def-go)
> (define (unchecked-go n x)
> (+ n 17))
> (define-syntax (def-go stx)
> (syntax-case stx ()
> [(_ go)
> (protect-syntax
> #'(define-syntax (go stx)
> (syntax-case stx ()
> [(_ x)
> (protect-syntax #'(unchecked-go 8 x))])))]))

When def-go is used inside another module to define go, and when the go-defining module is at a different protection level than the def-go-defining module, the generated macro’s use of protect-syntax is not right. The use of unchecked-go should be protected at the level of the def-go-defining module, not the go-defining module.

The solution is to define and use go-syntax-protect, instead:

> #lang racket
> (provide def-go)
> (define (unchecked-go n x)
> (+ n 17))
> (define-for-syntax go-syntax-protect
> (let ([insp (variable-reference->module-declaration-inspector
> (#%variable-reference))])
> (lambda (stx) (syntax-arm stx insp))))
> (define-syntax (def-go stx)
> (syntax-case stx ()
> [(_ go)
> (protect-syntax
> #'(define-syntax (go stx)
> (syntax-case stx ()
> [(_ x)
> (go-syntax-protect #'(unchecked-go 8 x))])))]))

##### 16.2.7.3`` Protected Exports

Sometimes, a module needs to export bindings to some modules—other modules that are at the same trust level as the exporting module—but prevent access from untrusted modules. Such exports should use the protect-out form in provide. For example, ffi/unsafe exports all of its unsafe bindings as protected in this sense.

Code inspectors, again, provide the mechanism for determining which modules are trusted and which are untrusted. When a module is declared, the value of current-code-inspector is associated to the module declaration. When a module is instantiated (i.e., when the body of the declaration is actually executed), a sub-inspector is created to guard the module’s exports. Access to the module’s protected exports requires a code inspector higher in the inspector hierarchy than the module’s instantiation inspector; note that a module’s declaration inspector is always higher than its instantiation inspector, so modules are declared with the same code inspector can access each other’s exports.

Syntax-object constants within a module, such as literal identifiers in a template, retain the inspector of their source module. In this way, a macro from a trusted module can be used within an untrusted module, and protected identifiers in the macro expansion still work, even through they ultimately appear in an untrusted module. Naturally, such identifiers should be armed, so that they cannot be extracted from the macro expansion and abused by untrusted code.

Compiled code from a ".zo" file is inherently untrustworthy, unfortunately, since it can be synthesized by means other than compile. When compiled code is written to a ".zo" file, syntax-object constants within the compiled code lose their inspectors. All syntax-object constants within compiled code acquire the enclosing module’s declaration-time inspector when the code is loaded.

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