Changes between Version 25 and Version 26 of SafeHaskell


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Timestamp:
Jan 18, 2011 9:02:06 PM (3 years ago)
Author:
David
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  • SafeHaskell

    v25 v26  
    1919 1. A new GHC option (`-XSafe`) enabling a "Safe" dialect of Haskell in which GHC rejects any source code that might produce unsafe effects or otherwise subvert the type system. 
    2020 
    21  2. Another new GHC option (`-XTrustworthy`) indicating that, though a module may invoke unsafe functions internally, the module's author claims the set of exported symbols cannot be used in an unsafe way. 
    22  
    23 The presence of either the `-XSafe` or `-XTrustworthy` option introduces a small extension to the syntax of import statements, adding a `safe` keyword: 
     21 2. Another new GHC option (`-XTrustworthy`) indicating that, though a module may invoke unsafe functions internally, the module's author claims the set of exported symbols cannot be used in an unsafe way.  (There is a corresponding `-XUntrustworthy` option to enable the language extension but negate `-XTrustworthy`.) 
     22 
     23The presence of any of the `-XSafe`, `-XTrustworthy`, or `-XUntrustworthy` options introduces a small extension to the syntax of import statements, adding a `safe` keyword: 
    2424   
    2525  impdecl -> `import` [`safe`] [`qualified`] modid [`as` modid] [impspec] 
    2626 
    27 Import declarations are either safe or unsafe, while modules are classified as either trusted or untrusted.  A safe import declaration causes compilation to fail if the imported module is not trusted. 
    28  
    29 In the Safe dialect, all import declarations are implicitly safe, regardless of the presence of the `safe` keyword.  When compiling with `-XTrustworthy` but not `-XSafe`, import declarations with the `safe` keyword are safe, while those without the keyword are unsafe.  A module can be trusted only if it was compiled with `-XSafe` or `-XTrustworthy`, but there is an additional restriction detailed below under '''Trusted modules''':  roughly speaking, for a module M to be trusted, any modules reachable from M and compiled with `-XTrustworthy` must reside in packages trusted by the user invoking the compiler. 
     27Import declarations are either safe or unsafe, while modules are classified as either trusted or untrusted.  An `import safe` declaration declares that the author of a module does not assume responsibility for the imported module's safety.  `import safe` causes compilation to fail if the imported module is not trusted.  Additionally, in the Safe dialect, ''all'' import declarations of untrusted modules cause compilation to fail, regardless of the presence of the `safe` keyword. 
    3028 
    3129== Safety Goal == 
     
    7270== Trusted modules == 
    7371 
    74 ----------------------- 
    75 '''SLPJ/SDM note'''.  This is an attempt to define what it means for a module, or a package, to be "trusted", and what consequences that trust has.  If successful it would replace the material that follows 
     72To determine what imports are allowed, we define what it means for a module, or a package, to be "trusted", and what consequences that trust has. 
    7673 
    7774 * A '''client''' is someone running GHC, typically the person compiling the application. 
    7875 
    79  * A '''package P is trusted by a client C''' iff one of these conditions hold 
    80      * C's package database records that P is trusted   
    81      * C's command-line flags say to trust it regardless (see `-trust`, `-distrust` below)[[BR]] 
     76 * A '''package P is trusted by a client C''' iff one of these conditions holds 
     77     * C's package database records that P is trusted (and command-line arguments do not override the database) 
     78     * C's command-line flags say to trust it regardless of the database (see `-trust`, `-distrust` below)[[BR]] 
    8279 It is up to C to decide what packages to trust; it is not a property of P. 
    8380 
    8481 * A '''module M from package P is trusted by a client C''' iff  
    85    * Either all of these hold: 
    86      * The module was compiled with `-XSafe` 
     82   * Either both of these hold: 
     83     * The module was compiled with `-XSafe` and without `-XUntrustworthy` 
    8784     * All of M's direct `imports` are trusted by C 
    8885   * Or all of these hold: 
     
    9390 * When a client C compiles a module M with 
    9491   * `-XSafe`: all M's `imports` must be trusted by C 
    95    * `-XTrustworthy`: all M's `safe imports` must be trusted by C[[BR]] 
     92   * `-XTrustworthy` or `-XUntrustworthy`, but not `-XSafe`: all M's `safe imports` must be trusted by C[[BR]] 
    9693 Otherwise the module is rejected. 
    9794 
     
    10198   * When the author of code marks it `-XTrustworthy` he takes on responsibility for the stafety of that code, ''under the assumption'' that `safe imports` are indeed safe. 
    10299 
    103  * When client C trusts package P, he expresses trust in the author of that code.  But since the author makes no guarantees about `safe imports`, C may need ot chase dependencies to decide which modules in P should be trusted by C. 
     100 * When client C trusts package P, he expresses trust in the author of that code.  But since the author makes no guarantees about `safe imports`, C may need to chase dependencies to decide which modules in P should be trusted by C. 
    104101 
    105102For example, suppose we have this setup: 
     
    119116Suppose client C decides to trust P.  Then does C trust M?  Well, M has a `safe` import, so P's author takes no responsibility.  So C must check whether `Buggle` is trusted by C.  Is it?  Well, it is compiled with `-XSafe`, so the code in `Buggle` is machine-checked to be OK, but again the author takes no responsibility for `Prelude`.  Ah, but `Prelude` comes from `base`, which C trusts, and is (let's say) compiled with `-XTrustworthy`. 
    120117 
    121 What about the import of `System.IO.Unsafe`.  C trust's P's author, and P's author takes responsibility for that import.  So C trusts M. 
    122  
    123 Notice that C didn't need to trust package `Wuggle`; the machine checking is enough.  C only needs to trust packages that have `-XTrustworthy` packages in them. 
    124  
    125 '''End of SLPJ/SDM note''' 
    126 ------------------------ 
    127  
    128 Recall that a safe import of a module M fails unless M is trusted. Whether or not M is trusted depends on the following factors: 
    129  
    130  - Which packages the user invoking the compiler trusts, 
    131  
    132  - What modules M depends on, and 
    133  
    134  - How M was compiled. 
    135  
    136 To capture the intentions of a user compiling potentially untrusted code, we introduce a notion of '''trusted package'''.  A trusted package is one that is declared trusted by command-line flags to the compiler.  The user must manually ensure that code in trusted packages does not violate the safety goal; the compiler assumes that responsibility for the rest of the program. 
    137  
    138 Next, we define a notion of a module's '''trust dependency set'''. The trust dependency set of module M consists of either a) the names and packages of all the modules that the user must manually verify to ensure M satisfies the safety goal, or b) the singleton set containing pseudo-module name `untrusted` in pseudo-package `_untrusted`. (`_untrusted` is never a trusted package, regardless of command-line options.)  More specifically, the trust dependency set of module M is computed as follows: 
    139  
    140  * If M was compiled with neither `-XSafe` nor `-XTrustworthy`, or, regardless of these options, if M contains the `{-# LANGUAGE Untrustworthy #-}` pragma, then M's trust dependency set consists solely of the `untrusted` pseudo-module. 
    141  
    142  * ''If M was compiled with `-XTrustworthy`'', then its trust dependency set includes M plus the union of the trust dependency sets of all modules M imports with the `safe` keyword. 
    143  
    144  * ''If M was compiled with `-XSafe` but not `-XTrustworthy`'', then its trust dependency set is the union of the trust dependency sets of all modules M imports. 
    145  
    146 Finally, we say an imported module M is '''trusted''' when every module in M's trust dependency set resides in a trusted package.  A safe import of module M fails unless M is trusted by this definition. 
     118What about the import of `System.IO.Unsafe`?  C trust's P's author, and P's author takes responsibility for that import.  So C trusts M. 
     119 
     120Notice that C didn't need to trust package `Wuggle`; the machine checking is enough.  C only needs to trust packages that have `-XTrustworthy` modules in them. 
    147121 
    148122=== Implementation details ===