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    1/*  Part of SWI-Prolog
    2
    3    Author:        Jan Wielemaker
    4    E-mail:        J.Wielemaker@vu.nl
    5    WWW:           http://www.swi-prolog.org
    6    Copyright (c)  2013-2020, VU University Amsterdam
    7                              CWI, Amsterdam
    8    All rights reserved.
    9
   10    Redistribution and use in source and binary forms, with or without
   11    modification, are permitted provided that the following conditions
   12    are met:
   13
   14    1. Redistributions of source code must retain the above copyright
   15       notice, this list of conditions and the following disclaimer.
   16
   17    2. Redistributions in binary form must reproduce the above copyright
   18       notice, this list of conditions and the following disclaimer in
   19       the documentation and/or other materials provided with the
   20       distribution.
   21
   22    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   23    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   24    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
   25    FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
   26    COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
   27    INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
   28    BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
   29    LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
   30    CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   31    LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
   32    ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
   33    POSSIBILITY OF SUCH DAMAGE.
   34*/
   35
   36:- module(sandbox,
   37          [ safe_goal/1,                % :Goal
   38            safe_call/1                 % :Goal
   39          ]).   40:- use_module(library(apply_macros),[expand_phrase/2]).   41:- use_module(library(apply),[maplist/2]).   42:- use_module(library(assoc),[empty_assoc/1,get_assoc/3,put_assoc/4]).   43:- use_module(library(debug),[debug/3,debugging/1]).   44:- use_module(library(error),
   45              [ must_be/2,
   46                instantiation_error/1,
   47                type_error/2,
   48                permission_error/3
   49              ]).   50:- use_module(library(lists),[append/3]).   51:- use_module(library(prolog_format),[format_types/2]).   52
   53:- multifile
   54    safe_primitive/1,               % Goal
   55    safe_meta_predicate/1,          % Name/Arity
   56    safe_meta/2,                    % Goal, Calls
   57    safe_meta/3,                    % Goal, Context, Calls
   58    safe_global_variable/1,         % Name
   59    safe_directive/1.               % Module:Goal
   60
   61% :- debug(sandbox).

Sandboxed Prolog code

Prolog is a full-featured Turing complete programming language in which it is easy to write programs that can harm your computer. On the other hand, Prolog is a logic based query language which can be exploited to query data interactively from, e.g., the web. This library provides safe_goal/1, which determines whether it is safe to call its argument.

See also
- http://www.swi-prolog.org/pldoc/package/pengines.html */
To be done
- Handling of ^ and // meta predicates
- Complete set of whitelisted predicates
   77:- meta_predicate
   78    safe_goal(:),
   79    safe_call(0).
 safe_call(:Goal)
Call Goal if it complies with the sandboxing rules. Before calling Goal, it performs expand_goal/2, followed by safe_goal/1. Expanding is done explicitly because situations in which safe_call/1 typically concern goals that are not known at compile time.
See also
- safe_goal/1.
   91safe_call(Goal0) :-
   92    expand_goal(Goal0, Goal),
   93    safe_goal(Goal),
   94    call(Goal).
 safe_goal(:Goal) is det
True if calling Goal provides no security risc. This implies that:
Errors
- instantiation_error if the analysis encounters a term in a callable position that is insufficiently instantiated to determine the predicate called.
- permission_error(call, sandboxed, Goal) if Goal is in the call-tree and not white-listed.
  118safe_goal(M:Goal) :-
  119    empty_assoc(Safe0),
  120    catch(safe(Goal, M, [], Safe0, _), E, true),
  121    !,
  122    nb_delete(sandbox_last_error),
  123    (   var(E)
  124    ->  true
  125    ;   throw(E)
  126    ).
  127safe_goal(_) :-
  128    nb_current(sandbox_last_error, E),
  129    !,
  130    nb_delete(sandbox_last_error),
  131    throw(E).
  132safe_goal(G) :-
  133    debug(sandbox(fail), 'safe_goal/1 failed for ~p', [G]),
  134    throw(error(instantiation_error, sandbox(G, []))).
 safe(+Goal, +Module, +Parents, +Safe0, -Safe) is semidet
Is true if Goal can only call safe code.
  141safe(V, _, Parents, _, _) :-
  142    var(V),
  143    !,
  144    Error = error(instantiation_error, sandbox(V, Parents)),
  145    nb_setval(sandbox_last_error, Error),
  146    throw(Error).
  147safe(M:G, _, Parents, Safe0, Safe) :-
  148    !,
  149    must_be(atom, M),
  150    must_be(callable, G),
  151    known_module(M:G, Parents),
  152    (   predicate_property(M:G, imported_from(M2))
  153    ->  true
  154    ;   M2 = M
  155    ),
  156    (   (   safe_primitive(M2:G)
  157        ;   safe_primitive(G),
  158            predicate_property(G, iso)
  159        )
  160    ->  Safe = Safe0
  161    ;   (   predicate_property(M:G, exported)
  162        ;   predicate_property(M:G, public)
  163        ;   predicate_property(M:G, multifile)
  164        ;   predicate_property(M:G, iso)
  165        ;   memberchk(M:_, Parents)
  166        )
  167    ->  safe(G, M, Parents, Safe0, Safe)
  168    ;   throw(error(permission_error(call, sandboxed, M:G),
  169                    sandbox(M:G, Parents)))
  170    ).
  171safe(G, _, Parents, _, _) :-
  172    debugging(sandbox(show)),
  173    length(Parents, Level),
  174    debug(sandbox(show), '[~D] SAFE ~q?', [Level, G]),
  175    fail.
  176safe(G, _, Parents, Safe, Safe) :-
  177    catch(safe_primitive(G),
  178          error(instantiation_error, _),
  179          rethrow_instantition_error([G|Parents])),
  180    predicate_property(G, iso),
  181    !.
  182safe(G, M, Parents, Safe, Safe) :-
  183    known_module(M:G, Parents),
  184    (   predicate_property(M:G, imported_from(M2))
  185    ->  true
  186    ;   M2 = M
  187    ),
  188    (   catch(safe_primitive(M2:G),
  189              error(instantiation_error, _),
  190              rethrow_instantition_error([M2:G|Parents]))
  191    ;   predicate_property(M2:G, number_of_rules(0))
  192    ),
  193    !.
  194safe(G, M, Parents, Safe0, Safe) :-
  195    predicate_property(G, iso),
  196    safe_meta_call(G, M, Called),
  197    !,
  198    add_iso_parent(G, Parents, Parents1),
  199    safe_list(Called, M, Parents1, Safe0, Safe).
  200safe(G, M, Parents, Safe0, Safe) :-
  201    (   predicate_property(M:G, imported_from(M2))
  202    ->  true
  203    ;   M2 = M
  204    ),
  205    safe_meta_call(M2:G, M, Called),
  206    !,
  207    safe_list(Called, M, Parents, Safe0, Safe).
  208safe(G, M, Parents, Safe0, Safe) :-
  209    goal_id(M:G, Id, Gen),
  210    (   get_assoc(Id, Safe0, _)
  211    ->  Safe = Safe0
  212    ;   put_assoc(Id, Safe0, true, Safe1),
  213        (   Gen == M:G
  214        ->  safe_clauses(Gen, M, [Id|Parents], Safe1, Safe)
  215        ;   catch(safe_clauses(Gen, M, [Id|Parents], Safe1, Safe),
  216                  error(instantiation_error, Ctx),
  217                  unsafe(Parents, Ctx))
  218        )
  219    ),
  220    !.
  221safe(G, M, Parents, _, _) :-
  222    debug(sandbox(fail),
  223          'safe/1 failed for ~p (parents:~p)', [M:G, Parents]),
  224    fail.
  225
  226unsafe(Parents, Var) :-
  227    var(Var),
  228    !,
  229    nb_setval(sandbox_last_error,
  230              error(instantiation_error, sandbox(_, Parents))),
  231    fail.
  232unsafe(_Parents, Ctx) :-
  233    Ctx = sandbox(_,_),
  234    nb_setval(sandbox_last_error,
  235              error(instantiation_error, Ctx)),
  236    fail.
  237
  238rethrow_instantition_error(Parents) :-
  239    throw(error(instantiation_error, sandbox(_, Parents))).
  240
  241safe_clauses(G, M, Parents, Safe0, Safe) :-
  242    predicate_property(M:G, interpreted),
  243    def_module(M:G, MD:QG),
  244    \+ compiled(MD:QG),
  245    !,
  246    findall(Ref-Body, clause(MD:QG, Body, Ref), Bodies),
  247    safe_bodies(Bodies, MD, Parents, Safe0, Safe).
  248safe_clauses(G, M, [_|Parents], _, _) :-
  249    predicate_property(M:G, visible),
  250    !,
  251    throw(error(permission_error(call, sandboxed, G),
  252                sandbox(M:G, Parents))).
  253safe_clauses(_, _, [G|Parents], _, _) :-
  254    throw(error(existence_error(procedure, G),
  255                sandbox(G, Parents))).
  256
  257compiled(system:(@(_,_))).
  258
  259known_module(M:_, _) :-
  260    current_module(M),
  261    !.
  262known_module(M:G, Parents) :-
  263    throw(error(permission_error(call, sandboxed, M:G),
  264                sandbox(M:G, Parents))).
  265
  266add_iso_parent(G, Parents, Parents) :-
  267    is_control(G),
  268    !.
  269add_iso_parent(G, Parents, [G|Parents]).
  270
  271is_control((_,_)).
  272is_control((_;_)).
  273is_control((_->_)).
  274is_control((_*->_)).
  275is_control(\+(_)).
 safe_bodies(+Bodies, +Module, +Parents, +Safe0, -Safe)
Verify the safety of bodies. If a clause was compiled with a qualified module, we consider execution of the body in a different module not a cross-module call.
  284safe_bodies([], _, _, Safe, Safe).
  285safe_bodies([Ref-H|T], M, Parents, Safe0, Safe) :-
  286    (   H = M2:H2, nonvar(M2),
  287        clause_property(Ref, module(M2))
  288    ->  copy_term(H2, H3),
  289        CM = M2
  290    ;   copy_term(H, H3),
  291        CM = M
  292    ),
  293    safe(H3, CM, Parents, Safe0, Safe1),
  294    safe_bodies(T, M, Parents, Safe1, Safe).
  295
  296def_module(M:G, MD:QG) :-
  297    predicate_property(M:G, imported_from(MD)),
  298    !,
  299    meta_qualify(MD:G, M, QG).
  300def_module(M:G, M:QG) :-
  301    meta_qualify(M:G, M, QG).
 safe_list(+Called, +Module, +Parents, +Safe0, -Safe)
Processed objects called through meta predicates. If the called object is in our current context we remove the module qualification to avoid the cross-module check.
  309safe_list([], _, _, Safe, Safe).
  310safe_list([H|T], M, Parents, Safe0, Safe) :-
  311    (   H = M2:H2,
  312        M == M2                             % in our context
  313    ->  copy_term(H2, H3)
  314    ;   copy_term(H, H3)                    % cross-module call
  315    ),
  316    safe(H3, M, Parents, Safe0, Safe1),
  317    safe_list(T, M, Parents, Safe1, Safe).
 meta_qualify(:G, +M, -QG) is det
Perform meta-qualification of the goal-argument
  323meta_qualify(MD:G, M, QG) :-
  324    predicate_property(MD:G, meta_predicate(Head)),
  325    !,
  326    G =.. [Name|Args],
  327    Head =.. [_|Q],
  328    qualify_args(Q, M, Args, QArgs),
  329    QG =.. [Name|QArgs].
  330meta_qualify(_:G, _, G).
  331
  332qualify_args([], _, [], []).
  333qualify_args([H|T], M, [A|AT], [Q|QT]) :-
  334    qualify_arg(H, M, A, Q),
  335    qualify_args(T, M, AT, QT).
  336
  337qualify_arg(S, M, A, Q) :-
  338    q_arg(S),
  339    !,
  340    qualify(A, M, Q).
  341qualify_arg(_, _, A, A).
  342
  343q_arg(I) :- integer(I), !.
  344q_arg(:).
  345q_arg(^).
  346q_arg(//).
  347
  348qualify(A, M, MZ:Q) :-
  349    strip_module(M:A, MZ, Q).
 goal_id(:Goal, -Id, -Gen) is nondet
Generate an identifier for the goal proven to be safe. We first try to prove the most general form of the goal. If this fails, we try to prove more specific versions.
To be done
- Do step-by-step generalisation instead of the current two levels (most general and most specific).
- We could also use variant_sha1 for the goal ids.
  361goal_id(M:Goal, M:Id, Gen) :-
  362    !,
  363    goal_id(Goal, Id, Gen).
  364goal_id(Var, _, _) :-
  365    var(Var),
  366    !,
  367    instantiation_error(Var).
  368goal_id(Atom, Atom, Atom) :-
  369    atom(Atom),
  370    !.
  371goal_id(Term, _, _) :-
  372    \+ compound(Term),
  373    !,
  374    type_error(callable, Term).
  375goal_id(Term, Skolem, Gen) :-           % most general form
  376    compound_name_arity(Term, Name, Arity),
  377    compound_name_arity(Skolem, Name, Arity),
  378    compound_name_arity(Gen, Name, Arity),
  379    copy_goal_args(1, Term, Skolem, Gen),
  380    (   Gen =@= Term
  381    ->  !                           % No more specific one; we can commit
  382    ;   true
  383    ),
  384    numbervars(Skolem, 0, _).
  385goal_id(Term, Skolem, Term) :-          % most specific form
  386    debug(sandbox(specify), 'Retrying with ~p', [Term]),
  387    copy_term(Term, Skolem),
  388    numbervars(Skolem, 0, _).
 copy_goal_args(+I, +Term, +Skolem, +Gen) is det
Create the most general form, but keep module qualified arguments because they will likely be called anyway.
  395copy_goal_args(I, Term, Skolem, Gen) :-
  396    arg(I, Term, TA),
  397    !,
  398    arg(I, Skolem, SA),
  399    arg(I, Gen, GA),
  400    copy_goal_arg(TA, SA, GA),
  401    I2 is I + 1,
  402    copy_goal_args(I2, Term, Skolem, Gen).
  403copy_goal_args(_, _, _, _).
  404
  405copy_goal_arg(Arg, SArg, Arg) :-
  406    copy_goal_arg(Arg),
  407    !,
  408    copy_term(Arg, SArg).
  409copy_goal_arg(_, _, _).
  410
  411copy_goal_arg(Var) :- var(Var), !, fail.
  412copy_goal_arg(_:_).
 verify_safe_declaration(+Decl)
See whether a safe declaration makes sense. That is, the predicate must be defined (such that the attacker cannot define the predicate), must be sufficiently instantiated and only ISO declared predicates may omit a module qualification.
To be done
- Verify safe_meta/2 declarations. It is a bit less clear what the rules are.
  424term_expansion(safe_primitive(Goal), Term) :-
  425    (   verify_safe_declaration(Goal)
  426    ->  Term = safe_primitive(Goal)
  427    ;   Term = []
  428    ).
  429
  430system:term_expansion(sandbox:safe_primitive(Goal), Term) :-
  431    \+ current_prolog_flag(xref, true),
  432    (   verify_safe_declaration(Goal)
  433    ->  Term = sandbox:safe_primitive(Goal)
  434    ;   Term = []
  435    ).
  436
  437verify_safe_declaration(Var) :-
  438    var(Var),
  439    !,
  440    instantiation_error(Var).
  441verify_safe_declaration(Module:Goal) :-
  442    !,
  443    must_be(atom, Module),
  444    must_be(callable, Goal),
  445    (   ok_meta(Module:Goal)
  446    ->  true
  447    ;   (   predicate_property(Module:Goal, visible)
  448        ->  true
  449        ;   predicate_property(Module:Goal, foreign)
  450        ),
  451        \+ predicate_property(Module:Goal, imported_from(_)),
  452        \+ predicate_property(Module:Goal, meta_predicate(_))
  453    ->  true
  454    ;   permission_error(declare, safe_goal, Module:Goal)
  455    ).
  456verify_safe_declaration(Goal) :-
  457    must_be(callable, Goal),
  458    (   predicate_property(system:Goal, iso),
  459        \+ predicate_property(system:Goal, meta_predicate())
  460    ->  true
  461    ;   permission_error(declare, safe_goal, Goal)
  462    ).
  463
  464ok_meta(system:assert(_)).
  465ok_meta(system:use_module(_,_)).
  466ok_meta(system:use_module(_)).
  467
  468verify_predefined_safe_declarations :-
  469    forall(clause(safe_primitive(Goal), _Body, Ref),
  470           ( catch(verify_safe_declaration(Goal), E, true),
  471             (   nonvar(E)
  472             ->  clause_property(Ref, file(File)),
  473                 clause_property(Ref, line_count(Line)),
  474                 print_message(error, bad_safe_declaration(Goal, File, Line))
  475             ;   true
  476             )
  477           )).
  478
  479:- initialization(verify_predefined_safe_declarations, now).
 safe_primitive(?Goal) is nondet
True if Goal is safe to call (i.e., cannot access dangerous system-resources and cannot upset other parts of the Prolog process). There are two types of facts. ISO built-ins are declared without a module prefix. This is safe because it is not allowed to (re-)define these primitives (i.e., give them an unsafe implementation) and the way around (redefine_system_predicate/1) is unsafe. The other group are module-qualified and only match if the system infers that the predicate is imported from the given module.
  493% First, all ISO system predicates that are considered safe
  494
  495safe_primitive(true).
  496safe_primitive(fail).
  497safe_primitive(system:false).
  498safe_primitive(repeat).
  499safe_primitive(!).
  500                                        % types
  501safe_primitive(var(_)).
  502safe_primitive(nonvar(_)).
  503safe_primitive(system:attvar(_)).
  504safe_primitive(integer(_)).
  505safe_primitive(float(_)).
  506safe_primitive(system:rational(_)).
  507safe_primitive(number(_)).
  508safe_primitive(atom(_)).
  509safe_primitive(system:blob(_,_)).
  510safe_primitive(system:string(_)).
  511safe_primitive(atomic(_)).
  512safe_primitive(compound(_)).
  513safe_primitive(callable(_)).
  514safe_primitive(ground(_)).
  515safe_primitive(system:cyclic_term(_)).
  516safe_primitive(acyclic_term(_)).
  517safe_primitive(system:is_stream(_)).
  518safe_primitive(system:'$is_char'(_)).
  519safe_primitive(system:'$is_char_code'(_)).
  520safe_primitive(system:'$is_char_list'(_,_)).
  521safe_primitive(system:'$is_code_list'(_,_)).
  522                                        % ordering
  523safe_primitive(@>(_,_)).
  524safe_primitive(@>=(_,_)).
  525safe_primitive(==(_,_)).
  526safe_primitive(@<(_,_)).
  527safe_primitive(@=<(_,_)).
  528safe_primitive(compare(_,_,_)).
  529safe_primitive(sort(_,_)).
  530safe_primitive(keysort(_,_)).
  531safe_primitive(system: =@=(_,_)).
  532safe_primitive(system:'$btree_find_node'(_,_,_,_,_)).
  533
  534                                        % unification and equivalence
  535safe_primitive(=(_,_)).
  536safe_primitive(\=(_,_)).
  537safe_primitive(system:'?='(_,_)).
  538safe_primitive(system:unifiable(_,_,_)).
  539safe_primitive(unify_with_occurs_check(_,_)).
  540safe_primitive(\==(_,_)).
  541                                        % arithmetic
  542safe_primitive(is(_,_)).
  543safe_primitive(>(_,_)).
  544safe_primitive(>=(_,_)).
  545safe_primitive(=:=(_,_)).
  546safe_primitive(=\=(_,_)).
  547safe_primitive(=<(_,_)).
  548safe_primitive(<(_,_)).
  549:- if(current_prolog_flag(bounded, false)).  550safe_primitive(system:nth_integer_root_and_remainder(_,_,_,_)).
  551:- endif.  552
  553                                        % term-handling
  554safe_primitive(arg(_,_,_)).
  555safe_primitive(system:setarg(_,_,_)).
  556safe_primitive(system:nb_setarg(_,_,_)).
  557safe_primitive(system:nb_linkarg(_,_,_)).
  558safe_primitive(functor(_,_,_)).
  559safe_primitive(_ =.. _).
  560safe_primitive(system:compound_name_arity(_,_,_)).
  561safe_primitive(system:compound_name_arguments(_,_,_)).
  562safe_primitive(system:'$filled_array'(_,_,_,_)).
  563safe_primitive(copy_term(_,_)).
  564safe_primitive(system:duplicate_term(_,_)).
  565safe_primitive(system:copy_term_nat(_,_)).
  566safe_primitive(system:size_abstract_term(_,_,_)).
  567safe_primitive(numbervars(_,_,_)).
  568safe_primitive(system:numbervars(_,_,_,_)).
  569safe_primitive(subsumes_term(_,_)).
  570safe_primitive(system:term_hash(_,_)).
  571safe_primitive(system:term_hash(_,_,_,_)).
  572safe_primitive(system:variant_sha1(_,_)).
  573safe_primitive(system:variant_hash(_,_)).
  574safe_primitive(system:'$term_size'(_,_,_)).
  575
  576                                        % dicts
  577safe_primitive(system:is_dict(_)).
  578safe_primitive(system:is_dict(_,_)).
  579safe_primitive(system:get_dict(_,_,_)).
  580safe_primitive(system:get_dict(_,_,_,_,_)).
  581safe_primitive(system:'$get_dict_ex'(_,_,_)).
  582safe_primitive(system:dict_create(_,_,_)).
  583safe_primitive(system:dict_pairs(_,_,_)).
  584safe_primitive(system:put_dict(_,_,_)).
  585safe_primitive(system:put_dict(_,_,_,_)).
  586safe_primitive(system:del_dict(_,_,_,_)).
  587safe_primitive(system:select_dict(_,_,_)).
  588safe_primitive(system:b_set_dict(_,_,_)).
  589safe_primitive(system:nb_set_dict(_,_,_)).
  590safe_primitive(system:nb_link_dict(_,_,_)).
  591safe_primitive(system:(:<(_,_))).
  592safe_primitive(system:(>:<(_,_))).
  593                                        % atoms
  594safe_primitive(atom_chars(_, _)).
  595safe_primitive(atom_codes(_, _)).
  596safe_primitive(sub_atom(_,_,_,_,_)).
  597safe_primitive(atom_concat(_,_,_)).
  598safe_primitive(atom_length(_,_)).
  599safe_primitive(char_code(_,_)).
  600safe_primitive(system:name(_,_)).
  601safe_primitive(system:atomic_concat(_,_,_)).
  602safe_primitive(system:atomic_list_concat(_,_)).
  603safe_primitive(system:atomic_list_concat(_,_,_)).
  604safe_primitive(system:downcase_atom(_,_)).
  605safe_primitive(system:upcase_atom(_,_)).
  606safe_primitive(system:char_type(_,_)).
  607safe_primitive(system:normalize_space(_,_)).
  608safe_primitive(system:sub_atom_icasechk(_,_,_)).
  609                                        % numbers
  610safe_primitive(number_codes(_,_)).
  611safe_primitive(number_chars(_,_)).
  612safe_primitive(system:atom_number(_,_)).
  613safe_primitive(system:code_type(_,_)).
  614                                        % strings
  615safe_primitive(system:atom_string(_,_)).
  616safe_primitive(system:number_string(_,_)).
  617safe_primitive(system:string_chars(_, _)).
  618safe_primitive(system:string_codes(_, _)).
  619safe_primitive(system:string_code(_,_,_)).
  620safe_primitive(system:sub_string(_,_,_,_,_)).
  621safe_primitive(system:split_string(_,_,_,_)).
  622safe_primitive(system:atomics_to_string(_,_,_)).
  623safe_primitive(system:atomics_to_string(_,_)).
  624safe_primitive(system:string_concat(_,_,_)).
  625safe_primitive(system:string_length(_,_)).
  626safe_primitive(system:string_lower(_,_)).
  627safe_primitive(system:string_upper(_,_)).
  628safe_primitive(system:term_string(_,_)).
  629safe_primitive('$syspreds':term_string(_,_,_)).
  630                                        % Lists
  631safe_primitive(length(_,_)).
  632                                        % exceptions
  633safe_primitive(throw(_)).
  634safe_primitive(system:abort).
  635                                        % misc
  636safe_primitive(current_prolog_flag(_,_)).
  637safe_primitive(current_op(_,_,_)).
  638safe_primitive(system:sleep(_)).
  639safe_primitive(system:thread_self(_)).
  640safe_primitive(system:get_time(_)).
  641safe_primitive(system:statistics(_,_)).
  642safe_primitive(system:thread_statistics(Id,_,_)) :-
  643    (   var(Id)
  644    ->  instantiation_error(Id)
  645    ;   thread_self(Id)
  646    ).
  647safe_primitive(system:thread_property(Id,_)) :-
  648    (   var(Id)
  649    ->  instantiation_error(Id)
  650    ;   thread_self(Id)
  651    ).
  652safe_primitive(system:format_time(_,_,_)).
  653safe_primitive(system:format_time(_,_,_,_)).
  654safe_primitive(system:date_time_stamp(_,_)).
  655safe_primitive(system:stamp_date_time(_,_,_)).
  656safe_primitive(system:strip_module(_,_,_)).
  657safe_primitive('$messages':message_to_string(_,_)).
  658safe_primitive(system:import_module(_,_)).
  659safe_primitive(system:file_base_name(_,_)).
  660safe_primitive(system:file_directory_name(_,_)).
  661safe_primitive(system:file_name_extension(_,_,_)).
  662
  663safe_primitive(clause(H,_)) :- safe_clause(H).
  664safe_primitive(asserta(X)) :- safe_assert(X).
  665safe_primitive(assertz(X)) :- safe_assert(X).
  666safe_primitive(retract(X)) :- safe_assert(X).
  667safe_primitive(retractall(X)) :- safe_assert(X).
  668
  669% We need to do data flow analysis to find the tag of the
  670% target key before we can conclude that functions on dicts
  671% are safe.
  672safe_primitive('$dicts':'.'(_,K,_)) :- atom(K).
  673safe_primitive('$dicts':'.'(_,K,_)) :-
  674    (   nonvar(K)
  675    ->  dict_built_in(K)
  676    ;   instantiation_error(K)
  677    ).
  678
  679dict_built_in(get(_)).
  680dict_built_in(put(_)).
  681dict_built_in(put(_,_)).
  682
  683% The non-ISO system predicates.  These can be redefined, so we must
  684% be careful to ensure the system ones are used.
  685
  686safe_primitive(system:false).
  687safe_primitive(system:cyclic_term(_)).
  688safe_primitive(system:msort(_,_)).
  689safe_primitive(system:sort(_,_,_,_)).
  690safe_primitive(system:between(_,_,_)).
  691safe_primitive(system:succ(_,_)).
  692safe_primitive(system:plus(_,_,_)).
  693safe_primitive(system:float_class(_,_)).
  694safe_primitive(system:term_variables(_,_)).
  695safe_primitive(system:term_variables(_,_,_)).
  696safe_primitive(system:'$term_size'(_,_,_)).
  697safe_primitive(system:atom_to_term(_,_,_)).
  698safe_primitive(system:term_to_atom(_,_)).
  699safe_primitive(system:atomic_list_concat(_,_,_)).
  700safe_primitive(system:atomic_list_concat(_,_)).
  701safe_primitive(system:downcase_atom(_,_)).
  702safe_primitive(system:upcase_atom(_,_)).
  703safe_primitive(system:is_list(_)).
  704safe_primitive(system:memberchk(_,_)).
  705safe_primitive(system:'$skip_list'(_,_,_)).
  706                                        % attributes
  707safe_primitive(system:get_attr(_,_,_)).
  708safe_primitive(system:get_attrs(_,_)).
  709safe_primitive(system:term_attvars(_,_)).
  710safe_primitive(system:del_attr(_,_)).
  711safe_primitive(system:del_attrs(_)).
  712safe_primitive('$attvar':copy_term(_,_,_)).
  713                                        % globals
  714safe_primitive(system:b_getval(_,_)).
  715safe_primitive(system:b_setval(Var,_)) :-
  716    safe_global_var(Var).
  717safe_primitive(system:nb_getval(_,_)).
  718safe_primitive('$syspreds':nb_setval(Var,_)) :-
  719    safe_global_var(Var).
  720safe_primitive(system:nb_linkval(Var,_)) :-
  721    safe_global_var(Var).
  722safe_primitive(system:nb_current(_,_)).
  723                                        % database
  724safe_primitive(system:assert(X)) :-
  725    safe_assert(X).
  726                                        % Output
  727safe_primitive(system:writeln(_)).
  728safe_primitive('$messages':print_message(_,_)).
  729
  730                                        % Stack limits (down)
  731safe_primitive('$syspreds':set_prolog_stack(Stack, limit(ByteExpr))) :-
  732    nonvar(Stack),
  733    stack_name(Stack),
  734    catch(Bytes is ByteExpr, _, fail),
  735    prolog_stack_property(Stack, limit(Current)),
  736    Bytes =< Current.
  737
  738stack_name(global).
  739stack_name(local).
  740stack_name(trail).
  741
  742safe_primitive('$tabling':abolish_all_tables).
  743safe_primitive('$tabling':'$wrap_tabled'(Module:_Head, _Mode)) :-
  744    prolog_load_context(module, Module),
  745    !.
  746safe_primitive('$tabling':'$moded_wrap_tabled'(Module:_Head,_,_,_)) :-
  747    prolog_load_context(module, Module),
  748    !.
  749
  750
  751% use_module/1.  We only allow for .pl files that are loaded from
  752% relative paths that do not contain /../
  753
  754safe_primitive(system:use_module(Spec, _Import)) :-
  755    safe_primitive(system:use_module(Spec)).
  756safe_primitive(system:use_module(Spec)) :-
  757    ground(Spec),
  758    (   atom(Spec)
  759    ->  Path = Spec
  760    ;   Spec =.. [_Alias, Segments],
  761        phrase(segments_to_path(Segments), List),
  762        atomic_list_concat(List, Path)
  763    ),
  764    \+ is_absolute_file_name(Path),
  765    \+ sub_atom(Path, _, _, _, '/../'),
  766    absolute_file_name(Spec, AbsFile,
  767                       [ access(read),
  768                         file_type(prolog),
  769                         file_errors(fail)
  770                       ]),
  771    file_name_extension(_, Ext, AbsFile),
  772    save_extension(Ext).
  773
  774% support predicates for safe_primitive, validating the safety of
  775% arguments to certain goals.
  776
  777segments_to_path(A/B) -->
  778    !,
  779    segments_to_path(A),
  780    [/],
  781    segments_to_path(B).
  782segments_to_path(X) -->
  783    [X].
  784
  785save_extension(pl).
 safe_assert(+Term) is semidet
True if assert(Term) is safe, which means it asserts in the current module. Cross-module asserts are considered unsafe. We only allow for adding facts. In theory, we could also allow for rules if we prove the safety of the body.
  794safe_assert(C) :- cyclic_term(C), !, fail.
  795safe_assert(X) :- var(X), !, fail.
  796safe_assert(_Head:-_Body) :- !, fail.
  797safe_assert(_:_) :- !, fail.
  798safe_assert(_).
 safe_clause(+Head) is semidet
Consider a call to clause safe if it does not try to cross a module boundary. Cross-module usage of clause/2 can extract private information from other modules.
  806safe_clause(H) :- var(H), !.
  807safe_clause(_:_) :- !, fail.
  808safe_clause(_).
 safe_global_var(+Name) is semidet
True if Name is a global variable to which assertion is considered safe.
  816safe_global_var(Name) :-
  817    var(Name),
  818    !,
  819    instantiation_error(Name).
  820safe_global_var(Name) :-
  821    safe_global_variable(Name).
 safe_global_variable(Name) is semidet
Declare the given global variable safe to write to.
 safe_meta(+Goal, -Called:list(callable)) is semidet
Hook. True if Goal is a meta-predicate that is considered safe iff all elements in Called are safe.
  833safe_meta(system:put_attr(V,M,A), Called) :-
  834    !,
  835    (   atom(M)
  836    ->  attr_hook_predicates([ attr_unify_hook(A, _),
  837                               attribute_goals(V,_,_),
  838                               project_attributes(_,_)
  839                             ], M, Called)
  840    ;   instantiation_error(M)
  841    ).
  842safe_meta(system:with_output_to(Output, G), [G]) :-
  843    safe_output(Output),
  844    !.
  845safe_meta(system:format(Format, Args), Calls) :-
  846    format_calls(Format, Args, Calls).
  847safe_meta(system:format(Output, Format, Args), Calls) :-
  848    safe_output(Output),
  849    format_calls(Format, Args, Calls).
  850safe_meta(prolog_debug:debug(_Term, Format, Args), Calls) :-
  851    format_calls(Format, Args, Calls).
  852safe_meta(system:set_prolog_flag(Flag, Value), []) :-
  853    atom(Flag),
  854    safe_prolog_flag(Flag, Value).
  855safe_meta('$attvar':freeze(_Var,Goal), [Goal]).
  856safe_meta(phrase(NT,Xs0,Xs), [Goal]) :- % phrase/2,3 and call_dcg/2,3
  857    expand_nt(NT,Xs0,Xs,Goal).
  858safe_meta(phrase(NT,Xs0), [Goal]) :-
  859    expand_nt(NT,Xs0,[],Goal).
  860safe_meta('$dcg':call_dcg(NT,Xs0,Xs), [Goal]) :-
  861    expand_nt(NT,Xs0,Xs,Goal).
  862safe_meta('$dcg':call_dcg(NT,Xs0), [Goal]) :-
  863    expand_nt(NT,Xs0,[],Goal).
  864safe_meta('$tabling':abolish_table_subgoals(V), []) :-
  865    \+ qualified(V).
  866safe_meta('$tabling':current_table(V, _), []) :-
  867    \+ qualified(V).
  868safe_meta('$tabling':tnot(G), [G]).
  869safe_meta('$tabling':not_exists(G), [G]).
  870
  871qualified(V) :-
  872    nonvar(V),
  873    V = _:_.
 attr_hook_predicates(+Hooks0, +Module, -Hooks) is det
Filter the defined hook implementations. This is safe because (1) calling an undefined predicate is not a safety issue, (2) the user an only assert in the current module and only predicates that have a safe body. This avoids the need to define attribute hooks solely for the purpose of making them safe.
  883attr_hook_predicates([], _, []).
  884attr_hook_predicates([H|T], M, Called) :-
  885    (   predicate_property(M:H, defined)
  886    ->  Called = [M:H|Rest]
  887    ;   Called = Rest
  888    ),
  889    attr_hook_predicates(T, M, Rest).
 expand_nt(+NT, ?Xs0, ?Xs, -NewGoal)
Similar to expand_phrase/2, but we do throw errors instead of failing if NT is not sufficiently instantiated.
  897expand_nt(NT, _Xs0, _Xs, _NewGoal) :-
  898    strip_module(NT, _, Plain),
  899    var(Plain),
  900    !,
  901    instantiation_error(Plain).
  902expand_nt(NT, Xs0, Xs, NewGoal) :-
  903    dcg_translate_rule((pseudo_nt --> NT),
  904                       (pseudo_nt(Xs0c,Xsc) :- NewGoal0)),
  905    (   var(Xsc), Xsc \== Xs0c
  906    ->  Xs = Xsc, NewGoal1 = NewGoal0
  907    ;   NewGoal1 = (NewGoal0, Xsc = Xs)
  908    ),
  909    (   var(Xs0c)
  910    ->  Xs0 = Xs0c,
  911        NewGoal = NewGoal1
  912    ;   NewGoal = ( Xs0 = Xs0c, NewGoal1 )
  913    ).
 safe_meta_call(+Goal, +Context, -Called:list(callable)) is semidet
True if Goal is a meta-predicate that is considered safe iff all elements in Called are safe.
  920safe_meta_call(Goal, _, _Called) :-
  921    debug(sandbox(meta), 'Safe meta ~p?', [Goal]),
  922    fail.
  923safe_meta_call(Goal, Context, Called) :-
  924    (   safe_meta(Goal, Called)
  925    ->  true
  926    ;   safe_meta(Goal, Context, Called)
  927    ),
  928    !.     % call hook
  929safe_meta_call(Goal, _, Called) :-
  930    Goal = M:Plain,
  931    compound(Plain),
  932    compound_name_arity(Plain, Name, Arity),
  933    safe_meta_predicate(M:Name/Arity),
  934    predicate_property(Goal, meta_predicate(Spec)),
  935    !,
  936    called(Spec, Plain, Called).
  937safe_meta_call(M:Goal, _, Called) :-
  938    !,
  939    generic_goal(Goal, Gen),
  940    safe_meta(M:Gen),
  941    called(Gen, Goal, Called).
  942safe_meta_call(Goal, _, Called) :-
  943    generic_goal(Goal, Gen),
  944    safe_meta(Gen),
  945    called(Gen, Goal, Called).
  946
  947called(Gen, Goal, Called) :-
  948    compound_name_arity(Goal, _, Arity),
  949    called(1, Arity, Gen, Goal, Called).
  950
  951called(I, Arity, Gen, Goal, Called) :-
  952    I =< Arity,
  953    !,
  954    arg(I, Gen, Spec),
  955    (   calling_meta_spec(Spec)
  956    ->  arg(I, Goal, Called0),
  957        extend(Spec, Called0, G),
  958        Called = [G|Rest]
  959    ;   Called = Rest
  960    ),
  961    I2 is I+1,
  962    called(I2, Arity, Gen, Goal, Rest).
  963called(_, _, _, _, []).
  964
  965generic_goal(G, Gen) :-
  966    functor(G, Name, Arity),
  967    functor(Gen, Name, Arity).
  968
  969calling_meta_spec(V) :- var(V), !, fail.
  970calling_meta_spec(I) :- integer(I), !.
  971calling_meta_spec(^).
  972calling_meta_spec(//).
  973
  974
  975extend(^, G, Plain) :-
  976    !,
  977    strip_existential(G, Plain).
  978extend(//, DCG, Goal) :-
  979    !,
  980    (   expand_phrase(call_dcg(DCG,_,_), Goal)
  981    ->  true
  982    ;   instantiation_error(DCG)    % Ask more instantiation.
  983    ).                              % might not help, but does not harm.
  984extend(0, G, G) :- !.
  985extend(I, M:G0, M:G) :-
  986    !,
  987    G0 =.. List,
  988    length(Extra, I),
  989    append(List, Extra, All),
  990    G =.. All.
  991extend(I, G0, G) :-
  992    G0 =.. List,
  993    length(Extra, I),
  994    append(List, Extra, All),
  995    G =.. All.
  996
  997strip_existential(Var, Var) :-
  998    var(Var),
  999    !.
 1000strip_existential(M:G0, M:G) :-
 1001    !,
 1002    strip_existential(G0, G).
 1003strip_existential(_^G0, G) :-
 1004    !,
 1005    strip_existential(G0, G).
 1006strip_existential(G, G).
 safe_meta(?Template)
 1010safe_meta((0,0)).
 1011safe_meta((0;0)).
 1012safe_meta((0->0)).
 1013safe_meta(system:(0*->0)).
 1014safe_meta(catch(0,*,0)).
 1015safe_meta(findall(*,0,*)).
 1016safe_meta('$bags':findall(*,0,*,*)).
 1017safe_meta(setof(*,^,*)).
 1018safe_meta(bagof(*,^,*)).
 1019safe_meta('$bags':findnsols(*,*,0,*)).
 1020safe_meta('$bags':findnsols(*,*,0,*,*)).
 1021safe_meta(system:call_cleanup(0,0)).
 1022safe_meta(system:setup_call_cleanup(0,0,0)).
 1023safe_meta(system:setup_call_catcher_cleanup(0,0,*,0)).
 1024safe_meta('$attvar':call_residue_vars(0,*)).
 1025safe_meta('$syspreds':call_with_inference_limit(0,*,*)).
 1026safe_meta('$syspreds':call_with_depth_limit(0,*,*)).
 1027safe_meta(^(*,0)).
 1028safe_meta(\+(0)).
 1029safe_meta(call(0)).
 1030safe_meta(call(1,*)).
 1031safe_meta(call(2,*,*)).
 1032safe_meta(call(3,*,*,*)).
 1033safe_meta(call(4,*,*,*,*)).
 1034safe_meta(call(5,*,*,*,*,*)).
 1035safe_meta(call(6,*,*,*,*,*,*)).
 1036safe_meta('$tabling':start_tabling(*,0)).
 1037safe_meta('$tabling':start_tabling(*,0,*,*)).
 safe_output(+Output)
True if something is a safe output argument for with_output_to/2 and friends. We do not want writing to streams.
 1044safe_output(Output) :-
 1045    var(Output),
 1046    !,
 1047    instantiation_error(Output).
 1048safe_output(atom(_)).
 1049safe_output(string(_)).
 1050safe_output(codes(_)).
 1051safe_output(codes(_,_)).
 1052safe_output(chars(_)).
 1053safe_output(chars(_,_)).
 1054safe_output(current_output).
 1055safe_output(current_error).
 format_calls(+Format, +FormatArgs, -Calls)
Find ~@ calls from Format and Args.
 1061:- public format_calls/3.                       % used in pengines_io
 1062
 1063format_calls(Format, _Args, _Calls) :-
 1064    var(Format),
 1065    !,
 1066    instantiation_error(Format).
 1067format_calls(Format, Args, Calls) :-
 1068    format_types(Format, Types),
 1069    (   format_callables(Types, Args, Calls)
 1070    ->  true
 1071    ;   throw(error(format_error(Format, Types, Args), _))
 1072    ).
 1073
 1074format_callables([], [], []).
 1075format_callables([callable|TT], [G|TA], [G|TG]) :-
 1076    !,
 1077    format_callables(TT, TA, TG).
 1078format_callables([_|TT], [_|TA], TG) :-
 1079    !,
 1080    format_callables(TT, TA, TG).
 1081
 1082
 1083                 /*******************************
 1084                 *    SAFE COMPILATION HOOKS    *
 1085                 *******************************/
 1086
 1087:- multifile
 1088    prolog:sandbox_allowed_directive/1,
 1089    prolog:sandbox_allowed_goal/1,
 1090    prolog:sandbox_allowed_expansion/1.
 prolog:sandbox_allowed_directive(:G) is det
Throws an exception if G is not considered a safe directive.
 1096prolog:sandbox_allowed_directive(Directive) :-
 1097    debug(sandbox(directive), 'Directive: ~p', [Directive]),
 1098    fail.
 1099prolog:sandbox_allowed_directive(Directive) :-
 1100    safe_directive(Directive),
 1101    !.
 1102prolog:sandbox_allowed_directive(M:PredAttr) :-
 1103    \+ prolog_load_context(module, M),
 1104    !,
 1105    debug(sandbox(directive), 'Cross-module directive', []),
 1106    permission_error(execute, sandboxed_directive, (:- M:PredAttr)).
 1107prolog:sandbox_allowed_directive(M:PredAttr) :-
 1108    safe_pattr(PredAttr),
 1109    !,
 1110    PredAttr =.. [Attr, Preds],
 1111    (   safe_pattr(Preds, Attr)
 1112    ->  true
 1113    ;   permission_error(execute, sandboxed_directive, (:- M:PredAttr))
 1114    ).
 1115prolog:sandbox_allowed_directive(_:Directive) :-
 1116    safe_source_directive(Directive),
 1117    !.
 1118prolog:sandbox_allowed_directive(_:Directive) :-
 1119    directive_loads_file(Directive, File),
 1120    !,
 1121    safe_path(File).
 1122prolog:sandbox_allowed_directive(G) :-
 1123    safe_goal(G).
 safe_directive(:Directive) is semidet
Hook to declare additional directives as safe. The argument is a term Module:Directive (without :- wrapper). In almost all cases, the implementation must verify that the Module is the current load context as illustrated below. This check is not performed by the system to allow for cases where particular cross-module directives are allowed.
sandbox:safe_directive(M:Directive) :-
    prolog_load_context(module, M),
    ...
 1141safe_pattr(dynamic(_)).
 1142safe_pattr(thread_local(_)).
 1143safe_pattr(volatile(_)).
 1144safe_pattr(discontiguous(_)).
 1145safe_pattr(multifile(_)).
 1146safe_pattr(public(_)).
 1147safe_pattr(meta_predicate(_)).
 1148safe_pattr(table(_)).
 1149safe_pattr(non_terminal(_)).
 1150
 1151safe_pattr(Var, _) :-
 1152    var(Var),
 1153    !,
 1154    instantiation_error(Var).
 1155safe_pattr((A,B), Attr) :-
 1156    !,
 1157    safe_pattr(A, Attr),
 1158    safe_pattr(B, Attr).
 1159safe_pattr(M:G, Attr) :-
 1160    !,
 1161    (   atom(M),
 1162        prolog_load_context(module, M)
 1163    ->  true
 1164    ;   Goal =.. [Attr,M:G],
 1165        permission_error(directive, sandboxed, (:- Goal))
 1166    ).
 1167safe_pattr(_, _).
 1168
 1169safe_source_directive(op(_,_,Name)) :-
 1170    !,
 1171    (   atom(Name)
 1172    ->  true
 1173    ;   is_list(Name),
 1174        maplist(atom, Name)
 1175    ).
 1176safe_source_directive(set_prolog_flag(Flag, Value)) :-
 1177    !,
 1178    atom(Flag), ground(Value),
 1179    safe_prolog_flag(Flag, Value).
 1180safe_source_directive(style_check(_)).
 1181safe_source_directive(initialization(_)).   % Checked at runtime
 1182safe_source_directive(initialization(_,_)). % Checked at runtime
 1183
 1184directive_loads_file(use_module(library(X)), X).
 1185directive_loads_file(use_module(library(X), _Imports), X).
 1186directive_loads_file(ensure_loaded(library(X)), X).
 1187directive_loads_file(include(X), X).
 1188
 1189safe_path(X) :-
 1190    var(X),
 1191    !,
 1192    instantiation_error(X).
 1193safe_path(X) :-
 1194    (   atom(X)
 1195    ;   string(X)
 1196    ),
 1197    !,
 1198    \+ sub_atom(X, 0, _, 0, '..'),
 1199    \+ sub_atom(X, 0, _, _, '/'),
 1200    \+ sub_atom(X, 0, _, _, '../'),
 1201    \+ sub_atom(X, _, _, 0, '/..'),
 1202    \+ sub_atom(X, _, _, _, '/../').
 1203safe_path(A/B) :-
 1204    !,
 1205    safe_path(A),
 1206    safe_path(B).
 safe_prolog_flag(+Flag, +Value) is det
True if it is safe to set the flag Flag to Value.
To be done
- If we can avoid that files are loaded after changing this flag, we can allow for more flags. The syntax flags are safe because they are registered with the module.
 1218% misc
 1219safe_prolog_flag(generate_debug_info, _).
 1220safe_prolog_flag(optimise, _).
 1221safe_prolog_flag(occurs_check, _).
 1222% syntax
 1223safe_prolog_flag(var_prefix, _).
 1224safe_prolog_flag(double_quotes, _).
 1225safe_prolog_flag(back_quotes, _).
 1226safe_prolog_flag(rational_syntax, _).
 1227% arithmetic
 1228safe_prolog_flag(prefer_rationals, _).
 1229safe_prolog_flag(float_overflow, _).
 1230safe_prolog_flag(float_zero_div, _).
 1231safe_prolog_flag(float_undefined, _).
 1232safe_prolog_flag(float_underflow, _).
 1233safe_prolog_flag(float_rounding, _).
 1234safe_prolog_flag(float_rounding, _).
 1235safe_prolog_flag(max_rational_size, _).
 1236safe_prolog_flag(max_rational_size_action, _).
 1237% tabling
 1238safe_prolog_flag(max_answers_for_subgoal,_).
 1239safe_prolog_flag(max_answers_for_subgoal_action,_).
 1240safe_prolog_flag(max_table_answer_size,_).
 1241safe_prolog_flag(max_table_answer_size_action,_).
 1242safe_prolog_flag(max_table_subgoal_size,_).
 1243safe_prolog_flag(max_table_subgoal_size_action,_).
 prolog:sandbox_allowed_expansion(:G) is det
Throws an exception if G is not considered a safe expansion goal. This deals with call-backs from the compiler for

Our assumption is that external expansion rules are coded safely and we only need to be careful if the sandboxed code defines expansion rules.

 1259prolog:sandbox_allowed_expansion(M:G) :-
 1260    prolog_load_context(module, M),
 1261    !,
 1262    debug(sandbox(expansion), 'Expand in ~p: ~p', [M, G]),
 1263    safe_goal(M:G).
 1264prolog:sandbox_allowed_expansion(_,_).
 prolog:sandbox_allowed_goal(:G) is det
Throw an exception if it is not safe to call G
 1270prolog:sandbox_allowed_goal(G) :-
 1271    safe_goal(G).
 1272
 1273
 1274                 /*******************************
 1275                 *            MESSAGES          *
 1276                 *******************************/
 1277
 1278:- multifile
 1279    prolog:message//1,
 1280    prolog:message_context//1,
 1281    prolog:error_message//1. 1282
 1283prolog:message(error(instantiation_error, Context)) -->
 1284    { nonvar(Context),
 1285      Context = sandbox(_Goal,Parents),
 1286      numbervars(Context, 1, _)
 1287    },
 1288    [ 'Sandbox restriction!'-[], nl,
 1289      'Could not derive which predicate may be called from'-[]
 1290    ],
 1291    (   { Parents == [] }
 1292    ->  [ 'Search space too large'-[] ]
 1293    ;   callers(Parents, 10)
 1294    ).
 1295
 1296prolog:message_context(sandbox(_G, [])) --> !.
 1297prolog:message_context(sandbox(_G, Parents)) -->
 1298    [ nl, 'Reachable from:'-[] ],
 1299    callers(Parents, 10).
 1300
 1301callers([], _) --> !.
 1302callers(_,  0) --> !.
 1303callers([G|Parents], Level) -->
 1304    { NextLevel is Level-1
 1305    },
 1306    [ nl, '\t  ~p'-[G] ],
 1307    callers(Parents, NextLevel).
 1308
 1309prolog:message(bad_safe_declaration(Goal, File, Line)) -->
 1310    [ '~w:~d: Invalid safe_primitive/1 declaration: ~p'-
 1311      [File, Line, Goal] ].
 1312
 1313prolog:error_message(format_error(Format, Types, Args)) -->
 1314    format_error(Format, Types, Args).
 1315
 1316format_error(Format, Types, Args) -->
 1317    { length(Types, TypeLen),
 1318      length(Args, ArgsLen),
 1319      (   TypeLen > ArgsLen
 1320      ->  Problem = 'not enough'
 1321      ;   Problem = 'too many'
 1322      )
 1323    },
 1324    [ 'format(~q): ~w arguments (found ~w, need ~w)'-
 1325      [Format, Problem, ArgsLen, TypeLen]
 1326    ]