------------------------------------------------------------------------------ -- -- -- GNAT COMPILER COMPONENTS -- -- -- -- S E M _ S P A R K -- -- -- -- S p e c -- -- -- -- Copyright (C) 2017-2018, Free Software Foundation, Inc. -- -- -- -- GNAT is free software; you can redistribute it and/or modify it under -- -- terms of the GNU General Public License as published by the Free Soft- -- -- ware Foundation; either version 3, or (at your option) any later ver- -- -- sion. GNAT is distributed in the hope that it will be useful, but WITH- -- -- OUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY -- -- or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License -- -- for more details. You should have received a copy of the GNU General -- -- Public License distributed with GNAT; see file COPYING3. If not, go to -- -- http://www.gnu.org/licenses for a complete copy of the license. -- -- -- -- GNAT was originally developed by the GNAT team at New York University. -- -- Extensive contributions were provided by Ada Core Technologies Inc. -- -- -- ------------------------------------------------------------------------------ -- This package implements an anti-aliasing analysis for access types. The -- rules that are enforced are defined in the anti-aliasing section of the -- SPARK RM 6.4.2 -- -- Check_Safe_Pointers is called by Gnat1drv, when GNATprove mode is -- activated. It does an analysis of the source code, looking for code that is -- considered as SPARK and launches another function called Analyze_Node that -- will do the whole analysis. -- -- A path is an abstraction of a name, of which all indices, slices (for -- indexed components) and function calls have been abstracted and all -- dereferences are made explicit. A path is the atomic element viewed by the -- analysis, with the notion of prefixes and extensions of different paths. -- -- The analysis explores the AST, and looks for different constructs -- that may involve aliasing. These main constructs are assignments -- (N_Assignment_Statement, N_Object_Declaration, ...), or calls -- (N_Procedure_Call_Statement, N_Entry_Call_Statement, N_Function_Call). -- The analysis checks the permissions of each construct and updates them -- according to the SPARK RM. This can follow three main different types -- of operations: move, borrow, and observe. ---------------------------- -- Deep and shallow types -- ---------------------------- -- The analysis focuses on objects that can cause problems in terms of pointer -- aliasing. These objects have types that are called deep. Deep types are -- defined as being either types with an access part or class-wide types -- (which may have an access part in a derived type). Non-deep types are -- called shallow. Some objects of shallow type may cause pointer aliasing -- problems when they are explicitely marked as aliased (and then the aliasing -- occurs when we take the Access to this object and store it in a pointer). ---------- -- Move -- ---------- -- Moves can happen at several points in the program: during assignment (and -- any similar statement such as object declaration with initial value), or -- during return statements. -- -- The underlying concept consists of transferring the ownership of any path -- on the right-hand side to the left-hand side. There are some details that -- should be taken into account so as not to transfer paths that appear only -- as intermediate results of a more complex expression. -- More specifically, the SPARK RM defines moved expressions, and any moved -- expression that points directly to a path is then checked and sees its -- permissions updated accordingly. ------------ -- Borrow -- ------------ -- Borrows can happen in subprogram calls. They consist of a temporary -- transfer of ownership from a caller to a callee. Expressions that can be -- borrowed can be found in either procedure or entry actual parameters, and -- consist of parameters of mode either "out" or "in out", or parameters of -- mode "in" that are of type nonconstant access-to-variable. We consider -- global variables as implicit parameters to subprograms, with their mode -- given by the Global contract associated to the subprogram. Note that the -- analysis looks for such a Global contract mentioning any global variable -- of deep type accessed directly in the subprogram, and it raises an error if -- there is no Global contract, or if the Global contract does not mention the -- variable. -- -- A borrow of a parameter X is equivalent in terms of aliasing to moving -- X'Access to the callee, and then assigning back X at the end of the call. -- -- Borrowed parameters should have read-write permission (or write-only for -- "out" parameters), and should all have read-write permission at the end -- of the call (this guarantee is ensured by the callee). ------------- -- Observe -- ------------- -- Observed parameters are all the other parameters that are not borrowed and -- that may cause problems with aliasing. They are considered as being sent to -- the callee with Read-Only permission, so that they can be aliased safely. -- This is the only construct that allows aliasing that does not prevent -- accessing the old path that is being aliased. However, this comes with -- the restriction that those aliased path cannot be written in the callee. -------------------- -- Implementation -- -------------------- -- The implementation is based on trees that represent the possible paths -- in the source code. Those trees can be unbounded in depth, hence they are -- represented using lazy data structures, whose laziness is handled manually. -- Each time an identifier is declared, its path is added to the permission -- environment as a tree with only one node, the declared identifier. Each -- time a path is checked or updated, we look in the tree at the adequate -- node, unfolding the tree whenever needed. -- For this, each node has several variables that indicate whether it is -- deep (Is_Node_Deep), what permission it has (Permission), and what is -- the lowest permission of all its descendants (Children_Permission). After -- unfolding the tree, we update the permissions of each node, deleting the -- Children_Permission, and specifying new ones for the leaves of the unfolded -- tree. -- After assigning a path, the descendants of the assigned path are dumped -- (and hence the tree is folded back), given that all descendants directly -- get read-write permission, which can be specified using the node's -- Children_Permission field. with Types; use Types; package Sem_SPARK is procedure Check_Safe_Pointers (N : Node_Id); -- The entry point of this package. It analyzes a node and reports errors -- when there are violations of aliasing rules. end Sem_SPARK;