symex_dereference.cpp

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/*******************************************************************\
 
Module: Symbolic Execution of ANSI-C
 
Author: Daniel Kroening, kroening@kroening.com
 
\*******************************************************************/
 
 
#include "goto_symex.h"
 
#include <util/arith_tools.h>
#include <util/byte_operators.h>
#include <util/c_types.h>
#include <util/exception_utils.h>
#include <util/expr_util.h>
#include <util/fresh_symbol.h>
#include <util/invariant.h>
#include <util/pointer_offset_size.h>
 
#include <pointer-analysis/value_set_dereference.h>
 
#include "expr_skeleton.h"
#include "path_storage.h"
#include "symex_assign.h"
#include "symex_dereference_state.h"
 
exprt goto_symext::address_arithmetic(
  const exprt &expr,
  statet &state,
  bool keep_array)
{
  exprt result;
 
  if(expr.id()==ID_byte_extract_little_endian ||
     expr.id()==ID_byte_extract_big_endian)
  {
    // address_of(byte_extract(op, offset, t)) is
    // address_of(op) + offset with adjustments for arrays
 
    const byte_extract_exprt &be=to_byte_extract_expr(expr);
 
    // recursive call
    result = address_arithmetic(be.op(), state, keep_array);
 
    if(be.op().type().id() == ID_array && result.id() == ID_address_of)
    {
      address_of_exprt &a=to_address_of_expr(result);
 
      // turn &a of type T[i][j] into &(a[0][0])
      for(const typet *t = &(to_type_with_subtype(a.type()).subtype());
          t->id() == ID_array && expr.type() != *t;
          t = &(to_array_type(*t).element_type()))
        a.object() = index_exprt(a.object(), from_integer(0, c_index_type()));
    }
 
    // do (expr.type() *)(((char *)op)+offset)
    result=typecast_exprt(result, pointer_type(char_type()));
 
    // there could be further dereferencing in the offset
    exprt offset=be.offset();
    dereference_rec(offset, state, false, false);
 
    result=plus_exprt(result, offset);
 
    // treat &array as &array[0]
    const typet &expr_type = expr.type();
    typet dest_type_subtype;
 
    if(expr_type.id()==ID_array && !keep_array)
      dest_type_subtype = to_array_type(expr_type).element_type();
    else
      dest_type_subtype=expr_type;
 
    result=typecast_exprt(result, pointer_type(dest_type_subtype));
  }
  else if(expr.id()==ID_index ||
          expr.id()==ID_member)
  {
    object_descriptor_exprt ode;
    ode.build(expr, ns);
 
    const byte_extract_exprt be =
      make_byte_extract(ode.root_object(), ode.offset(), expr.type());
 
    // recursive call
    result = address_arithmetic(be, state, keep_array);
 
    do_simplify(result);
  }
  else if(expr.id()==ID_dereference)
  {
    // ANSI-C guarantees &*p == p no matter what p is,
    // even if it's complete garbage
    // just grab the pointer, but be wary of further dereferencing
    // in the pointer itself
    result=to_dereference_expr(expr).pointer();
    dereference_rec(result, state, false, false);
  }
  else if(expr.id()==ID_if)
  {
    if_exprt if_expr=to_if_expr(expr);
 
    // the condition is not an address
    dereference_rec(if_expr.cond(), state, false, false);
 
    // recursive call
    if_expr.true_case() =
      address_arithmetic(if_expr.true_case(), state, keep_array);
    if_expr.false_case() =
      address_arithmetic(if_expr.false_case(), state, keep_array);
 
    result=if_expr;
  }
  else if(expr.id()==ID_symbol ||
          expr.id()==ID_string_constant ||
          expr.id()==ID_label ||
          expr.id()==ID_array)
  {
    // give up, just dereference
    result=expr;
    dereference_rec(result, state, false, false);
 
    // turn &array into &array[0]
    if(result.type().id() == ID_array && !keep_array)
      result = index_exprt(result, from_integer(0, c_index_type()));
 
    // handle field-sensitive SSA symbol
    mp_integer offset=0;
    if(is_ssa_expr(expr))
    {
      auto offset_opt = compute_pointer_offset(expr, ns);
      PRECONDITION(offset_opt.has_value());
      offset = *offset_opt;
    }
 
    if(offset>0)
    {
      const byte_extract_exprt be = make_byte_extract(
        to_ssa_expr(expr).get_l1_object(),
        from_integer(offset, c_index_type()),
        expr.type());
 
      result = address_arithmetic(be, state, keep_array);
 
      do_simplify(result);
    }
    else
      result=address_of_exprt(result);
  }
  else if(expr.id() == ID_typecast)
  {
    const typecast_exprt &tc_expr = to_typecast_expr(expr);
 
    result = address_arithmetic(tc_expr.op(), state, keep_array);
 
    // treat &array as &array[0]
    const typet &expr_type = expr.type();
    typet dest_type_subtype;
 
    if(expr_type.id() == ID_array && !keep_array)
      dest_type_subtype = to_array_type(expr_type).element_type();
    else
      dest_type_subtype = expr_type;
 
    result = typecast_exprt(result, pointer_type(dest_type_subtype));
  }
  else
    throw unsupported_operation_exceptiont(
      "goto_symext::address_arithmetic does not handle " + expr.id_string());
 
  const typet &expr_type = expr.type();
  INVARIANT(
    (expr_type.id() == ID_array && !keep_array) ||
      pointer_type(expr_type) == result.type(),
    "either non-persistent array or pointer to result");
 
  return result;
}
 
symbol_exprt
goto_symext::cache_dereference(exprt &dereference_result, statet &state)
{
  auto const cache_key = [&] {
    auto cache_key =
      state.field_sensitivity.apply(ns, state, dereference_result, false);
    if(auto let_expr = expr_try_dynamic_cast<let_exprt>(dereference_result))
    {
      let_expr->value() = state.rename<L2>(let_expr->value(), ns).get();
    }
    else
    {
      cache_key = state.rename<L2>(cache_key, ns).get();
    }
    return cache_key;
  }();
 
  if(auto cached = state.dereference_cache.find(cache_key))
  {
    return *cached;
  }
 
  auto cache_symbol_expr = get_fresh_aux_symbol(
                             cache_key.type(),
                             "symex",
                             "dereference_cache",
                             dereference_result.source_location(),
                             language_mode,
                             ns,
                             state.symbol_table)
                             .symbol_expr();
 
  // we need to lift possible lets
  // (come from the value set to avoid repeating complex pointer comparisons)
  auto cache_value = cache_key;
  lift_lets(state, cache_value);
 
  auto assign = symex_assignt{
    shadow_memory,
    state,
    symex_targett::assignment_typet::STATE,
    ns,
    symex_config,
    target};
 
  assign.assign_symbol(
    to_ssa_expr(state.rename<L1>(cache_symbol_expr, ns).get()),
    expr_skeletont{},
    cache_value,
    {});
 
  state.dereference_cache.insert(cache_key, cache_symbol_expr);
  return cache_symbol_expr;
}
 
void goto_symext::dereference_rec(
  exprt &expr,
  statet &state,
  bool write,
  bool is_in_binding_expression)
{
  if(expr.id()==ID_dereference)
  {
    bool expr_is_not_null = false;
 
    if(state.threads.size() == 1)
    {
      const irep_idt &expr_function = state.source.function_id;
      if(!expr_function.empty())
      {
        const dereference_exprt to_check =
          to_dereference_expr(get_original_name(expr));
 
        expr_is_not_null = path_storage.safe_pointers.at(expr_function)
                             .is_safe_dereference(to_check, state.source.pc);
      }
    }
 
    exprt tmp1;
    tmp1.swap(to_dereference_expr(expr).pointer());
 
    // first make sure there are no dereferences in there
    dereference_rec(tmp1, state, false, is_in_binding_expression);
 
    // Depending on the nature of the pointer expression, the recursive deref
    // operation might have introduced a construct such as
    // (x == &o1 ? o1 : o2).field, in which case we should simplify to push the
    // member operator inside the if, then apply field-sensitivity to yield
    // (x == &o1 ? o1..field : o2..field). value_set_dereferencet can then
    // apply the dereference operation to each of o1..field and o2..field
    // independently, as it special-cases the ternary conditional operator.
    // There may also be index operators in tmp1 which can now be resolved to
    // constant array cell references, so we replace symbols with constants
    // first, hoping for a transformation such as
    // (x == &o1 ? o1 : o2)[idx] =>
    // (x == &o1 ? o1 : o2)[2] =>
    // (x == &o1 ? o1[[2]] : o2[[2]])
    // Note we don't L2 rename non-constant symbols at this point, because the
    // value-set works in terms of L1 names and we don't want to ask it to
    // dereference an L2 pointer, which it would not have an entry for.
 
    tmp1 = state.rename<L1_WITH_CONSTANT_PROPAGATION>(tmp1, ns).get();
 
    do_simplify(tmp1);
 
    if(symex_config.run_validation_checks)
    {
      // make sure simplify has not re-introduced any dereferencing that
      // had previously been cleaned away
      INVARIANT(
        !has_subexpr(tmp1, ID_dereference),
        "simplify re-introduced dereferencing");
    }
 
    tmp1 = state.field_sensitivity.apply(ns, state, std::move(tmp1), false);
 
    // we need to set up some elaborate call-backs
    symex_dereference_statet symex_dereference_state(state, ns);
 
    value_set_dereferencet dereference(
      ns,
      state.symbol_table,
      symex_dereference_state,
      language_mode,
      expr_is_not_null,
      log.get_message_handler());
 
    // std::cout << "**** " << format(tmp1) << '\n';
    exprt tmp2 =
      dereference.dereference(tmp1, symex_config.show_points_to_sets);
    // std::cout << "**** " << format(tmp2) << '\n';
 
 
    // this may yield a new auto-object
    trigger_auto_object(tmp2, state);
 
    // Check various conditions for when we should try to cache the expression.
    // 1. Caching dereferences must be enabled.
    // 2. Do not cache inside LHS of writes.
    // 3. Do not cache inside quantifiers (references variables outside their
    //    scope).
    // 4. Only cache "complicated" expressions, i.e. of the form
    //     [let p = <expr> in ]
    //     (p == &something ? something : ...))
    // Otherwise we should just return it unchanged.
    if(
      symex_config.cache_dereferences && !write && !is_in_binding_expression &&
      (tmp2.id() == ID_if || tmp2.id() == ID_let))
    {
      expr = cache_dereference(tmp2, state);
    }
    else
    {
      expr = std::move(tmp2);
    }
  }
  else if(
    expr.id() == ID_index && to_index_expr(expr).array().id() == ID_member &&
    to_array_type(to_index_expr(expr).array().type()).size().is_zero())
  {
    // This is an expression of the form x.a[i],
    // where a is a zero-sized array. This gets
    // re-written into *(&x.a+i)
 
    index_exprt index_expr=to_index_expr(expr);
 
    address_of_exprt address_of_expr(index_expr.array());
    address_of_expr.type()=pointer_type(expr.type());
 
    dereference_exprt tmp{plus_exprt{address_of_expr, index_expr.index()}};
    tmp.add_source_location()=expr.source_location();
 
    // recursive call
    dereference_rec(tmp, state, write, is_in_binding_expression);
 
    expr.swap(tmp);
  }
  else if(expr.id()==ID_index &&
          to_index_expr(expr).array().type().id()==ID_pointer)
  {
    // old stuff, will go away
    UNREACHABLE;
  }
  else if(expr.id()==ID_address_of)
  {
    address_of_exprt &address_of_expr=to_address_of_expr(expr);
 
    exprt &object=address_of_expr.object();
 
    expr = address_arithmetic(
      object, state, to_pointer_type(expr.type()).base_type().id() == ID_array);
  }
  else if(expr.id()==ID_typecast)
  {
    exprt &tc_op=to_typecast_expr(expr).op();
 
    // turn &array into &array[0] when casting to pointer-to-element-type
    if(
      tc_op.id() == ID_address_of &&
      to_address_of_expr(tc_op).object().type().id() == ID_array &&
      expr.type() ==
        pointer_type(to_array_type(to_address_of_expr(tc_op).object().type())
                       .element_type()))
    {
      expr = address_of_exprt(index_exprt(
        to_address_of_expr(tc_op).object(), from_integer(0, c_index_type())));
 
      dereference_rec(expr, state, write, is_in_binding_expression);
    }
    else
    {
      dereference_rec(tc_op, state, write, is_in_binding_expression);
    }
  }
  else
  {
    bool is_binding_expression =
      can_cast_expr<binding_exprt>(expr) || can_cast_expr<let_exprt>(expr);
    Forall_operands(it, expr)
    {
      dereference_rec(
        *it, state, write, is_in_binding_expression || is_binding_expression);
    }
  }
}
 
static exprt
apply_to_objects_in_dereference(exprt e, const std::function<exprt(exprt)> &f)
{
  if(auto deref = expr_try_dynamic_cast<dereference_exprt>(e))
  {
    deref->op() = f(std::move(deref->op()));
    return *deref;
  }
 
  for(auto &sub : e.operands())
    sub = apply_to_objects_in_dereference(std::move(sub), f);
  return e;
}
 
void goto_symext::dereference(exprt &expr, statet &state, bool write)
{
  PRECONDITION(!state.call_stack().empty());
 
  // Symbols whose address is taken need to be renamed to level 1
  // in order to distinguish addresses of local variables
  // from different frames.
  expr = apply_to_objects_in_dereference(std::move(expr), [&](exprt e) {
    return state.field_sensitivity.apply(
      ns, state, state.rename<L1>(std::move(e), ns).get(), false);
  });
 
  // start the recursion!
  dereference_rec(expr, state, write, false);
  // dereferencing may introduce new symbol_exprt
  // (like __CPROVER_memory)
  expr = state.rename<L1>(std::move(expr), ns).get();
 
  // Dereferencing is likely to introduce new member-of-if constructs --
  // for example, "x->field" may have become "(x == &o1 ? o1 : o2).field."
  // Run expression simplification, which converts that to
  // (x == &o1 ? o1.field : o2.field))
  // before applying field sensitivity. Field sensitivity can then turn such
  // field-of-symbol expressions into atomic SSA expressions instead of having
  // to rewrite all of 'o1' otherwise.
  // Even without field sensitivity this can be beneficial: for example,
  // "(b ? s1 : s2).member := X" results in
  // (b ? s1 : s2) := (b ? s1 : s2) with (member := X)
  // and then
  // s1 := b ? ((b ? s1 : s2) with (member := X)) : s1
  // when all we need is
  // s1 := s1 with (member := X) [and guard b]
  // s2 := s2 with (member := X) [and guard !b]
  do_simplify(expr);
 
  if(symex_config.run_validation_checks)
  {
    // make sure simplify has not re-introduced any dereferencing that
    // had previously been cleaned away
    INVARIANT(
      !has_subexpr(expr, ID_dereference),
      "simplify re-introduced dereferencing");
  }
 
  expr = state.field_sensitivity.apply(ns, state, std::move(expr), write);
}