XML output: ensure expressions are valid XML 1.0

regression/cbmc/xml-trace2/main.c

@@ -0,0 +1,9 @@
+#include <assert.h>
+
+int main()
+{
+  char *c14 = "\x0E";
+  assert(c14[0] == 14);
+  assert(c14[1] == 0);
+  assert(c14[0] != 14);
+}

regression/cbmc/xml-trace2/test.desc

@@ -0,0 +1,8 @@
+CORE
+main.c
+--xml-ui
+<full_lhs_value( binary="0000001000000000000000000000000000000000000000000000000000000000")?>\{ 14, 0 \}</full_lhs_value>
+^EXIT=10$
+^SIGNAL=0$
+--
+<full_lhs_value( binary="0000001000000000000000000000000000000000000000000000000000000000")?>"\\&#14;"</full_lhs_value>

src/goto-programs/graphml_witness.cpp

@@ -18,6 +18,7 @@ Author: Daniel Kroening
 #include <util/pointer_predicates.h>
 #include <util/prefix.h>
 #include <util/ssa_expr.h>
+#include <util/string_constant.h>
 #include <util/string_container.h>
 
 #include <langapi/language_util.h>
@@ -57,6 +58,12 @@ void graphml_witnesst::remove_l0_l1(exprt &expr)
 
     return;
   }
+  else if(expr.id() == ID_string_constant)
+  {
+    std::string output_string = expr_to_string(ns, "", expr);
+    if(!xmlt::is_printable_xml(output_string))
+      expr = to_string_constant(expr).to_array_expr();
+  }
 
   Forall_operands(it, expr)
     remove_l0_l1(*it);

src/goto-programs/xml_goto_trace.cpp

@@ -13,6 +13,7 @@ Author: Daniel Kroening
 
 #include "xml_goto_trace.h"
 
+#include <util/string_constant.h>
 #include <util/symbol.h>
 #include <util/xml_irep.h>
 
@@ -23,6 +24,38 @@ Author: Daniel Kroening
 #include "structured_trace_util.h"
 #include "xml_expr.h"
 
+/// Rewrite all string constants to their array counterparts
+static void replace_string_constants_rec(exprt &expr)
+{
+  for(auto &op : expr.operands())
+    replace_string_constants_rec(op);
+
+  if(expr.id() == ID_string_constant)
+    expr = to_string_constant(expr).to_array_expr();
+}
+
+/// Given an expression \p expr, produce a string representation that is
+/// printable in XML 1.0. Produces an empty string if no valid XML 1.0 string
+/// representing \p expr can be generated.
+static std::string
+get_printable_xml(const namespacet &ns, const irep_idt &id, const exprt &expr)
+{
+  std::string result = from_expr(ns, id, expr);
+
+  if(!xmlt::is_printable_xml(result))
+  {
+    exprt new_expr = expr;
+    replace_string_constants_rec(new_expr);
+    result = from_expr(ns, id, new_expr);
+
+    // give up
+    if(!xmlt::is_printable_xml(result))
+      return {};
+  }
+
+  return result;
+}
+
 xmlt full_lhs_value(const goto_trace_stept &step, const namespacet &ns)
 {
   xmlt value_xml{"full_lhs_value"};
@@ -34,7 +67,7 @@ xmlt full_lhs_value(const goto_trace_stept &step, const namespacet &ns)
   const auto &lhs_object = step.get_lhs_object();
   const irep_idt identifier =
     lhs_object.has_value() ? lhs_object->get_identifier() : irep_idt();
-  value_xml.data = from_expr(ns, identifier, value);
+  value_xml.data = get_printable_xml(ns, identifier, value);
 
   const auto &bv_type = type_try_dynamic_cast<bitvector_typet>(value.type());
   const auto &constant = expr_try_dynamic_cast<constant_exprt>(value);
@@ -121,7 +154,7 @@ void convert(
       std::string full_lhs_string;
 
       if(step.full_lhs.is_not_nil())
-        full_lhs_string = from_expr(ns, identifier, step.full_lhs);
+        full_lhs_string = get_printable_xml(ns, identifier, step.full_lhs);
 
       xml_assignment.new_element("full_lhs").data = full_lhs_string;
       xml_assignment.new_element(full_lhs_value(step, ns));
@@ -158,7 +191,7 @@ void convert(
       for(const auto &arg : step.io_args)
       {
         xml_output.new_element("value").data =
-          from_expr(ns, step.function_id, arg);
+          get_printable_xml(ns, step.function_id, arg);
         xml_output.new_element("value_expression").new_element(xml(arg, ns));
       }
       break;
@@ -176,7 +209,7 @@ void convert(
       for(const auto &arg : step.io_args)
       {
         xml_input.new_element("value").data =
-          from_expr(ns, step.function_id, arg);
+          get_printable_xml(ns, step.function_id, arg);
         xml_input.new_element("value_expression").new_element(xml(arg, ns));
       }
 

src/util/xml.cpp

@@ -101,12 +101,17 @@ void xmlt::escape(const std::string &s, std::ostream &out)
       out << '\n';
       break;
 
+    case 0x9:  // TAB
+    case 0x7F: // DEL
+      out << "&#" << std::to_string((unsigned char)ch) << ';';
+      break;
+
     default:
-      // &#0; isn't allowed, but what shall we do?
-      if((ch>=0 && ch<' ') || ch==127)
-        out << "&#"+std::to_string((unsigned char)ch)+";";
-      else
-        out << ch;
+      DATA_INVARIANT(
+        ch >= ' ',
+        "XML does not support escaping non-printable character " +
+          std::to_string((unsigned char)ch));
+      out << ch;
     }
   }
 }
@@ -135,16 +140,34 @@ void xmlt::escape_attribute(const std::string &s, std::ostream &out)
       out << "&quot;";
       break;
 
+    case 0x9:  // TAB
+    case 0xA:  // LF
+    case 0xD:  // CR
+    case 0x7F: // DEL
+      out << "&#" << std::to_string((unsigned char)ch) << ';';
+      break;
+
     default:
-      // &#0; isn't allowed, but what shall we do?
-      if((ch>=0 && ch<' ') || ch==127)
-        out << "&#"+std::to_string((unsigned char)ch)+";";
-      else
-        out << ch;
+      DATA_INVARIANT(
+        ch >= ' ',
+        "XML does not support escaping non-printable character " +
+          std::to_string((unsigned char)ch));
+      out << ch;
     }
   }
 }
 
+bool xmlt::is_printable_xml(const std::string &s)
+{
+  for(const auto ch : s)
+  {
+    if(ch < 0x20 && ch != 0x9 && ch != 0xA && ch != 0xD)
+      return false;
+  }
+
+  return true;
+}
+
 void xmlt::do_indent(std::ostream &out, unsigned indent)
 {
   out << std::string(indent, ' ');

src/util/xml.h

@@ -123,6 +123,13 @@ class xmlt
 
   static void escape_attribute(const std::string &s, std::ostream &out);
 
+  /// Determine whether \p s does not contain any characters that cannot be
+  /// escaped in XML 1.0. See https://www.w3.org/TR/xml/#charsets for details.
+  /// \param s: string to verify
+  /// \return True if, and only if, all characters in \p s are taken from the
+  ///   charset permitted in XML 1.0.
+  static bool is_printable_xml(const std::string &s);
+
 protected:
   static void do_indent(
     std::ostream &out,