Variadic args/va_arg: handle unexpected va_list types

Our regression test has `void*` lists when we'd expected `void**`. This,
however, went unnoticed as we only tried to access the first element.
Attempts to access the second element would have been misaligned, and
failed an invariant as of 35cc503 (we don't handle pointer arithmetic
over void pointers in the back-end anymore).

We now make sure that we always increment by size-of-`void*`.

Fixes: #7706

Author: tautschnig

regression/cbmc/va_list2/main.c

@@ -11,14 +11,16 @@ void my_f(int first, ...)
 
   int v;
   v=__builtin_va_arg(args, int);
-  assert(v==2);
+  assert(v == 1);
+  v = __builtin_va_arg(args, int);
+  assert(v == 0);
 
   __builtin_va_end(args);
 }
 
 int main()
 {
-  my_f(1, 2);
+  my_f(2, 1, 0);
 }
 
 #else

src/ansi-c/goto-conversion/builtin_functions.cpp

@@ -20,6 +20,7 @@ Author: Daniel Kroening, kroening@kroening.com
 #include <util/mathematical_expr.h>
 #include <util/mathematical_types.h>
 #include <util/pointer_expr.h>
+#include <util/pointer_offset_size.h>
 #include <util/rational.h>
 #include <util/rational_tools.h>
 #include <util/simplify_expr.h>
@@ -1306,12 +1307,24 @@ void goto_convertt::do_function_call_symbol(
         goto_programt::make_assignment(lhs, rhs, function.source_location()));
     }
 
-    code_assignt assign{
-      list_arg, plus_exprt{list_arg, from_integer(1, pointer_diff_type())}};
-    assign.rhs().set(
-      ID_C_va_arg_type, to_code_type(function.type()).return_type());
+    exprt rhs;
+    if(list_arg.type() == pointer_type(pointer_type(empty_typet{})))
+      rhs = plus_exprt{list_arg, from_integer(1, pointer_diff_type())};
+    else
+    {
+      // handle unexpected va_list types by just enforcing pointer increments by
+      // size-of-void*
+      rhs = typecast_exprt{
+        plus_exprt{
+          typecast_exprt{list_arg, pointer_type(char_type())},
+          from_integer(
+            *pointer_offset_size(pointer_type(empty_typet{}), ns),
+            pointer_diff_type())},
+        list_arg.type()};
+    }
+    rhs.set(ID_C_va_arg_type, to_code_type(function.type()).return_type());
     dest.add(goto_programt::make_assignment(
-      std::move(assign), function.source_location()));
+      list_arg, std::move(rhs), function.source_location()));
   }
   else if(identifier == "__builtin_va_copy")
   {