Make ValueObject::Cast work for casts from smaller to larger structs in the cases where this currently can work.

lldb/source/Core/ValueObject.cpp

@@ -2744,18 +2744,30 @@ ValueObjectSP ValueObject::DoCast(const CompilerType &compiler_type) {
 
 ValueObjectSP ValueObject::Cast(const CompilerType &compiler_type) {
   // Only allow casts if the original type is equal or larger than the cast
-  // type.  We don't know how to fetch more data for all the ConstResult types,
-  // so we can't guarantee this will work:
+  // type, unless we know this is a load address.  Getting the size wrong for
+  // a host side storage could leak lldb memory, so we absolutely want to 
+  // prevent that.  We may not always get the right value, for instance if we
+  // have an expression result value that's copied into a storage location in
+  // the target may not have copied enough memory.  I'm not trying to fix that
+  // here, I'm just making Cast from a smaller to a larger possible in all the
+  // cases where that doesn't risk making a Value out of random lldb memory.
+  // You have to check the ValueObject's Value for the address types, since
+  // ValueObjects that use live addresses will tell you they fetch data from the
+  // live address, but once they are made, they actually don't.
+  // FIXME: Can we make ValueObject's with a live address fetch "more data" from
+  // the live address if it is still valid?
+
   Status error;
   CompilerType my_type = GetCompilerType();
 
   ExecutionContextScope *exe_scope
       = ExecutionContext(GetExecutionContextRef())
           .GetBestExecutionContextScope();
   if (compiler_type.GetByteSize(exe_scope)
-      <= GetCompilerType().GetByteSize(exe_scope)) {
+      <= GetCompilerType().GetByteSize(exe_scope) 
+      || m_value.GetValueType() == Value::ValueType::LoadAddress)
         return DoCast(compiler_type);
-  }
+
   error.SetErrorString("Can only cast to a type that is equal to or smaller "
                        "than the orignal type.");
 

lldb/test/API/python_api/value/TestValueAPI.py

@@ -148,14 +148,66 @@ def test(self):
 
         # Test some other cases of the Cast API.  We allow casts from one struct type
         # to another, which is a little weird, but we don't support casting from a
-        # smaller type to a larger as we often wouldn't know how to get the extra data:
-        val_f = target.EvaluateExpression("f")
-        bad_cast = val_s.Cast(val_f.GetType())
-        self.assertFailure(
-            bad_cast.GetError(),
-            "Can only cast to a type that is equal to or smaller than the orignal type.",
-        )
-        weird_cast = val_f.Cast(val_s.GetType())
+        # smaller type to a larger when the underlying data is not in the inferior,
+        # since then we have no way to fetch the out-of-bounds values.
+        # For an expression that references a variable, or a FindVariable result,
+        # or an SBValue made from an address and a type, we can get back to the target,
+        # so those will work.  Make sure they do and get the right extra values as well.
+
+        # We're casting everything to the type of "f", so get that first:
+        f_var = frame0.FindVariable("f")
+        self.assertSuccess(f_var.error, "Got f")
+        bigger_type = f_var.GetType()
+
+        # First try a value that we got from FindVariable
+        container = frame0.FindVariable("my_container")
+        self.assertSuccess(container.error, "Found my_container")
+        fv_small = container.GetValueForExpressionPath(".data.small")
+        self.assertSuccess(fv_small.error, "Found small in my_container")
+        fv_cast = fv_small.Cast(bigger_type)
+        self.assertSuccess(fv_cast.error, "Can cast up from FindVariable")
+        child_checks = [
+            ValueCheck(name="a", value="33", type="int"),
+            ValueCheck(name="b", value="44", type="int"),
+            ValueCheck(name="c", value="55", type="int"),
+        ]
+        cast_check = ValueCheck(type=bigger_type.name, children=child_checks)
+
+        # Now try one we made with expr.  This one should fail, because expr
+        # stores the "canonical value" in host memory, and doesn't know how
+        # to augment that from the live address.
+        expr_cont = frame0.EvaluateExpression("my_container")
+        self.assertSuccess(expr_cont.error, "Got my_container by expr")
+        expr_small = expr_cont.GetValueForExpressionPath(".data.small")
+        self.assertSuccess(expr_small.error, "Got small by expr")
+        expr_cast = expr_small.Cast(bigger_type)
+        self.assertFailure(expr_cast.error, msg="Cannot cast expr result")
+
+        # Now try one we made with CreateValueFromAddress.  That will succeed
+        # because this directly tracks the inferior memory.
+        small_addr = fv_small.addr
+        self.assertTrue(small_addr.IsValid())
+        small_type = fv_small.GetType()
+        vfa_small = target.CreateValueFromAddress(
+            "small_from_addr", small_addr, small_type
+        )
+        self.assertSuccess(vfa_small.error, "Made small from address")
+        vfa_cast = vfa_small.Cast(bigger_type)
+        self.assertSuccess(vfa_cast.error, "Made a cast from vfa_small")
+        cast_check.check_value(self, vfa_cast, "Cast of ValueFromAddress succeeds")
+
+        # Next try ValueObject created from data.  They should fail as there's no
+        # way to grow the data:
+        data_small = target.CreateValueFromData(
+            "small_from_data", fv_small.data, fv_small.type
+        )
+        self.assertSuccess(data_small.error, "Made a valid object from data")
+        data_cast = data_small.Cast(bigger_type)
+        self.assertFailure(data_cast.error, msg="Cannot cast data backed SBValue")
+
+        # Now check casting from a larger type to a smaller, we can always do this,
+        # so just test one case:
+        weird_cast = f_var.Cast(val_s.GetType())
         self.assertSuccess(weird_cast.GetError(), "Can cast from a larger to a smaller")
         self.assertEqual(
             weird_cast.GetChildMemberWithName("a").GetValueAsSigned(0),

lldb/test/API/python_api/value/main.c

@@ -22,7 +22,7 @@ const char *weekdays[5] = { "Monday",
 const char **g_table[2] = { days_of_week, weekdays };
 
 typedef int MyInt;
-
+  
 struct MyStruct
 {
   int a;
@@ -36,15 +36,26 @@ struct MyBiggerStruct
   int c;
 };
 
+struct Container
+{
+  int discriminator;
+  union Data {
+    struct MyStruct small;
+    struct MyBiggerStruct big;
+  } data;
+};
+
 int main (int argc, char const *argv[])
 {
     uint32_t uinthex = 0xE0A35F10;
     int32_t  sinthex = 0xE0A35F10;
 
     int i;
     MyInt a = 12345;
-    struct MyStruct s = { 11, 22 };
+    struct MyStruct s = {11, 22};
     struct MyBiggerStruct f = { 33, 44, 55 };
+    struct Container my_container;
+    my_container.data.big = f;
     int *my_int_ptr = &g_my_int;
     printf("my_int_ptr points to location %p\n", my_int_ptr);
     int *fixed_int_ptr = (int*)(void*)0xAA;