Make ValueObject::Cast work for casts from smaller to larger structs in the cases where this currently can work. (#84588)

The ValueObjectConstResult classes that back expression result variables
play a complicated game with where the data for their values is stored.
They try to make it appear as though they are still tied to the memory
in the target into which their value was written when the expression is
run, but they also keep a copy in the Host which they use after the
value is made (expression results are "history values" so that's how we
make sure they have "the value at the time of the expression".)

However, that means that if you ask them to cast themselves to a value
bigger than their original size, they don't have a way to get more
memory for that purpose. The same thing is true of ValueObjects backed
by DataExtractors, the data extractors don't know how to get more data
than they were made with in general.

The only place where we actually ask ValueObjects to sample outside
their captured bounds is when you do ValueObject::Cast from one
structure type to a bigger structure type. In
https://reviews.llvm.org/D153657 I handled this by just disallowing
casts from one structure value to a larger one. My reasoning at the time
was that the use case for this was to support discriminator based C
inheritance schemes, and you can't directly cast values in C, only
pointers, so this was not a natural way to handle those types. It seemed
logical that since you would have had to start with pointers in the
implementation, that's how you would write your lldb introspection code
as well.

Famous last words...

Turns out there are some heavy users of the SB API's who were relying on
this working, and this is a behavior change, so this patch makes this
work in the cases where it used to work before, while still disallowing
the cases we don't know how to support.

Note that if you had done this Cast operation before with either
expression results or value objects from data extractors, lldb would not
have returned the correct results, so the cases this patch outlaws are
ones that actually produce invalid results. So nobody should be using
Cast in these cases, or if they were, this patch will point out the bug
they hadn't yet noticed.

Author: jimingham

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;