[PATCH] object: Add sanity check on correct parent class

[Eric Blake <eblake@xxxxxxxxxx>]

Checking that the derived class is larger than the requested parent
class saves us from some obvious mistakes, but as written, it does not
catch all the cases; in particular, it is easy to forget to update a
VIR_CLASS_NEW when changing the 'parent' member from virObject to
virObjectLockabale, but where the size checks don't catch that.  Add a
parameter for one more layer of sanity checking.

Note that I did NOT change the fact that we require derived classes to
be larger (as the difference in size makes it easy to tell classes
apart), which means that even if a derived class has no functionality
to add (but rather exists for compiler-enforced type-safety), it must
still include a dummy member.  But I did fix the wording of the error
message to match the code.

Signed-off-by: Eric Blake <eblake@xxxxxxxxxx>
---

Here's hoping Coverity doesn't have a false-positive complaint about
the error message being a potential dereference of NULL (the only time
'parent == NULL' is when 'parentsize == 0', based on the fact that our
syntax checks forbid raw calls to virClassNew() except for "virObject"
itself - but Coverity likely won't see that).

 src/util/virobject.h | 5 ++++-
 src/util/virobject.c | 8 +++++---
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/src/util/virobject.h b/src/util/virobject.h
index d4ec943a43..757068fcc1 100644
--- a/src/util/virobject.h
+++ b/src/util/virobject.h
@@ -82,12 +82,15 @@ virClassPtr virClassForObjectRWLockable(void);
  */
 # define VIR_CLASS_NEW(name, prnt) \
     verify_expr(offsetof(name, parent) == 0, \
-      (name##Class = virClassNew(prnt, #name, sizeof(name), name##Dispose)))
+      (name##Class = virClassNew(prnt, #name, sizeof(name), \
+                                 sizeof(((name *)NULL)->parent), \
+                                 name##Dispose)))

 virClassPtr
 virClassNew(virClassPtr parent,
             const char *name,
             size_t objectSize,
+            size_t parentSize,
             virObjectDisposeCallback dispose)
     VIR_PARENT_REQUIRED ATTRIBUTE_NONNULL(2);

diff --git a/src/util/virobject.c b/src/util/virobject.c
index 3b28331ba7..b4ee068cb2 100644
--- a/src/util/virobject.c
+++ b/src/util/virobject.c
@@ -78,6 +78,7 @@ virObjectOnceInit(void)
     if (!(virObjectClass = virClassNew(NULL,
                                        "virObject",
                                        sizeof(virObject),
+                                       0,
                                        NULL)))
         return -1;

@@ -159,6 +160,7 @@ virClassPtr
 virClassNew(virClassPtr parent,
             const char *name,
             size_t objectSize,
+            size_t parentSize,
             virObjectDisposeCallback dispose)
 {
     virClassPtr klass;
@@ -167,10 +169,10 @@ virClassNew(virClassPtr parent,
         STRNEQ(name, "virObject")) {
         virReportInvalidNonNullArg(parent);
         return NULL;
-    } else if (parent &&
-               objectSize <= parent->objectSize) {
+    } else if (objectSize <= parentSize ||
+               parentSize != (parent ? parent->objectSize : 0)) {
         virReportInvalidArg(objectSize,
-                            _("object size %zu of %s is smaller than parent class %zu"),
+                            _("object size %zu of %s is not larger than parent class %zu"),
                             objectSize, name, parent->objectSize);
         return NULL;
     }
-- 
2.20.1

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