qapi: Do not cast function pointers

Using -fsanitize=undefined with Clang v18 causes an error if function
pointers are casted:

 qapi/qapi-clone-visitor.c:188:5: runtime error: call to function visit_type_SocketAddress through pointer to incorrect function type 'bool (*)(struct Visitor *, const char *, void **, struct Error **)'
 /tmp/qemu-ubsan/qapi/qapi-visit-sockets.c:487: note: visit_type_SocketAddress defined here
     #0 0x5642aa2f7f3b in qapi_clone qapi/qapi-clone-visitor.c:188:5
     #1 0x5642aa2c8ce5 in qio_channel_socket_listen_async io/channel-socket.c:285:18
     #2 0x5642aa2b8903 in test_io_channel_setup_async tests/unit/test-io-channel-socket.c:116:5
     #3 0x5642aa2b8204 in test_io_channel tests/unit/test-io-channel-socket.c:179:9
     #4 0x5642aa2b8129 in test_io_channel_ipv4 tests/unit/test-io-channel-socket.c:323:5
     ...

It also prevents enabling the strict mode of CFI which is currently
disabled with -fsanitize-cfi-icall-generalize-pointers.

The problematic casts are necessary to pass visit_type_T() and
visit_type_T_members() as callbacks to qapi_clone() and qapi_clone_members(),
respectively. Open-code these two functions to avoid the callbacks, and
thus the type casts.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2346
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240524-xkb-v4-3-2de564e5c859@daynix.com>
[thuth: Improve commit message according to Markus' suggestions]
Signed-off-by: Thomas Huth <thuth@redhat.com>

include/qapi/clone-visitor.h

@@ -11,6 +11,7 @@
 #ifndef QAPI_CLONE_VISITOR_H
 #define QAPI_CLONE_VISITOR_H
 
+#include "qapi/error.h"
 #include "qapi/visitor.h"
 
 /*
@@ -20,11 +21,8 @@
  */
 typedef struct QapiCloneVisitor QapiCloneVisitor;
 
-void *qapi_clone(const void *src, bool (*visit_type)(Visitor *, const char *,
-                                                     void **, Error **));
-void qapi_clone_members(void *dst, const void *src, size_t sz,
-                        bool (*visit_type_members)(Visitor *, void *,
-                                                   Error **));
+Visitor *qapi_clone_visitor_new(void);
+Visitor *qapi_clone_members_visitor_new(void);
 
 /*
  * Deep-clone QAPI object @src of the given @type, and return the result.
@@ -32,10 +30,18 @@ void qapi_clone_members(void *dst, const void *src, size_t sz,
  * Not usable on QAPI scalars (integers, strings, enums), nor on a
  * QAPI object that references the 'any' type.  Safe when @src is NULL.
  */
-#define QAPI_CLONE(type, src)                                           \
-    ((type *)qapi_clone(src,                                            \
-                        (bool (*)(Visitor *, const char *, void **,     \
-                                  Error **))visit_type_ ## type))
+#define QAPI_CLONE(type, src)                                   \
+    ({                                                          \
+        Visitor *v_;                                            \
+        type *dst_ = (type *) (src); /* Cast away const */      \
+                                                                \
+        if (dst_) {                                             \
+            v_ = qapi_clone_visitor_new();                      \
+            visit_type_ ## type(v_, NULL, &dst_, &error_abort); \
+            visit_free(v_);                                     \
+        }                                                       \
+        dst_;                                                   \
+    })
 
 /*
  * Copy deep clones of @type members from @src to @dst.
@@ -43,9 +49,14 @@ void qapi_clone_members(void *dst, const void *src, size_t sz,
  * Not usable on QAPI scalars (integers, strings, enums), nor on a
  * QAPI object that references the 'any' type.
  */
-#define QAPI_CLONE_MEMBERS(type, dst, src)                              \
-    qapi_clone_members(dst, src, sizeof(type),                          \
-                       (bool (*)(Visitor *, void *,                     \
-                                 Error **))visit_type_ ## type ## _members)
+#define QAPI_CLONE_MEMBERS(type, dst, src)                                \
+    ({                                                                    \
+        Visitor *v_;                                                      \
+                                                                          \
+        v_ = qapi_clone_members_visitor_new();                            \
+        *(type *)(dst) = *(src);                                          \
+        visit_type_ ## type ## _members(v_, (type *)(dst), &error_abort); \
+        visit_free(v_);                                                   \
+    })
 
 #endif

qapi/qapi-clone-visitor.c

@@ -149,7 +149,7 @@ static void qapi_clone_free(Visitor *v)
     g_free(v);
 }
 
-static Visitor *qapi_clone_visitor_new(void)
+Visitor *qapi_clone_visitor_new(void)
 {
     QapiCloneVisitor *v;
 
@@ -174,31 +174,9 @@ static Visitor *qapi_clone_visitor_new(void)
     return &v->visitor;
 }
 
-void *qapi_clone(const void *src, bool (*visit_type)(Visitor *, const char *,
-                                                     void **, Error **))
+Visitor *qapi_clone_members_visitor_new(void)
 {
-    Visitor *v;
-    void *dst = (void *) src; /* Cast away const */
-
-    if (!src) {
-        return NULL;
-    }
-
-    v = qapi_clone_visitor_new();
-    visit_type(v, NULL, &dst, &error_abort);
-    visit_free(v);
-    return dst;
-}
-
-void qapi_clone_members(void *dst, const void *src, size_t sz,
-                        bool (*visit_type_members)(Visitor *, void *,
-                                                   Error **))
-{
-    Visitor *v;
-
-    v = qapi_clone_visitor_new();
-    memcpy(dst, src, sz);
+    Visitor *v = qapi_clone_visitor_new();
     to_qcv(v)->depth++;
-    visit_type_members(v, dst, &error_abort);
-    visit_free(v);
+    return v;
 }