[PATCH] xenctrl_stubs.c: fix NULL dereference

["Edwin Török" <edvin.torok@citrix.com>]

From: Edwin Török <edwin.torok@cloud.com>

`Tag_cons` is `0` and is meant to be used as the tag argument for `caml_alloc`/`caml_alloc_small`
when constructing a non-empty list.
The empty list is `Val_emptylist` instead (which is really just `Val_int(0)`).

Assigning `0` to a list value like this is equivalent to assigning the naked pointer `NULL` to the field.
Naked pointers are not valid in OCaml 5, however even in OCaml <5.x any attempt to iterate on the list will lead to a segfault.

The list currently only has an opaque type, so no code would have reason to iterate on it currently,
but we shouldn't construct invalid OCaml values that might lead to a crash when exploring the type.

`Val_emptylist` is available since OCaml 3.01 as a constant.

Fixes: e5ac68a011 ("x86/hvm: Revert per-domain APIC acceleration support")

Signed-off-by: Edwin Török <edwin.torok@cloud.com>
---
 tools/ocaml/libs/xc/xenctrl_stubs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/ocaml/libs/xc/xenctrl_stubs.c b/tools/ocaml/libs/xc/xenctrl_stubs.c
index e4d9070f2d..3703f48c74 100644
--- a/tools/ocaml/libs/xc/xenctrl_stubs.c
+++ b/tools/ocaml/libs/xc/xenctrl_stubs.c
@@ -832,7 +832,7 @@ CAMLprim value physinfo_arch_caps(const xc_physinfo_t *info)
 
 	tag = 1; /* tag x86 */
 
-	arch_obj = Tag_cons;
+	arch_obj = Val_emptylist;
 
 #endif
 
-- 
2.41.0