Re: [PATCH] xenctrl_stubs.c: fix NULL dereference

[Christian Lindig <christian.lindig@xxxxxxxxx>]

> On 13 Jul 2023, at 09:30, Edwin Török <edvin.torok@xxxxxxxxxx> wrote:
> 
> From: Edwin Török <edwin.torok@xxxxxxxxx>
> 
> `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@xxxxxxxxx>
> ---
> 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
> 

Acked-by: Christian Lindig <christian.lindig@xxxxxxxxx>