Re: [PATCH] tools/ocaml: Factor out compatiblity handling

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 28/08/2024 3:15 pm, Edwin Torok wrote:
> On Wed, Aug 28, 2024 at 2:30 PM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 
> wrote:
>> ... rather than having each library implement its own subset.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> ---
>> CC: Christian Lindig <christian.lindig@xxxxxxxxxx>
>> CC: David Scott <dave@xxxxxxxxxx>
>> CC: Edwin Török <edwin.torok@xxxxxxxxx>
>> CC: Rob Hoes <Rob.Hoes@xxxxxxxxxx>
>> CC: Andrii Sultanov <andrii.sultanov@xxxxxxxxx>
>>
>> Broken out of a larger series, to help Andrii with his dynlib work.
>> ---
>>  tools/ocaml/libs/xc/Makefile        |  2 +-
>>  tools/ocaml/libs/xc/xenctrl_stubs.c | 13 +++----------
>>  tools/ocaml/libs/xen-caml-compat.h  | 23 +++++++++++++++++++++++
>>  3 files changed, 27 insertions(+), 11 deletions(-)
>>  create mode 100644 tools/ocaml/libs/xen-caml-compat.h
>>
>> diff --git a/tools/ocaml/libs/xc/Makefile b/tools/ocaml/libs/xc/Makefile
>> index 1d9fecb06ef2..cdf4d01dac52 100644
>> --- a/tools/ocaml/libs/xc/Makefile
>> +++ b/tools/ocaml/libs/xc/Makefile
>> @@ -2,7 +2,7 @@ OCAML_TOPLEVEL=$(CURDIR)/../..
>>  XEN_ROOT=$(OCAML_TOPLEVEL)/../..
>>  include $(OCAML_TOPLEVEL)/common.make
>>
>> -CFLAGS += -I../mmap $(CFLAGS_libxenctrl) $(CFLAGS_libxenguest)
>> +CFLAGS += -I../ -I../mmap $(CFLAGS_libxenctrl) $(CFLAGS_libxenguest)
>>  CFLAGS += $(APPEND_CFLAGS)
>>  OCAMLINCLUDE += -I ../mmap -I ../eventchn
>>
>> diff --git a/tools/ocaml/libs/xc/xenctrl_stubs.c 
>> b/tools/ocaml/libs/xc/xenctrl_stubs.c
>> index a52908012960..c78191f95abc 100644
>> --- a/tools/ocaml/libs/xc/xenctrl_stubs.c
>> +++ b/tools/ocaml/libs/xc/xenctrl_stubs.c
>> @@ -25,6 +25,8 @@
>>  #include <caml/fail.h>
>>  #include <caml/callback.h>
>>
>> +#include "xen-caml-compat.h"
>> +
>>  #include <sys/mman.h>
>>  #include <stdint.h>
>>  #include <string.h>
>> @@ -37,14 +39,6 @@
>>
>>  #include "mmap_stubs.h"
>>
>> -#ifndef Val_none
>> -#define Val_none (Val_int(0))
>> -#endif
>> -
>> -#ifndef Tag_some
>> -#define Tag_some 0
>> -#endif
>> -
>>  static inline xc_interface *xch_of_val(value v)
>>  {
>>         xc_interface *xch = *(xc_interface **)Data_custom_val(v);
>> @@ -744,8 +738,7 @@ CAMLprim value stub_xc_evtchn_status(value xch_val, 
>> value domid, value port)
>>         Store_field(result_status, 0, Val_int(status.vcpu));
>>         Store_field(result_status, 1, stat);
>>
>> -       result = caml_alloc_small(1, Tag_some);
>> -       Store_field(result, 0, result_status);
>> +       result = caml_alloc_some(result_status);
>>
>>         CAMLreturn(result);
>>  }
>> diff --git a/tools/ocaml/libs/xen-caml-compat.h 
>> b/tools/ocaml/libs/xen-caml-compat.h
>> new file mode 100644
>> index 000000000000..b4a0ca4ce90c
>> --- /dev/null
>> +++ b/tools/ocaml/libs/xen-caml-compat.h
>> @@ -0,0 +1,23 @@
>> +/* SPDX-License-Identifier: LGPL-2.1-only WITH OCaml-LGPL-linking-exception 
>> */
>> +#ifndef XEN_CAML_COMPAT_H
>> +#define XEN_CAML_COMPAT_H
>> +
>> +#ifndef Val_none /* Option handling.  Compat for Ocaml < 4.12 */
>> +
>> +#define Val_none Val_int(0)
>> +#define Tag_some 0
>> +#define Some_val(v) Field(v, 0)
>> +
>> +static inline value caml_alloc_some(value v)
>> +{
>> +    CAMLparam1(v);
>> +
>> +    value some = caml_alloc_small(1, Tag_some);
>> +    Store_field(some, 0, v);
> The compiler uses Field() rather than Store_field() here.
> I think using Store_field here can potentially read uninitialized
> data, because 'caml_alloc_small' gives you uninitialized memory
> that you must immediately fill with valid values.
> Looking at the implementation Store_field calls caml_modify which will
> read the old value to figure out whether it was in minor or major
> heap,
> and doing that on uninitialized data is unpredictable.
>
> We should follow what the manual says and use Field() when
> caml_alloc_small() is used, and use Store_field() when caml_alloc() is
> used,
> and not attempt to mix them:
> See https://ocaml.org/manual/5.2/intfc.html#ss:c-low-level-gc-harmony

Lovely, this got changed in Ocaml with no information or justification...

https://github.com/ocaml/ocaml/pull/9819

I'll resync this locally, but I shaltn't repost.

~Andrew