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

[Edwin Torok <edwin.torok@xxxxxxxxx>]

On Wed, Aug 28, 2024 at 3:15 PM Edwin Torok <edwin.torok@xxxxxxxxx> 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

Which probably means we've got a bunch of other pre-existing bugs like
these that we need to fix,
as otherwise we do quite a lot of operations on uninitialized data...

>
> > +
> > +    CAMLreturn(some);
> > +}
> > +
> > +#endif /* !Val_none */
> > +
> > +#endif /* XEN_CAML_COMPAT_H */
> >
> > base-commit: 75c64db3722f0245137a1e8cfd3435f4790d0fd7
> > --
> > 2.39.2
> >