Libguestfs - Jan 2017 - Re: [nbdkit PATCH v3 1/4] plugins: Don't use bogus errno from non-C plugins

On Thu, Jan 26, 2017 at 08:58:34PM -0600, Eric Blake
wrote:
> diff --git a/plugins/ocaml/ocaml.c b/plugins/ocaml/ocaml.c
> index e2b433e..5d7aeeb 100644
> --- a/plugins/ocaml/ocaml.c
> +++ b/plugins/ocaml/ocaml.c
> @@ -499,6 +499,17 @@ SET(pwrite)
>  SET(flush)
>  SET(trim)
> 
> +/* We can't guarantee that errno is stable across language binding
> + * glue code, so this callback is implemented in C only, and not
> + * exposed in OCaml.
> + */
> +static int
> +plugin_ocaml_errno_is_reliable (void *handle)
> +{
> +  return 0;
> +}
Actually OCaml is a real compiled language, and the call from C to
OCaml code (via caml_callback_exn) is a short piece of asm which
preserves errno.

However you'll need to save errno around caml_enter_blocking_section
since that unblocks and processes signals.

IOW:

  static int
  pread_wrapper (void *h, void *buf, uint32_t count, uint64_t offset)
  {
    CAMLparam0 ();
    CAMLlocal3 (rv, strv, offsetv);
+   int saved_errno;
...
    rv = caml_callback3_exn (pread_fn, *(value *) h, strv, offsetv);
+   saved_errno = errno;
    if (Is_exception_result (rv)) {
      nbdkit_error ("%s", caml_format_exception (Extract_exception
(rv)));
      caml_enter_blocking_section ();
+     errno = saved_errno;
      CAMLreturnT (int, -1);
    }

Rich.

-- 
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On 01/27/2017 04:11 AM, Richard W.M. Jones wrote:
> 
> Actually OCaml is a real compiled language, and the call from C to
> OCaml code (via caml_callback_exn) is a short piece of asm which
> preserves errno.
Which shows my lack of familiarity with OCaml; but I'm guessing that
also means that OCaml comes with easy ways to directly set errno so that
it will be visible from C.
> 
> However you'll need to save errno around caml_enter_blocking_section
> since that unblocks and processes signals.
> 
> IOW:
> 
>   static int
>   pread_wrapper (void *h, void *buf, uint32_t count, uint64_t offset)
>   {
>     CAMLparam0 ();
>     CAMLlocal3 (rv, strv, offsetv);
> +   int saved_errno;
> ...
>     rv = caml_callback3_exn (pread_fn, *(value *) h, strv, offsetv);
> +   saved_errno = errno;
>     if (Is_exception_result (rv)) {
>       nbdkit_error ("%s", caml_format_exception
(Extract_exception (rv)));
>       caml_enter_blocking_section ();
> +     errno = saved_errno;
>       CAMLreturnT (int, -1);
Okay, I'll respin the patch along these lines.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

On Fri, Jan 27, 2017 at 08:01:17AM -0600, Eric Blake
wrote:
> On 01/27/2017 04:11 AM, Richard W.M. Jones wrote:
> > 
> > Actually OCaml is a real compiled language, and the call from C to
> > OCaml code (via caml_callback_exn) is a short piece of asm which
> > preserves errno.
> 
> Which shows my lack of familiarity with OCaml; but I'm guessing that
> also means that OCaml comes with easy ways to directly set errno so that
> it will be visible from C.
Actually no, that would be too easy.  Failing system calls (in OCaml
code) are turned into exceptions.  The exception contains something
which has a 1-1 mapping with the errno, but is not the errno.  In
libguestfs we had to do some mappings for those, so it's a pain.

So in fact I'm not sure if errno is preserved properly or if we should
map the Is_exception_result.

Need to think about this - at the moment I'm in the middle of some
customer crisis.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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