[Libguestfs] [PATCH 08/11] ocaml: do not try to malloc 0 elements in get_all_event_callbacks

[Pino Toscano]

On Friday, 3 March 2017 16:09:33 CET Richard W.M. Jones wrote:
> On Fri, Mar 03, 2017 at 03:33:02PM +0100, Pino Toscano wrote:
> > In case there are no event handlers registered with the handle,
> > get_all_event_callbacks will count 0 elements, trying to malloc a buffer
> > of that size.  POSIX says that this can result in either a null pointer,
> > or an unusable pointer.  Since we assume a null pointer means failure,
> > then always add a null element at the end, so we do not rely on
> > implementation-defined behaviour of malloc.
> > 
> > The output parameter 'len_rtn' already keeps the number of valid items
> > in the returned array, so there are no behaviour changes for callers of
> > get_all_event_callbacks.
> > ---
> >  ocaml/guestfs-c.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/ocaml/guestfs-c.c b/ocaml/guestfs-c.c
> > index 9042752..f14eee3 100644
> > --- a/ocaml/guestfs-c.c
> > +++ b/ocaml/guestfs-c.c
> > @@ -311,7 +311,7 @@ get_all_event_callbacks (guestfs_h *g, size_t *len_rtn)
> >    }
> >  
> >    /* Copy them into the return array. */
> > -  r = malloc (sizeof (value *) * (*len_rtn));
> > +  r = malloc (sizeof (value *) * (*len_rtn + 1));
> >    if (r == NULL) caml_raise_out_of_memory ();
> 
> Isn't it better to fix this by doing:
> 
>    r = malloc (sizeof (value *) * (*len_rtn));
> -  if (r == NULL) caml_raise_out_of_memory ();
> +  if (*len_rtn > 0 && r == NULL) caml_raise_out_of_memory ();
> 
> (same comment in the following patches)

This could work for OCaml and Ruby, because NULL as return value is not
mishandled, and they throw already errors.  In the Python
implementation, NULL short-circuits the guestfs_int_py_close flow.

I'll change further the implementations, so when there are no handlers
no memory is allocated at all.

Thanks,
-- 
Pino Toscano
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