Re: g_function_info_invoke() may truncate return values

[Keri Harris <keri gentoo org>]

I've lodged bug report #253.


Thanks

Keri

[1] https://gitlab.gnome.org/GNOME/gobject-introspection/issues/253

On 20/12/18 03:53, philip chimento gmail com wrote:
> Thanks for the detailed analysis. I agree, it seems quite plausible that 
> that's happening. Would you mind filing a bug report 
> (https://gitlab.gnome.org/GNOME/gobject-introspection/issues)? 
> Additionally if you are willing to look into fixing it, add me on the 
> merge request and I will be happy to review the fix.
>
> Regards,
> Philip C
>
> On Wed, Dec 19, 2018 at 2:14 PM Keri Harris via gir-devel-list 
> <gir-devel-list gnome org <mailto:gir-devel-list gnome org>> wrote:
>
>     Hello,
>
>     I've run into an issue with gobject-introspection where return values
>     from functions can in some circumstances be truncated. An example of
>     such a function is gdk_pixbuf_read_pixels() [1] which has the signature:
>
>           const guint8 *
>           gdk_pixbuf_read_pixels (const GdkPixbuf *pixbuf);
>
>     Calling gdk_pixbuf_read_pixels() directly and comparing the result when
>     the function is called via g_function_info_invoke() is illustrative of
>     the truncation problem:
>
>           Calling gdk_pixbuf_read_pixels() directly...
>           retval: 0x7fc14fd4a010    // healthy return value
>           Calling gdk_pixbuf_read_pixels() via g_function_info_invoke()...
>           ret_arg->v_pointer: 0x10  // truncated return value
>
>     I think the problem arises in gicallableinfo.c when we translate back
>     and forth between the GI <-> FFI data types. There are two places in
>     which this translation fails:
>
>     1. translating return-arg GITypeInfo -> ffi_type values
>
>     In g_callable_info_invoke() we call:
>
>           rinfo = g_callable_info_get_return_type ((GICallableInfo *)info);
>
>     but this does not consider whether the return value is a pointer; a
>     pointer-to-integral type is mapped to a integral type. e.g. a
>     guint8* is
>     mapped to a ffi_type_uint8.
>
>     2. translating ffi_return_value -> GIArgument return value
>
>     In gi_type_info_extract_ffi_return_value() we don't consider whether
>     the
>     return value is a pointer. In the case where the size represented by
>     the
>     type-info tag is less than the size of a pointer then we'll truncate
>     the
>     return value. For example, in the above case of
>     gdk_pixbuf_read_pixels()
>     we execute:
>
>           switch (g_type_info_get_tag (return_info)) {
>           ...
>           case GI_TYPE_TAG_UINT8:
>               arg->v_uint8 = (guint8) ffi_value->v_ulong;
>               break;
>
>     ffi_value->v_pointer should hold the value of a (const guint8 *) but we
>     instead look at ffi_value->v_ulong, and then cast it to a guint8.
>
>
>     Thanks
>
>     Keri
>
>     [1]
>     https://developer.gnome.org/gdk-pixbuf/2.36/gdk-pixbuf-The-GdkPixbuf-Structure.html#gdk-pixbuf-read-pixels
>     _______________________________________________
>     gir-devel-list mailing list
>     gir-devel-list gnome org <mailto:gir-devel-list gnome org>
>     https://mail.gnome.org/mailman/listinfo/gir-devel-list