Re: wide function pointer type

[Martin Uecker <ma.uecker@gmail.com>]

I would be interested to learn about ABI issues
I might not be aware of. 

In particular, I would be good to know whether
implementing a perfectly forwarding stub for
a variadic functions that loads the static
is possible on all architectures. I assume so,
but I am not entirely sure.

If not, we might need to restrict conversion for
regular functions pointers to wide pointers to only
allow functions without variadic args, or define
a new ABI for wide pointers, but this then
partially defeats the point (on the affected
architecture).

Martin


Am Montag, den 18.10.2021, 07:33 +0200 schrieb Martin Uecker:
> Hi Anthony,
> 
> Am Sonntag, den 17.10.2021, 19:35 -0400 schrieb Anthony Green:
> > Hi Martin,
> > 
> >   I haven't read the whole proposal yet, but I'll try to this week.  I
> > did, however, read the first paragraph, and it says:  "Trying to use
> > regular function pointers for callbacks with data requires run-time
> > code generation (e.g. nested functions in GCC or XL C, and closures in
> > libffi [1]) but this is complex, inefficient, and problematic from a
> > security point of view."
> > 
> >   This is no longer true for libffi on the important Linux platforms.
> > We use static code templates that are placed in memory at a fixed,
> > predetermined distance from the data they reference.  There is no
> > runtime code generation.  Well, to be fair, perhaps it is still
> > complex, but it's far more secure than before.
> > 
> >   Thanks for sharing.  I'm not a language expert, but I know others here are.
> 
> Thank you for your comments! I had to submit this to WG14 to meet
> the deadline for C23. So the final version (so far) is here:
> 
>  http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2787.pdf
> 
> Of course, it is still possible to modify the proposal later.
> 
> It is great to hear that libffi can create trampolines
> without run-time code generation! We previously discussed
> such an approach also for GCC, but nothing was implemented.
> 
> 
> Martin
> 
> 
> > AG
> > 
> > On Sun, Oct 10, 2021 at 7:32 AM Martin Uecker via Libffi-discuss
> > <libffi-discuss@sourceware.org> wrote:
> > > 
> > > Hi all,
> > > 
> > > I will propose a wide function pointer type (actually
> > > a wide function type) to WG14 for C23 as a common
> > > type for callbacks, closures, which now require an
> > > additional void pointer argument in C APIs. This
> > > is intended to be compatible with ABIs with now
> > > use a static chain register.
> > > 
> > > An early draft can be found here:
> > > 
> > > http://wwwuser.gwdg.de/~muecker1/wide_v4.pdf
> > > 
> > > 
> > > I thought you might be interested and I would love
> > > to hear your feedback.
> > > 
> > > Martin
> > > 
> > > 
> > >