[PATCH v2 6/6] libcamera: shared_mem_object: Remove is_standard_layout restriction

[Naushir Patuck]

On Thu, 9 May 2024 at 12:31, Laurent Pinchart <
laurent.pinchart at ideasonboard.com> wrote:

> On Thu, May 09, 2024 at 12:22:56PM +0100, Kieran Bingham wrote:
> > Quoting Laurent Pinchart (2024-05-09 12:16:15)
> > > On Thu, May 09, 2024 at 11:49:41AM +0200, Jacopo Mondi wrote:
> > > > On Wed, May 08, 2024 at 09:04:01AM GMT, Naushir Patuck wrote:
> > > > > The shared_mem_object may be used to construct complex classes, so
> > > > > remove the standard layout type restriction.
> > > > >
> > > > > Signed-off-by: Naushir Patuck <naush at raspberrypi.com>
> > > >
> > > > This looks fine to me, but maybe better check on why this was
> > > > introduced in first place.
> > >
> > > It was added there because sharing objects containing pointers between
> > > different processes isn't safe. Naush, what object do you need to store
> > > in shared memory that has a non-standard layout, but is still safe to
> > > share ?
> >
> > Oh, interesting point. I wonder if that's why we've seen people get
> > segfaults if the RPi IPA runs in isolation (as then these buffers are
> > shared, and could potentially contain invalid pointers?)
>
> On Pi4 the only shared buffer I'm aware of contains the LSC table, so
> that should be fine. I haven't checked Pi5.
>

The Pi 4 LS tables use a dmabuf and FD between the pipeline handler and
IPA, so that's ok.

>
> The std::is_standard_layout() check doesn't catch pointers
> unfortunately, but it catches C++ objects that can contain pointers
> (such as containers from the C++ standard library). It may also catch
> objects that are fine, if that's the case here, we can try to relax the
> check.
>

Eek, this can be nasty.  Frontend is safe, but Backend is not.  We don't
use any pointers but have a couple of std::vectors, these can easily be
removed.  It does have a few std::map types that are problematic.  We only
ever access these maps and vectors from the pipeline handler side, so
it "works", but potentially fragile.  I don't know what to do here...


>
> > > > Reviewed-by: Jacopo Mondi <jacopo.mondi at ideasonboard.com>
> > > >
> > > > > ---
> > > > >  include/libcamera/internal/shared_mem_object.h | 2 +-
> > > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/include/libcamera/internal/shared_mem_object.h
> b/include/libcamera/internal/shared_mem_object.h
> > > > > index 9b1d939302a8..c9c0482062bd 100644
> > > > > --- a/include/libcamera/internal/shared_mem_object.h
> > > > > +++ b/include/libcamera/internal/shared_mem_object.h
> > > > > @@ -56,7 +56,7 @@ private:
> > > > >     Span<uint8_t> mem_;
> > > > >  };
> > > > >
> > > > > -template<class T, typename =
> std::enable_if_t<std::is_standard_layout<T>::value>>
> > > > > +template<class T>
> > > > >  class SharedMemObject : public SharedMem
> > > > >  {
> > > > >  public:
>
> --
> Regards,
>
> Laurent Pinchart
>
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