Re: [PATCH] drm/syncobj: Fix handle <-> fd ioctls with dirty stack

[David Laight]

On Tue, 3 Mar 2026 18:44:59 +0100
Julian Orth <ju.orth@xxxxxxxxx> wrote:

> On Tue, Mar 3, 2026 at 6:41 PM Maarten Lankhorst
> <maarten.lankhorst@xxxxxxxxxxxxxxx> wrote:
> >
> > My point is that it works for old userspace without problems. It's only
> > newly compiled userspace with new headers that will run into problems.
> > The code as written would have continued to work, but if you update to
> > the new header and don't initialise the new members then it's a userspace
> > problem. It should not be worked around in the kernel, since it's newly
> > written bad userspace code, not old bad userspace code that stopped working
> > when the kernel changed.  
> 
> But it's not newly written. The example is, say, 5 year old code. The
> binary that was compiled 5 years ago works fine as you say. But if you
> take the same code and just run gcc again, the new binary will no
> longer work.
> 

Worse, the recompiled version may well work when you test it, and even
when deployed.
But you'll get non-obvious random failures - a support nightmare.

Probably best code is something like:
	case OLD_IOCTL_CODE:
		if (ioc->flag & NEW_FLAG)
			return -EINVAL;
		/* FALLTHROUGH *.
	case NEW_IOCTL_CODE:
		if (!(ioc->flag & NEW_FLAG))
			ioc->new_field = 0;

    David