Hi David,
I'm a bit surprised that this patch was ignored. I had sort of assumed
the response would be, "good god, I can't believe we're doing this.
Applied to drm-next!" rather than crickets, but maybe it just got buried
under a lot of other patches. So I thought I'd poke about this again.
The original message is reproduced in full below.
Regards,
Jason
On Sat, Nov 05, 2022 at 11:20:12PM +0100, Jason A. Donenfeld wrote:
> This reverts 26b1d3b527e7 ("drm/atomic: Take the atomic toys away from
> X"), a rootkit-like kludge that has no business being inside of a
> general purpose kernel. It's the type of debugging hack I'll use
> momentarily but never commit, or a sort of babbies-first-process-hider
> malware trick.
>
> The backstory is that some userspace code -- xorg-server -- has a
> modesetting DDX that isn't really coded right. With nobody wanting to
> maintain X11 anymore, rather than fixing the buggy code, the kernel was
> adjusted to avoid having to touch X11. A bummer, but fair enough: if the
> kernel doesn't want to support some userspace API any more, the right
> thing to do is to arrange for a graceful fallback where userspace thinks
> it's not available in a manageable way.
>
> However, the *way* it goes about doing that is just to check
> `current->comm[0] == 'X'`, and disable it for only that case. So that
> means it's *not* simply a matter of the kernel not wanting to support a
> particular userspace API anymore, but rather it's the kernel not wanting
> to support xorg-server, in theory, but actually, it turns out, that's
> all processes that begin with 'X'.
>
> Playing games with current->comm like this is obviously wrong, and it's
> pretty shocking that this ever got committed.
>
> Fortunately, since this was committed, somebody did actually disable
> the userspace side by default in X11:
> https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/180 and
> this was three years ago. So userspace is mostly fine now for ordinary
> default usage. And people who opt into this -- since it does actually
> work fine for many use cases on i915 -- ostensibly know what they're
> getting themselves into (my case).
>
> So let's just revert this `comm[0] == 'X'` business entirely, but still
> allow for `value == 2`, in case anybody actually started working on that
> part elsewhere.
>
> Fixes: 26b1d3b527e7 ("drm/atomic: Take the atomic toys away from X")
> Cc: Daniel Vetter <daniel.vet...@intel.com>
> Cc: Peter Zijlstra <pet...@infradead.org>
> Cc: Ilia Mirkin <imir...@alum.mit.edu>
> Cc: Maarten Lankhorst <maarten.lankho...@linux.intel.com>
> Cc: Nicholas Kazlauskas <nicholas.kazlaus...@amd.com>
> Cc: Christian Brauner <brau...@kernel.org>
> Cc: Michel Dänzer <mic...@daenzer.net>
> Cc: Alex Deucher <alexdeuc...@gmail.com>
> Cc: Adam Jackson <a...@redhat.com>
> Cc: Greg Kroah-Hartman <gre...@linuxfoundation.org>
> Cc: Sean Paul <s...@poorly.run>
> Cc: David Airlie <airl...@linux.ie>
> Cc: Rob Clark <robdcl...@gmail.com>
> Cc: Sultan Alsawaf <sul...@kerneltoast.com>
> Signed-off-by: Jason A. Donenfeld <ja...@zx2c4.com>
> ---
> drivers/gpu/drm/drm_ioctl.c | 5 -----
> 1 file changed, 5 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
> index ca2a6e6101dc..017f31e67179 100644
> --- a/drivers/gpu/drm/drm_ioctl.c
> +++ b/drivers/gpu/drm/drm_ioctl.c
> @@ -336,11 +336,6 @@ drm_setclientcap(struct drm_device *dev, void *data,
> struct drm_file *file_priv)
> case DRM_CLIENT_CAP_ATOMIC:
> if (!drm_core_check_feature(dev, DRIVER_ATOMIC))
> return -EOPNOTSUPP;
> - /* The modesetting DDX has a totally broken idea of atomic. */
> - if (current->comm[0] == 'X' && req->value == 1) {
> - pr_info("broken atomic modeset userspace detected,
> disabling atomic\n");
> - return -EOPNOTSUPP;
> - }
> if (req->value > 2)
> return -EINVAL;
> file_priv->atomic = req->value;
> --
> 2.38.1
>
