drm/atomic: do not branch based on the value of current->comm[0]

  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.vetter@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Michel Dänzer <michel@daenzer.net>
Cc: Alex Deucher <alexdeucher@gmail.com>
Cc: Adam Jackson <ajax@redhat.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Sultan Alsawaf <sultan@kerneltoast.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 drivers/gpu/drm/drm_ioctl.c | 5 -----
 1 file changed, 5 deletions(-)
  

Hey again,

On Wed, Nov 16, 2022 at 01:36:00AM +0100, Jason A. Donenfeld wrote:
> I'm a bit surprised that this patch was ignored. I had sort of assumed

Mystery solved: this message to you bounced from this linux.ie address I
somehow wound up with in the recipients list. Fixing now by using the
one in MAINTAINERS. Sorry about that.

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.vetter@intel.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Ilia Mirkin <imirkin@alum.mit.edu>
> > Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > Cc: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
> > Cc: Christian Brauner <brauner@kernel.org>
> > Cc: Michel Dänzer <michel@daenzer.net>
> > Cc: Alex Deucher <alexdeucher@gmail.com>
> > Cc: Adam Jackson <ajax@redhat.com>
> > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > Cc: Sean Paul <sean@poorly.run>
> > Cc: David Airlie <airlied@linux.ie>
> > Cc: Rob Clark <robdclark@gmail.com>
> > Cc: Sultan Alsawaf <sultan@kerneltoast.com>
> > Signed-off-by: Jason A. Donenfeld <Jason@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
  

On Sun, 6 Nov 2022 at 08:21, Jason A. Donenfeld <Jason@zx2c4.com> 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.

I've been ignoring this because I don't really want to reintroduce a
regression for deployed X servers. I don't see the value.

You use a lot of what I'd call overly not backed up language. Why is
it obviously wrong though? Is it "playing games" or is it accessing
the comm to see if the process starts with X.

Do we have lots of userspace processes starting with X that access
this specific piece of the drm modesetting API. I suppose we might and
if we have complaints about that I'd say let's try to fix it better.

Sometimes engineering is hard, It was a big enough problem that a big
enough hammer was used.

I'd hope @Daniel Vetter can comment as well since the original patch was his.

Dave.
  

Am 2022-11-05 23:20, schrieb Jason A. Donenfeld:
> This reverts 26b1d3b527e7 ("drm/atomic: Take the atomic toys away from 
> X")

I'm in favor of reverting this commit. I've tried to get allowing to 
enable atomic in Xorg again in there in the past: 
https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/533

I've no illusions of getting this through though, after all mostly the 
same people control what's merged into Xorg, what drm stuff gets into 
the kernel and who disabled it in the kernel in the first place. And 
there doesn't seem much interest in dealing with anything Xorg either, 
in the merge request I linked, someone even called Xorg "abandonware". 
This is also why I didn't respond here until now.

I do see value in enabling this. When I looked at this 2 years ago, 
there were situations where enabling atomic brought clear improvements, 
and I would expect that it can still improve performance on some special 
systems. I think the users should have the option to use it if they want 
or need to.

There is also the concern that this may cause a regression, but I would 
argue, that there never was a regression to be fixed here in the first 
place. There may have been that one broken application in the past, but 
it was just that, a broken application, not something broken by the 
kernel. I do not think the kernel should modify it's behavior just to 
work around bugs in a specific program, which have always existed, and 
didn't come from a changer in behavior of the kernel APIs. If a program 
was written wrongly, the program should be fixed, and in case of Xorg, I 
think it is fixed already.

This probably won't mean much coming from me, but:
Acked-by: Daniel Abrecht <public@danielabrecht.ch>
  

On Wed, Nov 16, 2022 at 01:49:43PM +1000, Dave Airlie wrote:
> On Sun, 6 Nov 2022 at 08:21, Jason A. Donenfeld <Jason@zx2c4.com> 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.
> 
> I've been ignoring this because I don't really want to reintroduce a
> regression for deployed X servers. I don't see the value.
> 
> You use a lot of what I'd call overly not backed up language. Why is
> it obviously wrong though? Is it "playing games" or is it accessing
> the comm to see if the process starts with X.
> 
> Do we have lots of userspace processes starting with X that access
> this specific piece of the drm modesetting API. I suppose we might and
> if we have complaints about that I'd say let's try to fix it better.
> 
> Sometimes engineering is hard, It was a big enough problem that a big
> enough hammer was used.
> 
> I'd hope @Daniel Vetter can comment as well since the original patch was his.

Frankly I refrained from replying when I've seen the patch originally
because I didn't manage to come up with a nice&constructive reply like you
did here. The only thing novel here is the amount of backhanded insults
folded into the commit message.

I very much welcome constructive contributions that actually solve the
problem here, or at least move it forward a bit. This patch is neither.

What might be an option is a tainting module option that disables this
check, since the amount of people willing&able to fix up Xorg is still
zero. But that would need to come with a proper commit message and all
that, and ideally a pile of acks from people who insist they really want
this and need it.
-Daniel