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

On Wed, Nov 16, 2022 at 01:49:43PM +1000, Dave Airlie wrote:

On Sun, 6 Nov 2022 at 08:21, Jason A. Donenfeld [off-list ref] wrote:

quoted

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
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch