From patchwork Sat Nov 5 22:20:12 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: "Jason A. Donenfeld" X-Patchwork-Id: 13033260 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from gabe.freedesktop.org (gabe.freedesktop.org [131.252.210.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id AAFD1C433FE for ; Sat, 5 Nov 2022 22:21:02 +0000 (UTC) Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id 8F25610E0F9; Sat, 5 Nov 2022 22:21:01 +0000 (UTC) Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by gabe.freedesktop.org (Postfix) with ESMTPS id A921710E0F9 for ; Sat, 5 Nov 2022 22:20:57 +0000 (UTC) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 0F7C060B58; Sat, 5 Nov 2022 22:20:57 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id BEF3EC433D7; Sat, 5 Nov 2022 22:20:54 +0000 (UTC) Authentication-Results: smtp.kernel.org; dkim=pass (1024-bit key) header.d=zx2c4.com header.i=@zx2c4.com header.b="NiluAsAW" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=zx2c4.com; s=20210105; t=1667686852; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding; bh=GokT8QcZOOF1Sj+w1E78zsAC5vdLp3dZxLPEamRZIQc=; b=NiluAsAWGU9q2YBTgfIqH7nw2j4QXLI9zVeGdjIIsiTzIQF6J69JxnxlXhrGVgPJYsTlNf m2iE8C353WC3NFDPhvgr3yf7e8dr5Up3F5jyaGu5chUUnWd3HCjhX+7PdW1ogHNlCnFFUk +6XrN1/n8nwHh1xBkhJAboxHjNS2tOg= Received: by mail.zx2c4.com (ZX2C4 Mail Server) with ESMTPSA id 969eda74 (TLSv1.3:TLS_AES_256_GCM_SHA384:256:NO); Sat, 5 Nov 2022 22:20:52 +0000 (UTC) From: "Jason A. Donenfeld" To: dri-devel@lists.freedesktop.org, linux-api@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH] drm/atomic: do not branch based on the value of current->comm[0] Date: Sat, 5 Nov 2022 23:20:12 +0100 Message-Id: <20221105222012.4226-1-Jason@zx2c4.com> MIME-Version: 1.0 X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: "Jason A. Donenfeld" , Peter Zijlstra , Greg Kroah-Hartman , =?utf-8?q?Michel_D=C3=A4nz?= =?utf-8?q?er?= , Christian Brauner , David Airlie , Daniel Vetter , Sultan Alsawaf , Sean Paul , Nicholas Kazlauskas Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" 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 Cc: Peter Zijlstra Cc: Ilia Mirkin Cc: Maarten Lankhorst Cc: Nicholas Kazlauskas Cc: Christian Brauner Cc: Michel Dänzer Cc: Alex Deucher Cc: Adam Jackson Cc: Greg Kroah-Hartman Cc: Sean Paul Cc: David Airlie Cc: Rob Clark Cc: Sultan Alsawaf Signed-off-by: Jason A. Donenfeld Acked-by: Daniel Abrecht --- 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;