linux-kernel - [PATCH v4 3/5] drm: lock drm_global_mutex earlier in the ioctl handler

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Message-Id: <20210820100251.448346-4-desmondcheongzx@gmail.com>
Date:   Fri, 20 Aug 2021 18:02:49 +0800
From:   Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>
To:     maarten.lankhorst@...ux.intel.com, mripard@...nel.org,
        tzimmermann@...e.de, airlied@...ux.ie, daniel@...ll.ch,
        sumit.semwal@...aro.org, christian.koenig@....com
Cc:     Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        intel-gfx@...ts.freedesktop.org, linux-media@...r.kernel.org,
        linaro-mm-sig@...ts.linaro.org, skhan@...uxfoundation.org,
        gregkh@...uxfoundation.org,
        linux-kernel-mentees@...ts.linuxfoundation.org
Subject: [PATCH v4 3/5] drm: lock drm_global_mutex earlier in the ioctl handler

In a future patch, a read lock on drm_device.master_rwsem is
held in the ioctl handler before the check for ioctl
permissions. However, this inverts the lock hierarchy of
drm_global_mutex --> master_rwsem.

To avoid this, we do some prep work to grab the drm_global_mutex
before checking for ioctl permissions.

Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>
---
 drivers/gpu/drm/drm_ioctl.c | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
index d25713b09b80..158629d88319 100644
--- a/drivers/gpu/drm/drm_ioctl.c
+++ b/drivers/gpu/drm/drm_ioctl.c
@@ -772,19 +772,19 @@ long drm_ioctl_kernel(struct file *file, drm_ioctl_t *func, void *kdata,
 	if (drm_dev_is_unplugged(dev))
 		return -ENODEV;
 
+	/* Enforce sane locking for modern driver ioctls. */
+	if (unlikely(drm_core_check_feature(dev, DRIVER_LEGACY)) && !(flags & DRM_UNLOCKED))
+		mutex_lock(&drm_global_mutex);
+
 	retcode = drm_ioctl_permit(flags, file_priv);
 	if (unlikely(retcode))
-		return retcode;
+		goto out;
 
-	/* Enforce sane locking for modern driver ioctls. */
-	if (likely(!drm_core_check_feature(dev, DRIVER_LEGACY)) ||
-	    (flags & DRM_UNLOCKED))
-		retcode = func(dev, kdata, file_priv);
-	else {
-		mutex_lock(&drm_global_mutex);
-		retcode = func(dev, kdata, file_priv);
+	retcode = func(dev, kdata, file_priv);
+
+out:
+	if (unlikely(drm_core_check_feature(dev, DRIVER_LEGACY)) && !(flags & DRM_UNLOCKED))
 		mutex_unlock(&drm_global_mutex);
-	}
 	return retcode;
 }
 EXPORT_SYMBOL(drm_ioctl_kernel);
-- 
2.25.1