linux-kernel - Re: [PATCH 1/2] drm: Add might_fault to drm_modeset

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Message-ID: <03f7e2ad-fd5c-4da7-a14c-34c2c158c513@amd.com>
Date: Wed, 10 Jul 2024 13:38:56 +0200
From: Christian König <christian.koenig@....com>
To: Daniel Vetter <daniel.vetter@...ll.ch>,
 DRI Development <dri-devel@...ts.freedesktop.org>,
 LKML <linux-kernel@...r.kernel.org>
Cc: Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
 Daniel Vetter <daniel.vetter@...el.com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 Sumit Semwal <sumit.semwal@...aro.org>, linux-media@...r.kernel.org,
 linaro-mm-sig@...ts.linaro.org
Subject: Re: [PATCH 1/2] drm: Add might_fault to drm_modeset_lock priming

Am 10.07.24 um 11:31 schrieb Daniel Vetter:
> We already teach lockdep that dma_resv nests within drm_modeset_lock,
> but there's a lot more: All drm kms ioctl rely on being able to
> put/get_user while holding modeset locks, so we really need a
> might_fault in there too to complete the picture. Add it.

Mhm, lockdep should be able to deduce that when there might be faults 
under the dma_resv lock there might also be faults under the 
drm_modeset_lock.

>
> Motivated by a syzbot report that blew up on bcachefs doing an
> unconditional console_lock way deep in the locking hierarchy, and
> lockdep only noticing the depency loop in a drm ioctl instead of much
> earlier. This annotation will make sure such issues have a much harder
> time escaping.
>
> References: https://lore.kernel.org/dri-devel/00000000000073db8b061cd43496@google.com/
> Signed-off-by: Daniel Vetter <daniel.vetter@...el.com>
> Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
> Cc: Maxime Ripard <mripard@...nel.org>
> Cc: Thomas Zimmermann <tzimmermann@...e.de>
> Cc: Sumit Semwal <sumit.semwal@...aro.org>
> Cc: "Christian König" <christian.koenig@....com>
> Cc: linux-media@...r.kernel.org
> Cc: linaro-mm-sig@...ts.linaro.org

On the other hand pointing it out explicitly doesn't hurts us at all, so 
Reviewed-by: Christian König <christian.koenig@....com>.

Regards,
Christian.

> ---
>   drivers/gpu/drm/drm_mode_config.c | 2 ++
>   1 file changed, 2 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_mode_config.c b/drivers/gpu/drm/drm_mode_config.c
> index 568972258222..37d2e0a4ef4b 100644
> --- a/drivers/gpu/drm/drm_mode_config.c
> +++ b/drivers/gpu/drm/drm_mode_config.c
> @@ -456,6 +456,8 @@ int drmm_mode_config_init(struct drm_device *dev)
>   		if (ret == -EDEADLK)
>   			ret = drm_modeset_backoff(&modeset_ctx);
>   
> +		might_fault();
> +
>   		ww_acquire_init(&resv_ctx, &reservation_ww_class);
>   		ret = dma_resv_lock(&resv, &resv_ctx);
>   		if (ret == -EDEADLK)