linux-kernel - Re: [PATCH 1/3] drm/i915: allocate mock file pointer dynamically

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Date:   Tue, 21 Mar 2017 10:16:52 +0000
From:   Chris Wilson <chris@...is-wilson.co.uk>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Daniel Vetter <daniel.vetter@...el.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        David Airlie <airlied@...ux.ie>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...el.com>,
        Matthew Auld <matthew.auld@...el.com>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] drm/i915: allocate mock file pointer dynamically

On Mon, Mar 20, 2017 at 10:40:27AM +0100, Arnd Bergmann wrote:
> A struct file is a bit too large to put on the kernel stack in general
> and triggers a warning for low settings of CONFIG_FRAME_WARN:
> 
> drivers/gpu/drm/i915/selftests/mock_drm.c: In function 'mock_file':
> drivers/gpu/drm/i915/selftests/mock_drm.c:46:1: error: the frame size of 1328 bytes is larger than 1280 bytes [-Werror=frame-larger-than=]
> drivers/gpu/drm/i915/selftests/mock_drm.c: In function 'mock_file_free':
> drivers/gpu/drm/i915/selftests/mock_drm.c:54:1: error: the frame size of 1312 bytes is larger than 1280 bytes [-Werror=frame-larger-than=]
> 
> It's also slightly dangerous to leave a reference to a stack object
> in the drm_file structure after leaving the stack frame.
> This changes the code to just allocate the object dynamically
> and release it when we are done with it.
> 
> Fixes: 66d9cb5d805a ("drm/i915: Mock the GEM device for self-testing")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  drivers/gpu/drm/i915/selftests/mock_drm.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/selftests/mock_drm.c b/drivers/gpu/drm/i915/selftests/mock_drm.c
> index 113dec05c7dc..18514065c93d 100644
> --- a/drivers/gpu/drm/i915/selftests/mock_drm.c
> +++ b/drivers/gpu/drm/i915/selftests/mock_drm.c
> @@ -32,15 +32,15 @@ static inline struct inode fake_inode(struct drm_i915_private *i915)
>  struct drm_file *mock_file(struct drm_i915_private *i915)
>  {
>  	struct inode inode = fake_inode(i915);
> -	struct file filp = {};
> +	struct file *filp = kzalloc(sizeof(struct file), GFP_KERNEL);
>  	struct drm_file *file;
>  	int err;
>  
> -	err = drm_open(&inode, &filp);
> +	err = drm_open(&inode, filp);
>  	if (unlikely(err))
>  		return ERR_PTR(err);
>  
> -	file = filp.private_data;
> +	file = filp->private_data;

What I don't like about this approach is that we leak the unwanted
inode/file. We only want the drm_file here and any access to above that
inside i915.ko is fubar.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre