[Intel-gfx] [PATCH v3 2/2] drm/i915: On fb alloc failure, unref gem object where it gets refed

[Chris Wilson]

On Sat, Jul 04, 2015 at 11:50:58AM +0200, Lukas Wunner wrote:
> Currently when allocating a framebuffer fails, the gem object gets
> unrefed at the bottom of the call chain in __intel_framebuffer_create,
> not where it gets refed, which is in intel_framebuffer_create_for_mode
> (via i915_gem_alloc_object) and in intel_user_framebuffer_create
> (via drm_gem_object_lookup).
> 
> This invites mistakes: As discovered by Tvrtko Ursulin, a double unref
> has sneaked into intelfb_alloc (which calls __intel_framebuffer_create).
> 
> As suggested by Ville Syrjälä, improve code clarity by moving the unref
> away from __intel_framebuffer_create to where the gem object gets refed.
> 
> Signed-off-by: Lukas Wunner <lukas at wunner.de>
> Fixes: a8bb6818270c ("drm/i915: Fix error path leak in fbdev fb
>     allocation")
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> ---
>  drivers/gpu/drm/i915/intel_display.c | 21 ++++++++++++++-------
>  1 file changed, 14 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> index 9079fcd..9499003 100644
> --- a/drivers/gpu/drm/i915/intel_display.c
> +++ b/drivers/gpu/drm/i915/intel_display.c
> @@ -8876,20 +8876,17 @@ __intel_framebuffer_create(struct drm_device *dev,
>  	int ret;
>  
>  	intel_fb = kzalloc(sizeof(*intel_fb), GFP_KERNEL);
> -	if (!intel_fb) {
> -		drm_gem_object_unreference(&obj->base);
> +	if (!intel_fb)
>  		return ERR_PTR(-ENOMEM);
> -	}
>  
>  	ret = intel_framebuffer_init(dev, intel_fb, mode_cmd, obj);
>  	if (ret)
>  		goto err;
>  
>  	return &intel_fb->base;
> +
>  err:
> -	drm_gem_object_unreference(&obj->base);
>  	kfree(intel_fb);
> -
>  	return ERR_PTR(ret);
>  }
>  
> @@ -8929,6 +8926,7 @@ intel_framebuffer_create_for_mode(struct drm_device *dev,
>  				  struct drm_display_mode *mode,
>  				  int depth, int bpp)
>  {
> +	struct drm_framebuffer *fb;
>  	struct drm_i915_gem_object *obj;
>  	struct drm_mode_fb_cmd2 mode_cmd = { 0 };
>  
> @@ -8943,7 +8941,11 @@ intel_framebuffer_create_for_mode(struct drm_device *dev,
>  								bpp);
>  	mode_cmd.pixel_format = drm_mode_legacy_fb_format(bpp, depth);
>  
> -	return intel_framebuffer_create(dev, &mode_cmd, obj);
> +	fb = intel_framebuffer_create(dev, &mode_cmd, obj);
> +	if (IS_ERR(fb))
> +		drm_gem_object_unreference(&obj->base);

This needs to be drm_gem_object_unreference_unlocked(). It's much
simpler if you just document this as consuming the obj reference. If you
want to fix it, you have to move the struct_mutex into the caller i.e.
eliminate intel_framebuffer_create() and call
__intel_framebuffer_create().
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre