[PATCH v2 5/7] Revert "drm/gem-framebuffer: Use dma_buf from GEM object instance"

[Thomas Zimmermann]

This reverts commit cce16fcd7446dcff7480cd9d2b6417075ed81065.

The dma_buf field in struct drm_gem_object is not stable over the
object instance's lifetime. The field becomes NULL when user space
releases the final GEM handle on the buffer object. This resulted
in a NULL-pointer deref.

Workarounds in commit 5307dce878d4 ("drm/gem: Acquire references on
GEM handles for framebuffers") and commit f6bfc9afc751 ("drm/framebuffer:
Acquire internal references on GEM handles") only solved the problem
partially. They especially don't work for buffer objects without a DRM
framebuffer associated.

Hence, this revert to going back to using .import_attach->dmabuf.

Signed-off-by: Thomas Zimmermann <tzimmerm...@suse.de>
Reviewed-by: Simona Vetter <simona.vet...@ffwll.ch>
---
 drivers/gpu/drm/drm_gem_framebuffer_helper.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem_framebuffer_helper.c 
b/drivers/gpu/drm/drm_gem_framebuffer_helper.c
index 618ce725cd75..fefb2a0f6b40 100644
--- a/drivers/gpu/drm/drm_gem_framebuffer_helper.c
+++ b/drivers/gpu/drm/drm_gem_framebuffer_helper.c
@@ -420,6 +420,7 @@ EXPORT_SYMBOL(drm_gem_fb_vunmap);
 static void __drm_gem_fb_end_cpu_access(struct drm_framebuffer *fb, enum 
dma_data_direction dir,
                                        unsigned int num_planes)
 {
+       struct dma_buf_attachment *import_attach;
        struct drm_gem_object *obj;
        int ret;
 
@@ -428,9 +429,10 @@ static void __drm_gem_fb_end_cpu_access(struct 
drm_framebuffer *fb, enum dma_dat
                obj = drm_gem_fb_get_obj(fb, num_planes);
                if (!obj)
                        continue;
+               import_attach = obj->import_attach;
                if (!drm_gem_is_imported(obj))
                        continue;
-               ret = dma_buf_end_cpu_access(obj->dma_buf, dir);
+               ret = dma_buf_end_cpu_access(import_attach->dmabuf, dir);
                if (ret)
                        drm_err(fb->dev, "dma_buf_end_cpu_access(%u, %d) 
failed: %d\n",
                                ret, num_planes, dir);
@@ -453,6 +455,7 @@ static void __drm_gem_fb_end_cpu_access(struct 
drm_framebuffer *fb, enum dma_dat
  */
 int drm_gem_fb_begin_cpu_access(struct drm_framebuffer *fb, enum 
dma_data_direction dir)
 {
+       struct dma_buf_attachment *import_attach;
        struct drm_gem_object *obj;
        unsigned int i;
        int ret;
@@ -463,9 +466,10 @@ int drm_gem_fb_begin_cpu_access(struct drm_framebuffer 
*fb, enum dma_data_direct
                        ret = -EINVAL;
                        goto err___drm_gem_fb_end_cpu_access;
                }
+               import_attach = obj->import_attach;
                if (!drm_gem_is_imported(obj))
                        continue;
-               ret = dma_buf_begin_cpu_access(obj->dma_buf, dir);
+               ret = dma_buf_begin_cpu_access(import_attach->dmabuf, dir);
                if (ret)
                        goto err___drm_gem_fb_end_cpu_access;
        }
-- 
2.50.0