Re: [PATCH v1 2/2] drm/gem: Check whether object is NULL in drm_gem_vunmap()

Christian König <christian.koenig@xxxxxxx> · Thu, 27 Oct 2022 09:40:27 +0200

Am 27.10.22 um 09:28 schrieb Dan Carpenter:
On Thu, Oct 27, 2022 at 08:17:31AM +0200, Christian König wrote:

Am 27.10.22 um 00:46 schrieb Dmitry Osipenko:
The drm_gem_vunmap() will crash with a NULL dereference if the passed
object pointer is NULL. It wasn't a problem before we added the locking
support to drm_gem_vunmap function because the mapping argument was always
NULL together with the object. Make drm_gem_vunmap() functions to handle
the NULL pointers better.

Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
Link: https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fdri-devel%2FY1kFEGxT8MVlf32V%40kili%2F&amp;data=05%7C01%7Cchristian.koenig%40amd.com%7C50d3b7e10eca4c7fb72108dab7ecd646%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C638024525139985844%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&amp;sdata=ET0TRHscb1bQVVHgBkmYvTQyV2Q6WqcMC83LDlrY5ZM%3D&amp;reserved=0  Fixes:
79e2cf2e7a19 ("drm/gem: Take reservation lock for vmap/vunmap
operations")
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx>
---
   drivers/gpu/drm/drm_gem.c | 9 ++++++---
   1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
index b8db675e7fb5..ee0a246ff4ac 100644
--- a/drivers/gpu/drm/drm_gem.c
+++ b/drivers/gpu/drm/drm_gem.c
@@ -1175,11 +1175,11 @@ EXPORT_SYMBOL(drm_gem_vmap);
   void drm_gem_vunmap(struct drm_gem_object *obj, struct iosys_map *map)
   {
-	dma_resv_assert_held(obj->resv);
-
-	if (iosys_map_is_null(map))
+	if (!obj || iosys_map_is_null(map))
   		return;
I'm not very keen about that. Calling a function with all parameters NULL
doesn't make much sense and is clearly a coding error. Hiding that somehow
doesn't help but rather makes things worse.

The only execption to that are things like kfree() or *_put() which work
with the lifetime of objects.

Why is the static checker complaining about that in the first place?

drivers/gpu/drm/drm_client.c:240 drm_client_buffer_delete()
warn: variable dereferenced before check 'buffer->gem' (see line 238)

Well than rather modify drm_client_buffer_delete() instead.

Calling drm_gem_vunmap() with obj NULL is certainly not a good idea if 
we even check that for drm_gem_object_put().

Interesting that the static checkers complain about that :)

Regards,
Christian.


regards,
dan carpenter








