Am 21.03.22 um 16:44 schrieb Robert Beckett:
On 21/03/2022 09:51, Christian König wrote:
Am 18.03.22 um 20:50 schrieb Robert Beckett:
when allocating a resource in place it is common to free the buffer's
resource, then allocate a new resource in a different placement.
e.g. amdgpu_bo_create_kernel_at calls ttm_resource_free, then calls
ttm_bo_mem_space.
Well yes I'm working the drivers towards this, but NAK at the moment.
Currently bo->resource is never expected to be NULL.
And yes I'm searching for this bug in amdgpu for quite a while. Where
exactly does that happen?
in my case, I am writing new code for i915 that does this. I will
switch it to allocate the new resource first, then free the old one if
successful.
For the existing amd case, see
https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgit.kernel.org%2Fpub%2Fscm%2Flinux%2Fkernel%2Fgit%2Ftorvalds%2Flinux.git%2Ftree%2Fdrivers%2Fgpu%2Fdrm%2Famd%2Famdgpu%2Famdgpu_object.c%3Fh%3Dv5.17%23n384&data=04%7C01%7Cchristian.koenig%40amd.com%7C81f91d39683e4991181008da0b51a8d0%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637834742606744919%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=mjHPvBStFM7tsYf%2BL9fYCWqddZuIaeza6BsRF3fAmao%3D&reserved=0
amdgpu_bo_create_kernel_at calls ttm_resource_free, then calls
ttm_bo_mem_space. If the ttm_bo_mem_space call fails (e.g. due to
memory pressure), then the error path will try to deref bo->resource,
which will be null at that point.
Yeah, but that's a special handling only used during driver startup. We
somehow have this on systems with DMA-buf sharing as well.
to fix this, I honestly don't see a reason to not also have the safety
check for null there. It could check early and return an error if it
is null. I think that defensive programming here makes sense, better
than a null deref if someone programs it wrong.
Having it here is fine, the problem is you need to have that at tons of
other places as well.
Maybe I should send you my WIP patch set for this? If you handle all the
other cases as well I'm perfectly fine with this.
Regards,
Christian.
Amdgpu is supposed to allocate a new resource first, then do a swap
and the free the old one.
Thanks,
Christian.
In this situation, bo->resource will be null as it is cleared during
the initial freeing of the previous resource.
This leads to a null deref.
Fixes: d3116756a710 (drm/ttm: rename bo->mem and make it a pointer)
Signed-off-by: Robert Beckett <bob.beckett@xxxxxxxxxxxxx>
---
drivers/gpu/drm/ttm/ttm_bo.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/ttm/ttm_bo.c
b/drivers/gpu/drm/ttm/ttm_bo.c
index db3dc7ef5382..62b29ee7d040 100644
--- a/drivers/gpu/drm/ttm/ttm_bo.c
+++ b/drivers/gpu/drm/ttm/ttm_bo.c
@@ -875,7 +875,7 @@ int ttm_bo_mem_space(struct ttm_buffer_object *bo,
}
error:
- if (bo->resource->mem_type == TTM_PL_SYSTEM && !bo->pin_count)
+ if (bo->resource && bo->resource->mem_type == TTM_PL_SYSTEM &&
!bo->pin_count)
ttm_bo_move_to_lru_tail_unlocked(bo);
return ret;
