On 30.06.25 13:34, Thomas Zimmermann wrote:
> Hi
>
> Am 30.06.25 um 10:49 schrieb Christian König:
>> On 30.06.25 10:36, Thomas Zimmermann wrote:
>>> A GEM handle can be released while the GEM buffer object is attached
>>> to a DRM framebuffer. This leads to the release of the dma-buf backing
>>> the buffer object, if any. [1] Trying to use the framebuffer in further
>>> mode-setting operations leads to a segmentation fault. Most easily
>>> happens with driver that use shadow planes for vmap-ing the dma-buf
>>> during a page flip. An example is shown below.
>>>
>>> [ 156.791968] ------------[ cut here ]------------
>>> [ 156.796830] WARNING: CPU: 2 PID: 2255 at drivers/dma-buf/dma-buf.c:1527
>>> dma_buf_vmap+0x224/0x430
>>> [...]
>>> [ 156.942028] RIP: 0010:dma_buf_vmap+0x224/0x430
>>> [ 157.043420] Call Trace:
>>> [ 157.045898] <TASK>
>>> [ 157.048030] ? show_trace_log_lvl+0x1af/0x2c0
>>> [ 157.052436] ? show_trace_log_lvl+0x1af/0x2c0
>>> [ 157.056836] ? show_trace_log_lvl+0x1af/0x2c0
>>> [ 157.061253] ? drm_gem_shmem_vmap+0x74/0x710
>>> [ 157.065567] ? dma_buf_vmap+0x224/0x430
>>> [ 157.069446] ? __warn.cold+0x58/0xe4
>>> [ 157.073061] ? dma_buf_vmap+0x224/0x430
>>> [ 157.077111] ? report_bug+0x1dd/0x390
>>> [ 157.080842] ? handle_bug+0x5e/0xa0
>>> [ 157.084389] ? exc_invalid_op+0x14/0x50
>>> [ 157.088291] ? asm_exc_invalid_op+0x16/0x20
>>> [ 157.092548] ? dma_buf_vmap+0x224/0x430
>>> [ 157.096663] ? dma_resv_get_singleton+0x6d/0x230
>>> [ 157.101341] ? __pfx_dma_buf_vmap+0x10/0x10
>>> [ 157.105588] ? __pfx_dma_resv_get_singleton+0x10/0x10
>>> [ 157.110697] drm_gem_shmem_vmap+0x74/0x710
>>> [ 157.114866] drm_gem_vmap+0xa9/0x1b0
>>> [ 157.118763] drm_gem_vmap_unlocked+0x46/0xa0
>>> [ 157.123086] drm_gem_fb_vmap+0xab/0x300
>>> [ 157.126979] drm_atomic_helper_prepare_planes.part.0+0x487/0xb10
>>> [ 157.133032] ? lockdep_init_map_type+0x19d/0x880
>>> [ 157.137701] drm_atomic_helper_commit+0x13d/0x2e0
>>> [ 157.142671] ? drm_atomic_nonblocking_commit+0xa0/0x180
>>> [ 157.147988] drm_mode_atomic_ioctl+0x766/0xe40
>>> [...]
>>> [ 157.346424] ---[ end trace 0000000000000000 ]---
>>>
>>> Acquiring GEM handles for the framebuffer's GEM buffer objects prevents
>>> this from happening. The framebuffer's cleanup later puts the handle
>>> references.
>>>
>>> Commit 1a148af06000 ("drm/gem-shmem: Use dma_buf from GEM object
>>> instance") triggers the segmentation fault easily by using the dma-buf
>>> field more widely. The underlying issue with reference counting has
>>> been present before.
>>>
>>> v2:
>>> - acquire the handle instead of the BO (Christian)
>>> - fix comment style (Christian)
>>> - drop the Fixes tag (Christian)
>>> - rename err_ gotos
>>> - add missing Link tag
>>>
>>> Suggested-by: Christian König <christian.koe...@amd.com>
>>> Signed-off-by: Thomas Zimmermann <tzimmerm...@suse.de>
>> Reviewed-by: Christian König <christian.koe...@amd.com>
>
> Thanks a lot
>
>>
>> But I strongly suggest to let the different CI systems take a look as well,
>> we already had to much fun with that.
>
> I can wait a bit longer for reports, but the patch fixes a regression in
> v6.15. I'd rather see it merged soon-ish.
Yeah, agree. I just want to make sure that we don't have a case where we never
create a handle for a BO, but still try to have a FB for it.
I'm pretty sure such cases don't exists any more, but who knows?
Anyway feel free to push it to drm-misc-fixes as soon as possible, just keep it
in the back of your mind to keep an eye on it.
Regards,
Christian.
>
> Best regards
> Thomas
>
>>
>> Regards,
>> Christian.
>>
>>> Link:
>>> https://elixir.bootlin.com/linux/v6.15/source/drivers/gpu/drm/drm_gem.c#L241
>>> # [1]
>>> Cc: Thomas Zimmermann <tzimmerm...@suse.de>
>>> Cc: Anusha Srivatsa <asriv...@redhat.com>
>>> Cc: Christian König <christian.koe...@amd.com>
>>> Cc: Maarten Lankhorst <maarten.lankho...@linux.intel.com>
>>> Cc: Maxime Ripard <mrip...@kernel.org>
>>> Cc: Sumit Semwal <sumit.sem...@linaro.org>
>>> Cc: "Christian König" <christian.koe...@amd.com>
>>> Cc: linux-me...@vger.kernel.org
>>> Cc: dri-devel@lists.freedesktop.org
>>> Cc: linaro-mm-...@lists.linaro.org
>>> Cc: <sta...@vger.kernel.org>
>>> ---
>>> drivers/gpu/drm/drm_gem.c | 44 ++++++++++++++++++--
>>> drivers/gpu/drm/drm_gem_framebuffer_helper.c | 16 +++----
>>> drivers/gpu/drm/drm_internal.h | 2 +
>>> 3 files changed, 51 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
>>> index 19d50d254fe6..bc505d938b3e 100644
>>> --- a/drivers/gpu/drm/drm_gem.c
>>> +++ b/drivers/gpu/drm/drm_gem.c
>>> @@ -213,6 +213,35 @@ void drm_gem_private_object_fini(struct drm_gem_object
>>> *obj)
>>> }
>>> EXPORT_SYMBOL(drm_gem_private_object_fini);
>>> +static void drm_gem_object_handle_get(struct drm_gem_object *obj)
>>> +{
>>> + struct drm_device *dev = obj->dev;
>>> +
>>> + drm_WARN_ON(dev, !mutex_is_locked(&dev->object_name_lock));
>>> +
>>> + if (obj->handle_count++ == 0)
>>> + drm_gem_object_get(obj);
>>> +}
>>> +
>>> +/**
>>> + * drm_gem_object_handle_get_unlocked - acquire reference on user-space
>>> handles
>>> + * @obj: GEM object
>>> + *
>>> + * Acquires a reference on the GEM buffer object's handle. Required
>>> + * to keep the GEM object alive. Call drm_gem_object_handle_put_unlocked()
>>> + * to release the reference.
>>> + */
>>> +void drm_gem_object_handle_get_unlocked(struct drm_gem_object *obj)
>>> +{
>>> + struct drm_device *dev = obj->dev;
>>> +
>>> + guard(mutex)(&dev->object_name_lock);
>>> +
>>> + drm_WARN_ON(dev, !obj->handle_count); /* first ref taken in
>>> create-tail helper */
>>> + drm_gem_object_handle_get(obj);
>>> +}
>>> +EXPORT_SYMBOL(drm_gem_object_handle_get_unlocked);
>>> +
>>> /**
>>> * drm_gem_object_handle_free - release resources bound to userspace
>>> handles
>>> * @obj: GEM object to clean up.
>>> @@ -243,8 +272,14 @@ static void
>>> drm_gem_object_exported_dma_buf_free(struct drm_gem_object *obj)
>>> }
>>> }
>>> -static void
>>> -drm_gem_object_handle_put_unlocked(struct drm_gem_object *obj)
>>> +/**
>>> + * drm_gem_object_handle_put_unlocked - releases reference on user-space
>>> handles
>>> + * @obj: GEM object
>>> + *
>>> + * Releases a reference on the GEM buffer object's handle. Possibly
>>> releases
>>> + * the GEM buffer object and associated dma-buf objects.
>>> + */
>>> +void drm_gem_object_handle_put_unlocked(struct drm_gem_object *obj)
>>> {
>>> struct drm_device *dev = obj->dev;
>>> bool final = false;
>>> @@ -269,6 +304,7 @@ drm_gem_object_handle_put_unlocked(struct
>>> drm_gem_object *obj)
>>> if (final)
>>> drm_gem_object_put(obj);
>>> }
>>> +EXPORT_SYMBOL(drm_gem_object_handle_put_unlocked);
>>> /*
>>> * Called at device or object close to release the file's
>>> @@ -390,8 +426,8 @@ drm_gem_handle_create_tail(struct drm_file *file_priv,
>>> int ret;
>>> WARN_ON(!mutex_is_locked(&dev->object_name_lock));
>>> - if (obj->handle_count++ == 0)
>>> - drm_gem_object_get(obj);
>>> +
>>> + drm_gem_object_handle_get(obj);
>>> /*
>>> * Get the user-visible handle using idr. Preload and perform
>>> diff --git a/drivers/gpu/drm/drm_gem_framebuffer_helper.c
>>> b/drivers/gpu/drm/drm_gem_framebuffer_helper.c
>>> index 618ce725cd75..c60d0044d036 100644
>>> --- a/drivers/gpu/drm/drm_gem_framebuffer_helper.c
>>> +++ b/drivers/gpu/drm/drm_gem_framebuffer_helper.c
>>> @@ -100,7 +100,7 @@ void drm_gem_fb_destroy(struct drm_framebuffer *fb)
>>> unsigned int i;
>>> for (i = 0; i < fb->format->num_planes; i++)
>>> - drm_gem_object_put(fb->obj[i]);
>>> + drm_gem_object_handle_put_unlocked(fb->obj[i]);
>>> drm_framebuffer_cleanup(fb);
>>> kfree(fb);
>>> @@ -183,8 +183,10 @@ int drm_gem_fb_init_with_funcs(struct drm_device *dev,
>>> if (!objs[i]) {
>>> drm_dbg_kms(dev, "Failed to lookup GEM object\n");
>>> ret = -ENOENT;
>>> - goto err_gem_object_put;
>>> + goto err_gem_object_handle_put_unlocked;
>>> }
>>> + drm_gem_object_handle_get_unlocked(objs[i]);
>>> + drm_gem_object_put(objs[i]);
>>> min_size = (height - 1) * mode_cmd->pitches[i]
>>> + drm_format_info_min_pitch(info, i, width)
>>> @@ -194,22 +196,22 @@ int drm_gem_fb_init_with_funcs(struct drm_device *dev,
>>> drm_dbg_kms(dev,
>>> "GEM object size (%zu) smaller than minimum size (%u)
>>> for plane %d\n",
>>> objs[i]->size, min_size, i);
>>> - drm_gem_object_put(objs[i]);
>>> + drm_gem_object_handle_put_unlocked(objs[i]);
>>> ret = -EINVAL;
>>> - goto err_gem_object_put;
>>> + goto err_gem_object_handle_put_unlocked;
>>> }
>>> }
>>> ret = drm_gem_fb_init(dev, fb, mode_cmd, objs, i, funcs);
>>> if (ret)
>>> - goto err_gem_object_put;
>>> + goto err_gem_object_handle_put_unlocked;
>>> return 0;
>>> -err_gem_object_put:
>>> +err_gem_object_handle_put_unlocked:
>>> while (i > 0) {
>>> --i;
>>> - drm_gem_object_put(objs[i]);
>>> + drm_gem_object_handle_put_unlocked(objs[i]);
>>> }
>>> return ret;
>>> }
>>> diff --git a/drivers/gpu/drm/drm_internal.h b/drivers/gpu/drm/drm_internal.h
>>> index 442eb31351dd..f7b414a813ae 100644
>>> --- a/drivers/gpu/drm/drm_internal.h
>>> +++ b/drivers/gpu/drm/drm_internal.h
>>> @@ -161,6 +161,8 @@ void drm_sysfs_lease_event(struct drm_device *dev);
>>> /* drm_gem.c */
>>> int drm_gem_init(struct drm_device *dev);
>>> +void drm_gem_object_handle_get_unlocked(struct drm_gem_object *obj);
>>> +void drm_gem_object_handle_put_unlocked(struct drm_gem_object *obj);
>>> int drm_gem_handle_create_tail(struct drm_file *file_priv,
>>> struct drm_gem_object *obj,
>>> u32 *handlep);
>
