New subject: Warnings in next-20250703 caused by commit 582111e630f5

30 Jun 2025

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.koenig@amd.com
Signed-off-by: Thomas Zimmermann tzimmermann@suse.de
Link: https://elixir.bootlin.com/linux/v6.15/source/drivers/gpu/drm/drm_gem.c#L241 # [1]
Cc: Thomas Zimmermann tzimmermann@suse.de
Cc: Anusha Srivatsa asrivats@redhat.com
Cc: Christian König christian.koenig@amd.com
Cc: Maarten Lankhorst maarten.lankhorst@linux.intel.com
Cc: Maxime Ripard mripard@kernel.org
Cc: Sumit Semwal sumit.semwal@linaro.org
Cc: "Christian König" christian.koenig@amd.com
Cc: linux-media@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: linaro-mm-sig@lists.linaro.org
Cc: stable@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);
-- 
2.50.0



    

[PATCH v2] drm/gem: Acquire references on GEM handles for framebuffers