Hi On Tue, Jul 16, 2013 at 9:12 AM, Daniel Vetter <daniel.vetter@xxxxxxxx> wrote: > This is the 2nd attempt, I've always been a bit dissatisified with the > tricky nature of the first one: > > http://lists.freedesktop.org/archives/dri-devel/2012-July/025451.html > > The issue is that the flink ioctl can race with calling gem_close on > the last gem handle. In that case we'll end up with a zero handle > count, but an flink name (and it's corresponding reference). Which > results in a neat space leak. > > In my first attempt I've solved this by rechecking the handle count. > But fundamentally the issue is that ->handle_count isn't your usual > refcount - it can be resurrected from 0 among other things. > > For those special beasts atomic_t often suggest way more ordering that > it actually guarantees. To prevent being tricked by those hairy > semantics take the easy way out and simply protect the handle with the > existing dev->object_name_lock. > > With that change implemented it's dead easy to fix the flink vs. gem > close reace: When we try to create the name we simply have to check > whether there's still officially a gem handle around and if not refuse > to create the flink name. Since the handle count decrement and flink > name destruction is now also protected by that lock the reace is gone > and we can't ever leak the flink reference again. > > Outside of the drm core only the exynos driver looks at the handle > count, and tbh I have no idea why (it's just for debug dmesg output > luckily). > > I've considered inlining the drm_gem_object_handle_free, but I plan to > add more name-like things (like the exported dma_buf) to this scheme, > so it's clearer to leave the handle freeing in its own function. > > v2: Fix up the error path handling in handle_create and make it more > robust by simply calling object_handle_unreference. > > v3: Fix up the handle_unreference logic bug - atomic_dec_and_test > retursn 1 for 0. Oops. > > Cc: Inki Dae <inki.dae@xxxxxxxxxxx> > Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxx> > --- > drivers/gpu/drm/drm_gem.c | 34 ++++++++++++++++++++------------- > drivers/gpu/drm/drm_info.c | 2 +- > drivers/gpu/drm/exynos/exynos_drm_gem.c | 2 +- > include/drm/drmP.h | 12 ++++++++++-- > 4 files changed, 33 insertions(+), 17 deletions(-) > > diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c > index b07519e..14c70b5 100644 > --- a/drivers/gpu/drm/drm_gem.c > +++ b/drivers/gpu/drm/drm_gem.c > @@ -140,7 +140,7 @@ int drm_gem_object_init(struct drm_device *dev, > return PTR_ERR(obj->filp); > > kref_init(&obj->refcount); > - atomic_set(&obj->handle_count, 0); > + obj->handle_count = 0; > obj->size = size; > > return 0; > @@ -161,7 +161,7 @@ int drm_gem_private_object_init(struct drm_device *dev, > obj->filp = NULL; > > kref_init(&obj->refcount); > - atomic_set(&obj->handle_count, 0); > + obj->handle_count = 0; > obj->size = size; > > return 0; > @@ -227,11 +227,9 @@ static void drm_gem_object_handle_free(struct drm_gem_object *obj) > struct drm_device *dev = obj->dev; > > /* Remove any name for this object */ > - spin_lock(&dev->object_name_lock); > if (obj->name) { > idr_remove(&dev->object_name_idr, obj->name); > obj->name = 0; > - spin_unlock(&dev->object_name_lock); > /* > * The object name held a reference to this object, drop > * that now. > @@ -239,15 +237,13 @@ static void drm_gem_object_handle_free(struct drm_gem_object *obj) > * This cannot be the last reference, since the handle holds one too. > */ > kref_put(&obj->refcount, drm_gem_object_ref_bug); > - } else > - spin_unlock(&dev->object_name_lock); > - > + } > } > > void > drm_gem_object_handle_unreference_unlocked(struct drm_gem_object *obj) > { > - if (WARN_ON(atomic_read(&obj->handle_count) == 0)) > + if (WARN_ON(obj->handle_count == 0)) > return; > > /* > @@ -256,8 +252,11 @@ drm_gem_object_handle_unreference_unlocked(struct drm_gem_object *obj) > * checked for a name > */ > > - if (atomic_dec_and_test(&obj->handle_count)) > + spin_lock(&obj->dev->object_name_lock); > + if (--obj->handle_count == 0) > drm_gem_object_handle_free(obj); If you inline this here, you can actually drop the huge comment for "caller still holds reference" as you call "object_unreference()" below, anyway. And with this patch, gem_object_handle_free() is pretty small, anyway. I don't actually understand what you try to say in the commit message about new name-like stuff, but if you reuse it, it's fine. > + spin_unlock(&obj->dev->object_name_lock); > + > drm_gem_object_unreference_unlocked(obj); > } > > @@ -321,18 +320,21 @@ drm_gem_handle_create(struct drm_file *file_priv, > * allocation under our spinlock. > */ > idr_preload(GFP_KERNEL); > + spin_lock(&dev->object_name_lock); > spin_lock(&file_priv->table_lock); > > ret = idr_alloc(&file_priv->object_idr, obj, 1, 0, GFP_NOWAIT); > - > + drm_gem_object_reference(obj); > + obj->handle_count++; > spin_unlock(&file_priv->table_lock); > + spin_unlock(&dev->object_name_lock); > idr_preload_end(); > - if (ret < 0) > + if (ret < 0) { > + drm_gem_object_handle_unreference_unlocked(obj); > return ret; > + } The locking order isn't really documented. What's wrong with: idr_preload(GFP_KERNEL); spin_lock(&file_priv->table_lock); ret = idr_alloc(&file_priv->object_idr, obj, 1, 0, GFP_NOWAIT); spin_unlock(&file_priv->table_lock); idr_preload_end(); if (ret < 0) return ret; spin_lock(&dev->object_name_lock); obj->handle_count++; spin_unlock(&dev->object_name_lock); drm_gem_object_reference(obj); This is safe against flink() as we don't care whether flink() fails if user-space isn't even aware of the handle, yet. And if user-space already has a handle, then "handle_count" is >0, anyway. And gem_object_handle_unreference can only be called if another handle exists (thus, handle_count > 0). Or am I missing something? > *handlep = ret; > > - drm_gem_object_reference(obj); > - atomic_inc(&obj->handle_count); > > if (dev->driver->gem_open_object) { > ret = dev->driver->gem_open_object(obj, file_priv); > @@ -499,6 +501,12 @@ drm_gem_flink_ioctl(struct drm_device *dev, void *data, > > idr_preload(GFP_KERNEL); > spin_lock(&dev->object_name_lock); > + /* prevent races with concurrent gem_close. */ > + if (obj->handle_count == 0) { > + ret = -ENOENT; > + goto err; spin_unlock(&dev->object_name_lock); ? Aside from the spin_unlock(), it's all a matter of taste, so: Reviewed-by: David Herrmann <dh.herrmann@xxxxxxxxx> Cheers David > + } > + > if (!obj->name) { > ret = idr_alloc(&dev->object_name_idr, obj, 1, 0, GFP_NOWAIT); > if (ret < 0) > diff --git a/drivers/gpu/drm/drm_info.c b/drivers/gpu/drm/drm_info.c > index d4b20ce..f4b348c 100644 > --- a/drivers/gpu/drm/drm_info.c > +++ b/drivers/gpu/drm/drm_info.c > @@ -207,7 +207,7 @@ static int drm_gem_one_name_info(int id, void *ptr, void *data) > > seq_printf(m, "%6d %8zd %7d %8d\n", > obj->name, obj->size, > - atomic_read(&obj->handle_count), > + obj->handle_count, > atomic_read(&obj->refcount.refcount)); > return 0; > } > diff --git a/drivers/gpu/drm/exynos/exynos_drm_gem.c b/drivers/gpu/drm/exynos/exynos_drm_gem.c > index 24c22a8..16963ca 100644 > --- a/drivers/gpu/drm/exynos/exynos_drm_gem.c > +++ b/drivers/gpu/drm/exynos/exynos_drm_gem.c > @@ -135,7 +135,7 @@ void exynos_drm_gem_destroy(struct exynos_drm_gem_obj *exynos_gem_obj) > obj = &exynos_gem_obj->base; > buf = exynos_gem_obj->buffer; > > - DRM_DEBUG_KMS("handle count = %d\n", atomic_read(&obj->handle_count)); > + DRM_DEBUG_KMS("handle count = %d\n", obj->handle_count); > > /* > * do not release memory region from exporter. > diff --git a/include/drm/drmP.h b/include/drm/drmP.h > index 2fb83b4..25da8e0 100644 > --- a/include/drm/drmP.h > +++ b/include/drm/drmP.h > @@ -634,8 +634,16 @@ struct drm_gem_object { > /** Reference count of this object */ > struct kref refcount; > > - /** Handle count of this object. Each handle also holds a reference */ > - atomic_t handle_count; /* number of handles on this object */ > + /** > + * handle_count - gem file_priv handle count of this object > + * > + * Each handle also holds a reference. Note that when the handle_count > + * drops to 0 any global names (e.g. the id in the flink namespace) will > + * be cleared. > + * > + * Protected by dev->object_name_lock. > + * */ > + unsigned handle_count; > > /** Related drm device */ > struct drm_device *dev; > -- > 1.8.3.2 > > _______________________________________________ > dri-devel mailing list > dri-devel@xxxxxxxxxxxxxxxxxxxxx > http://lists.freedesktop.org/mailman/listinfo/dri-devel _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/dri-devel
