| Message ID | 20250325-b4-panel-refcounting-v1-2-4e2bf5d19c5d@redhat.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | drm/panel: Panel Refcounting infrastructure | expand |
Hi Anusha, kernel test robot noticed the following build warnings: [auto build test WARNING on c8ba07caaecc622a9922cda49f24790821af8a71] url: https://github.com/intel-lab-lkp/linux/commits/Anusha-Srivatsa/drm-panel-Add-new-helpers-for-refcounted-panel-allocatons/20250326-012651 base: c8ba07caaecc622a9922cda49f24790821af8a71 patch link: https://lore.kernel.org/r/20250325-b4-panel-refcounting-v1-2-4e2bf5d19c5d%40redhat.com patch subject: [PATCH 2/5] drm/panel: Add refcount support config: s390-randconfig-002-20250326 (https://download.01.org/0day-ci/archive/20250326/202503260820.3wTF0Zap-lkp@intel.com/config) compiler: clang version 15.0.7 (https://github.com/llvm/llvm-project 8dfdcc7b7bf66834a761bd8de445840ef68e4d1a) reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250326/202503260820.3wTF0Zap-lkp@intel.com/reproduce) If you fix the issue in a separate patch/commit (i.e. not just a new version of the same patch/commit), kindly add following tags | Reported-by: kernel test robot <lkp@intel.com> | Closes: https://lore.kernel.org/oe-kbuild-all/202503260820.3wTF0Zap-lkp@intel.com/ All warnings (new ones prefixed by >>): >> drivers/gpu/drm/drm_panel.c:408: warning: expecting prototype for drm_bridge_put_void(). Prototype was for drm_panel_put_void() instead vim +408 drivers/gpu/drm/drm_panel.c 398 399 /** 400 * drm_bridge_put_void - wrapper to drm_bridge_put() taking a void pointer 401 * 402 * @data: pointer to @struct drm_bridge, cast to a void pointer 403 * 404 * Wrapper of drm_bridge_put() to be used when a function taking a void 405 * pointer is needed, for example as a devm action. 406 */ 407 static void drm_panel_put_void(void *data) > 408 { 409 struct drm_panel *panel = (struct drm_panel *)data; 410 411 drm_panel_put(panel); 412 } 413
On Tue, 25 Mar 2025 13:24:09 -0400 Anusha Srivatsa <asrivats@redhat.com> wrote: > Allocate panel via reference counting. > Add _get() and _put() helper functions > to ensure panel allocations are refcounted. > Avoid use after free by ensuring panel is > valid and can be usable till the last reference > is put. This avoids use-after-free "panel is valid and can be usable" is not totally correct. When there are still references held, you ensure the panel struct is still _allocated_, not necessarily valid and usable. Minor nit: you are wrapping at less than 50 columns, which is a bit tight. I think 75 is the expected value for wrapping. > Signed-off-by: Anusha Srivatsa <asrivats@redhat.com> [...] > +/** > + * drm_panel_get - Acquire a panel reference > + * @panel: DRM panel > + * > + * This function increments the panel's refcount. > + * > + */ Not sure it's mandatory, but documenting the returned value is good practice , e.g.: * Returns: * Pointer to @panel. > +/** > + * drm_panel_put - Release a panel reference > + * @panel: DRM panel > + * > + * This function decrements the panel's reference count and frees the > + * object if the reference count drops to zero. > + */ > +struct drm_panel *drm_panel_put(struct drm_panel *panel) > +{ > + if (!panel) > + return panel; > + > + kref_put(&panel->refcount, __drm_panel_free); > + > + return panel; While this is using the same structure as my bridge work, I now realize the _put() should probably not return the panel (or bridge) pointer, it should just be a void function. The reason is the pointer might have been freed when _put() returns (depending on the refcount) so that pointer value might be dangling and whoever calls _put() must not use that pointer anymore. Other get/put APIs do this, e.g. of_node_get/put(). So I'm going to change drm_bridge_put() to return void, unless there are good reasons to keep it and that I'm missing. > @@ -280,7 +291,10 @@ void *__devm_drm_panel_alloc(struct device *dev, size_t size, size_t offset, > * @member: the name of the &drm_panel within @type > * @funcs: callbacks for this panel > * @connector_type: connector type of the driver > - * The returned refcount is initialised to 1 > + * > + * The returned refcount is initialised to 1. This reference will > + * be automatically dropped via devm (by calling > + * drm_bridge_put()) when @dev is removed. ^^^^^^ "panel". Same in a few other places in this patch. Please search in all this series in case there are more. It's easy to forget about replacing some of those in the comments. :) Luca
On Tue, Mar 25, 2025 at 01:24:09PM -0400, Anusha Srivatsa wrote: > Allocate panel via reference counting. > Add _get() and _put() helper functions > to ensure panel allocations are refcounted. > Avoid use after free by ensuring panel is > valid and can be usable till the last reference > is put. This avoids use-after-free > > Signed-off-by: Anusha Srivatsa <asrivats@redhat.com> > --- > drivers/gpu/drm/drm_panel.c | 65 ++++++++++++++++++++++++++++++++++++++++++++- > include/drm/drm_panel.h | 19 ++++++++++++- > 2 files changed, 82 insertions(+), 2 deletions(-) > > diff --git a/drivers/gpu/drm/drm_panel.c b/drivers/gpu/drm/drm_panel.c > index bdeab5710ee324dc1742fbc77582250960556308..079c3c666a2ddc99a0051d1a3c9ba65d986dd003 100644 > --- a/drivers/gpu/drm/drm_panel.c > +++ b/drivers/gpu/drm/drm_panel.c > @@ -355,24 +355,87 @@ struct drm_panel *of_drm_find_panel(const struct device_node *np) > } > EXPORT_SYMBOL(of_drm_find_panel); > > +static void __drm_panel_free(struct kref *kref) > +{ > + struct drm_panel *panel = container_of(kref, struct drm_panel, refcount); > + > + kfree(panel->container); > +} > + > +/** > + * drm_panel_get - Acquire a panel reference > + * @panel: DRM panel > + * > + * This function increments the panel's refcount. > + * > + */ > +struct drm_panel *drm_panel_get(struct drm_panel *panel) > +{ > + if (!panel) > + return panel; > + > + kref_get(&panel->refcount); > + > + return panel; > +} This should be exported > +/** > + * drm_panel_put - Release a panel reference > + * @panel: DRM panel > + * > + * This function decrements the panel's reference count and frees the > + * object if the reference count drops to zero. > + */ > +struct drm_panel *drm_panel_put(struct drm_panel *panel) > +{ > + if (!panel) > + return panel; > + > + kref_put(&panel->refcount, __drm_panel_free); > + > + return panel; > +} Ditto, > +/** > + * drm_bridge_put_void - wrapper to drm_bridge_put() taking a void pointer > + * > + * @data: pointer to @struct drm_bridge, cast to a void pointer > + * > + * Wrapper of drm_bridge_put() to be used when a function taking a void > + * pointer is needed, for example as a devm action. > + */ > +static void drm_panel_put_void(void *data) > +{ > + struct drm_panel *panel = (struct drm_panel *)data; > + > + drm_panel_put(panel); > +} You can drop the documentation on that one. Looks good otherwise, Maxime
On Wed, Mar 26, 2025 at 10:23:04AM +0100, Luca Ceresoli wrote: > On Tue, 25 Mar 2025 13:24:09 -0400 > Anusha Srivatsa <asrivats@redhat.com> wrote: > > > Allocate panel via reference counting. > > Add _get() and _put() helper functions > > to ensure panel allocations are refcounted. > > Avoid use after free by ensuring panel is > > valid and can be usable till the last reference > > is put. This avoids use-after-free > > "panel is valid and can be usable" is not totally correct. When there > are still references held, you ensure the panel struct is still > _allocated_, not necessarily valid and usable. I guess "panel pointer is valid" is a better wording indeed. > > +/** > > + * drm_panel_put - Release a panel reference > > + * @panel: DRM panel > > + * > > + * This function decrements the panel's reference count and frees the > > + * object if the reference count drops to zero. > > + */ > > +struct drm_panel *drm_panel_put(struct drm_panel *panel) > > +{ > > + if (!panel) > > + return panel; > > + > > + kref_put(&panel->refcount, __drm_panel_free); > > + > > + return panel; > > While this is using the same structure as my bridge work, I now realize > the _put() should probably not return the panel (or bridge) pointer, it > should just be a void function. The reason is the pointer might have > been freed when _put() returns (depending on the refcount) so that > pointer value might be dangling and whoever calls _put() must not use > that pointer anymore. > > Other get/put APIs do this, e.g. of_node_get/put(). > > So I'm going to change drm_bridge_put() to return void, unless there > are good reasons to keep it and that I'm missing. Oh, right, definitely. Maxime
diff --git a/drivers/gpu/drm/drm_panel.c b/drivers/gpu/drm/drm_panel.c index bdeab5710ee324dc1742fbc77582250960556308..079c3c666a2ddc99a0051d1a3c9ba65d986dd003 100644 --- a/drivers/gpu/drm/drm_panel.c +++ b/drivers/gpu/drm/drm_panel.c @@ -355,24 +355,87 @@ struct drm_panel *of_drm_find_panel(const struct device_node *np) } EXPORT_SYMBOL(of_drm_find_panel); +static void __drm_panel_free(struct kref *kref) +{ + struct drm_panel *panel = container_of(kref, struct drm_panel, refcount); + + kfree(panel->container); +} + +/** + * drm_panel_get - Acquire a panel reference + * @panel: DRM panel + * + * This function increments the panel's refcount. + * + */ +struct drm_panel *drm_panel_get(struct drm_panel *panel) +{ + if (!panel) + return panel; + + kref_get(&panel->refcount); + + return panel; +} + +/** + * drm_panel_put - Release a panel reference + * @panel: DRM panel + * + * This function decrements the panel's reference count and frees the + * object if the reference count drops to zero. + */ +struct drm_panel *drm_panel_put(struct drm_panel *panel) +{ + if (!panel) + return panel; + + kref_put(&panel->refcount, __drm_panel_free); + + return panel; +} + +/** + * drm_bridge_put_void - wrapper to drm_bridge_put() taking a void pointer + * + * @data: pointer to @struct drm_bridge, cast to a void pointer + * + * Wrapper of drm_bridge_put() to be used when a function taking a void + * pointer is needed, for example as a devm action. + */ +static void drm_panel_put_void(void *data) +{ + struct drm_panel *panel = (struct drm_panel *)data; + + drm_panel_put(panel); +} + void *__devm_drm_panel_alloc(struct device *dev, size_t size, size_t offset, const struct drm_panel_funcs *funcs, int connector_type) { void *container; struct drm_panel *panel; + int err; if (!funcs) { dev_warn(dev, "Missing funcs pointer\n"); return ERR_PTR(-EINVAL); } - container = devm_kzalloc(dev, size, GFP_KERNEL); + container = kzalloc(size, GFP_KERNEL); if (!container) return ERR_PTR(-ENOMEM); panel = container + offset; + panel->container = container; panel->funcs = funcs; + kref_init(&panel->refcount); + + err = devm_add_action_or_reset(dev, drm_panel_put_void, panel); + if (err) + return ERR_PTR(err); drm_panel_init(panel, dev, funcs, connector_type); diff --git a/include/drm/drm_panel.h b/include/drm/drm_panel.h index 63fb1dbe15a0556e7484bc18737a6b1f4c208b0c..af81d596f385567a12cf9e08dff9443ce4d97ec0 100644 --- a/include/drm/drm_panel.h +++ b/include/drm/drm_panel.h @@ -267,6 +267,17 @@ struct drm_panel { * If true then the panel has been enabled. */ bool enabled; + + /** + * @container: Pointer to the private driver struct embedding this + * @struct drm_panel. + */ + void *container; + + /** + * @refcount: reference count of users referencing this bridge. + */ + struct kref refcount; }; void *__devm_drm_panel_alloc(struct device *dev, size_t size, size_t offset, @@ -280,7 +291,10 @@ void *__devm_drm_panel_alloc(struct device *dev, size_t size, size_t offset, * @member: the name of the &drm_panel within @type * @funcs: callbacks for this panel * @connector_type: connector type of the driver - * The returned refcount is initialised to 1 + * + * The returned refcount is initialised to 1. This reference will + * be automatically dropped via devm (by calling + * drm_bridge_put()) when @dev is removed. * * Returns: * Pointer to new panel, or ERR_PTR on failure. @@ -294,6 +308,9 @@ void drm_panel_init(struct drm_panel *panel, struct device *dev, const struct drm_panel_funcs *funcs, int connector_type); +struct drm_panel *drm_panel_get(struct drm_panel *panel); +struct drm_panel *drm_panel_put(struct drm_panel *panel); + void drm_panel_add(struct drm_panel *panel); void drm_panel_remove(struct drm_panel *panel);
Allocate panel via reference counting. Add _get() and _put() helper functions to ensure panel allocations are refcounted. Avoid use after free by ensuring panel is valid and can be usable till the last reference is put. This avoids use-after-free Signed-off-by: Anusha Srivatsa <asrivats@redhat.com> --- drivers/gpu/drm/drm_panel.c | 65 ++++++++++++++++++++++++++++++++++++++++++++- include/drm/drm_panel.h | 19 ++++++++++++- 2 files changed, 82 insertions(+), 2 deletions(-)