Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider: - [High] dma-buf: dma_fence_get() uses kref_get_unless_zero() breaking API contract - [High] dma-buf/sw_sync: sync_timeline_signal() leaves zero-refcount pt linked --
commit 6ff0958843bbbad778cdc3d38a038557ac00bd49 Author: Edward Adam Davis <[email protected]> dma-buf/sw_sync: Optimize the retrieval of fence reference counts This commit changes dma_fence_get() to use kref_get_unless_zero() and updates sync_timeline_signal() to skip fences when dma_fence_get() returns NULL. This is intended to avoid a refcount addition-on-zero when racing with fence release in sw_sync. > diff --git a/drivers/dma-buf/sw_sync.c b/drivers/dma-buf/sw_sync.c > index 8df20b0218a9e..ad60ad5e5cd9e 100644 > --- a/drivers/dma-buf/sw_sync.c > +++ b/drivers/dma-buf/sw_sync.c > @@ -221,7 +221,8 @@ static void sync_timeline_signal(struct sync_timeline > *obj, unsigned int inc) > if (!timeline_fence_signaled(&pt->base)) > break; > > - dma_fence_get(&pt->base); > + if (!dma_fence_get(&pt->base)) > + continue; [Severity: High] When acquiring a reference to pt->base fails because its refcount has already dropped to zero on another CPU, is it safe to continue without unlinking pt from obj->pt_list and obj->pt_tree? Continuing here leaves a fence with pt->base.seqno <= obj->value linked at the head of obj->pt_list and in obj->pt_tree until timeline_fence_release() acquires obj->lock: drivers/dma-buf/sw_sync.c:timeline_fence_release() { ... dma_fence_lock_irqsave(fence, flags); if (!list_empty(&pt->link)) { list_del(&pt->link); rb_erase(&pt->node, &parent->pt_tree); } dma_fence_unlock_irqrestore(fence, flags); ... } Before timeline_fence_release() takes obj->lock, leaving this stale entry linked can cause several regressions: 1. In sw_sync_ioctl_inc(), increments larger than INT_MAX call sync_timeline_signal(obj, INT_MAX) in a loop (or consecutive SW_SYNC_IOC_INC calls may run): while (value > INT_MAX) { sync_timeline_signal(obj, INT_MAX); value -= INT_MAX; } sync_timeline_signal(obj, value); If obj->value advances by more than INT_MAX past the stale pt->base.seqno at the head of obj->pt_list, the signed difference in timeline_fence_signaled() -> __dma_fence_is_later() wraps positive. timeline_fence_signaled(&pt->base) then returns false at the stale head entry, causing sync_timeline_signal() to break out of the loop early without signaling active fences later in obj->pt_list. 2. In sync_pt_create(), rbtree insertion assumes all nodes in obj->pt_tree have seqno in (obj->value, obj->value + INT_MAX]: parent = *p; other = rb_entry(parent, typeof(*pt), node); cmp = value - other->base.seqno; If a stale pt with seqno <= obj->value remains in obj->pt_tree and a new unsignaled fence is created with value in (pt->base.seqno + INT_MAX, obj->value + INT_MAX], cmp overflows a signed 32-bit int to a negative value. That inserts the new fence to the left of the stale pt and ahead of earlier active fences in obj->pt_list, corrupting the list and tree ordering even after timeline_fence_release() later removes the stale pt. 3. If sync_info_debugfs_show() -> sync_print_obj() -> sync_print_fence() runs while the skipped pt is still on obj->pt_list, dma_fence_get_status_locked() calls dma_fence_is_signaled_locked(&pt->base), which sees timeline_fence_signaled(&pt->base) is true and sets DMA_FENCE_FLAG_SIGNALED_BIT on pt->base. If sw_sync_debugfs_release() then runs before timeline_fence_release() acquires obj->lock: list_for_each_entry_safe(pt, next, &obj->pt_list, link) { dma_fence_set_error(&pt->base, -ENOENT); dma_fence_signal_locked(&pt->base); } dma_fence_set_error() triggers WARN_ON(test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)). Since timeline_fence_release() checks !list_empty(&pt->link) under obj->lock, should sync_timeline_signal() unlink pt with list_del_init(&pt->link) and rb_erase(&pt->node, &obj->pt_tree) before continuing when acquiring the fence reference fails? > > list_move_tail(&pt->link, &signalled); > rb_erase(&pt->node, &obj->pt_tree); > diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h > index dd07d128adc3b..4d885c8e1d4f5 100644 > --- a/include/linux/dma-fence.h > +++ b/include/linux/dma-fence.h > @@ -320,8 +320,9 @@ static inline void dma_fence_put(struct dma_fence *fence) > static inline struct dma_fence *dma_fence_get(struct dma_fence *fence) > { > if (fence) > - kref_get(&fence->refcount); > - return fence; > + if (kref_get_unless_zero(&fence->refcount)) > + return fence; > + return NULL; > } [Severity: High] Does changing dma_fence_get() to use kref_get_unless_zero() break the dma_fence_get() API contract across the kernel? dma_fence_get() is meant for callers that already hold a valid reference to a fence, and its kernel-doc comment right above states that it returns the same fence with its refcount increased by 1. Replacing kref_get() with kref_get_unless_zero() here suppresses refcount_t addition-on-zero warnings and saturation when a caller has a genuine use-after-free bug. Because dma_fence_get() is not annotated with __must_check, callers that do not check its return value (such as dma_resv_add_fence(), dma_buf_poll(), dma_fence_array_enable_signaling(), dma_fence_chain_enable_signaling(), drm_syncobj_add_point(), and drm_syncobj_replace_fence()) will silently fail to take a reference and later underflow the refcount in dma_fence_put(). Other callers that pass a non-NULL fence and dereference or pass the return value to container_of() (such as i915_request_get(), sync_file_create(), dma_resv_replace_fences(), and __dma_fence_unwrap_merge()) can unexpectedly receive NULL. include/linux/dma-fence.h already provides dma_fence_get_rcu() right below dma_fence_get() for acquiring a reference when the refcount might be zero: static inline struct dma_fence *dma_fence_get_rcu(struct dma_fence *fence) { if (kref_get_unless_zero(&fence->refcount)) return fence; else return NULL; } And drivers/dma-buf/sw_sync.c already uses dma_fence_get_rcu() in sync_pt_create() for this exact situation: } else { if (dma_fence_get_rcu(&other->base)) { sync_timeline_put(obj); kfree(pt); pt = other; goto unlock; } p = &parent->rb_left; } Should dma_fence_get() be left unchanged and sync_timeline_signal() call dma_fence_get_rcu(&pt->base) instead? -- Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=1
