From 921b0a863382b70234aeb4bd589c10328e9ff042 Mon Sep 17 00:00:00 2001 From: Thomas Hebb Date: Thu, 6 Jan 2022 03:44:55 -0800 Subject: input/seat: unset has_focus when focus_stack becomes empty We currently track the focus of a seat in two ways: we use a list called focus_stack to track the order in which nodes have been focused, with the first node representing what's currently focused, and we use a variable called has_focus to indicate whether anything has focus--i.e. whether we should actually treat that first node as focused at any given time. In a number of places, we treat has_focus as implying that a focused node exists. If it's true, we attempt to dereference the return value of seat_get_focus(), our helper function for getting the first node in focus_list, with no further checks. But this isn't quite correct with the current implementation of seat_get_focus(): not only does it return NULL when has_focus is false, it also returns NULL when focus_stack contains no items. In most cases, focus_stack never becomes empty and so this doesn't matter at all. Since focus_stack stores a history of focused nodes, we rarely remove nodes from it. The exception to this is when a node itself goes away. In that case, we call seat_node_destroy() to remove it from focus_stack and free it. But we don't unset has_focus if we've removed the final node! This lets us get into a state where has_focus is true but seat_get_focus() returns NULL, leading to a segfault when we try to dereference it. Fix the issue both by updating has_focus in seat_node_destroy() and by adding an assertion in seat_get_focus() that ensures focus_stack and has_focus are in sync, which will make it easier to track down similar issues in the future. Fixes #6395. [1] There's some discussion in #1585 from when this was implemented about whether has_focus is actually necessary; it's possible we could remove it entirely, but for the moment this is the architecture we have. --- sway/input/seat.c | 15 ++++++++++++--- 1 file changed, 12 insertions(+), 3 deletions(-) diff --git a/sway/input/seat.c b/sway/input/seat.c index c5c8459e..ce933b66 100644 --- a/sway/input/seat.c +++ b/sway/input/seat.c @@ -51,6 +51,16 @@ static void seat_device_destroy(struct sway_seat_device *seat_device) { static void seat_node_destroy(struct sway_seat_node *seat_node) { wl_list_remove(&seat_node->destroy.link); wl_list_remove(&seat_node->link); + + /* + * This is the only time we remove items from the focus stack without + * immediately re-adding them. If we just removed the last thing, + * mark that nothing has focus anymore. + */ + if (wl_list_empty(&seat_node->seat->focus_stack)) { + seat_node->seat->has_focus = false; + } + free(seat_node); } @@ -1415,9 +1425,8 @@ struct sway_node *seat_get_focus(struct sway_seat *seat) { if (!seat->has_focus) { return NULL; } - if (wl_list_empty(&seat->focus_stack)) { - return NULL; - } + sway_assert(!wl_list_empty(&seat->focus_stack), + "focus_stack is empty, but has_focus is true"); struct sway_seat_node *current = wl_container_of(seat->focus_stack.next, current, link); return current->node; -- cgit v1.2.3