On Mon, Jul 25, 2022 at 09:49:19AM -0400, Rik van Riel wrote:
> When a KLP fails to apply, klp_reverse_transition will clear the
> TIF_PATCH_PENDING flag on all tasks, except for newly created tasks
> which are not on the task list yet.
This paragraph and $SUBJECT both talk about a reverse transition. Isn't
it also possible to race on a normal (forward) transition?
> Meanwhile, fork will copy over the TIF_PATCH_PENDING flag from the
> parent to the child early on, in dup_task_struct -> setup_thread_stack.
>
> Much later, klp_copy_process will set child->patch_state to match
> that of the parent.
>
> However, the parent's patch_state may have been changed by KLP loading
> or unloading since it was initially copied over into the child.
>
> This results in the KLP code occasionally hitting this warning in
> klp_complete_transition:
>
> for_each_process_thread(g, task) {
> WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING));
> task->patch_state = KLP_UNDEFINED;
> }
>
> This patch will set, or clear, the TIF_PATCH_PENDING flag in the child
> process depending on whether or not it is needed at the time
> klp_copy_process is called, at a point in copy_process where the
> tasklist_lock is held exclusively, preventing races with the KLP
> code.
Use imperative language, i.e. no "This patch". See
Documentation/process/submitting-patches.rst
>
> The KLP code does have a few places where the state is changed
> without the tasklist_lock held, but those should not cause
> problems because klp_update_patch_state(current) cannot be
> called while the current task is in the middle of fork,
> klp_check_and_switch_task() which is called under the pi_lock,
> which prevents rescheduling, and manipulation of the patch
> state of idle tasks, which do not fork.
>
> This should prevent this warning from triggering again in the
> future.
>
Fixes: d83a7cb375ee ("livepatch: change to a per-task consistency model")
> Signed-off-by: Rik van Riel <riel@xxxxxxxxxxx>
> Reported-by: Breno Leitao <leitao@xxxxxxxxxx>
> Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>
With the above minor things fixed:
Acked-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
--
Josh
