Re: [PATCH 06/17] kthread: cfi: disable callback pointer check with modules

[Christoph Hellwig]

On Thu, Mar 11, 2021 at 04:49:08PM -0800, Sami Tolvanen wrote:
> With CONFIG_CFI_CLANG, a callback function passed to
> __kthread_queue_delayed_work from a module points to a jump table
> entry defined in the module instead of the one used in the core
> kernel, which breaks function address equality in this check:
> 
>   WARN_ON_ONCE(timer->function != kthread_delayed_work_timer_fn);
> 
> Disable the warning when CFI and modules are enabled.
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
> ---
>  kernel/kthread.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index 1578973c5740..af5fee350586 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -963,7 +963,13 @@ static void __kthread_queue_delayed_work(struct kthread_worker *worker,
>  	struct timer_list *timer = &dwork->timer;
>  	struct kthread_work *work = &dwork->work;
>  
> -	WARN_ON_ONCE(timer->function != kthread_delayed_work_timer_fn);
> +	/*
> +	 * With CFI, timer->function can point to a jump table entry in a module,

you keep spewing this comment line that has exactly 81 characters and
thus badly messes up read it with a normal termina everywhere.

Maybe instead of fixing that in ever duplication hide the whole check in
a well documented helper (which would have to be a macro due to the
typing involved).