linux-kernel - Re: [PATCH v2 03/11] tty: kbd: reduce stack size with KASAN

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Date:   Wed, 14 Jun 2017 23:28:34 +0200
From:   Samuel Thibault <samuel.thibault@...-lyon.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        kasan-dev@...glegroups.com, Dmitry Vyukov <dvyukov@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Arend van Spriel <arend.vanspriel@...adcom.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>
Subject: Re: [PATCH v2 03/11] tty: kbd: reduce stack size with KASAN

Hello,

Arnd Bergmann, on mer. 14 juin 2017 23:15:38 +0200, wrote:
> As reported by kernelci, some functions in the VT code use significant
> amounts of kernel stack when local variables get inlined into the caller
> multiple times:
> 
> drivers/tty/vt/keyboard.c: In function 'kbd_keycode':
> drivers/tty/vt/keyboard.c:1452:1: error: the frame size of 2240 bytes is larger than 2048 bytes [-Werror=frame-larger-than=]
> 
> Annotating those functions as noinline_if_stackbloat prevents the inlining
> and reduces the overall stack usage in this driver.


> --- a/drivers/tty/vt/keyboard.c
> +++ b/drivers/tty/vt/keyboard.c
> @@ -301,13 +301,13 @@ int kbd_rate(struct kbd_repeat *rpt)
>  /*
>   * Helper Functions.
>   */
> -static void put_queue(struct vc_data *vc, int ch)
> +static noinline_if_stackbloat void put_queue(struct vc_data *vc, int ch)
>  {
>  	tty_insert_flip_char(&vc->port, ch, 0);
>  	tty_schedule_flip(&vc->port);
>  }

I'm surprised that this be able generate so much stack use: the
tty_insert_flip_char inline only uses a pointer and an int.

And I'm surprised that multiple inlines can accumulate stack usage.

I however agree that it's a bad idea to inline it in functions where
it's called so many times (and we're talking about the keyboard anyway).

> -static void puts_queue(struct vc_data *vc, char *cp)
> +static noinline_if_stackbloat void puts_queue(struct vc_data *vc, char *cp)

I don't see why, it's only called once in the callers. k_fn, however, is
called several times in k_pad, so that could be why, but then it's
rather be the inlining of k_fn which is a bad idea.

> -static void fn_send_intr(struct vc_data *vc)
> +static noinline_if_stackbloat void fn_send_intr(struct vc_data *vc)

This one is only referenced, not called, I don't see how that could pose
problem.

Samuel