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

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Message-ID: <20170614221644.sws7xgm5cgiia6oo@var.youpi.perso.aquilenet.fr>
Date:   Thu, 15 Jun 2017 00:16:44 +0200
From:   Samuel Thibault <samuel.thibault@...-lyon.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Networking <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <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

Arnd Bergmann, on mer. 14 juin 2017 23:56:39 +0200, wrote:
> > 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.
> 
> It's called by applkey, which in turn is called by k_pad(),

k_pad calls applkey twice only. Is that really to be considered bloat?

> >> -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.
> 
> I was surprised by this as well, but it seems that gcc these days is
> smart enough to turn the indirect function calls for k_handler[type]
> and/or f_handler[value] into inlines again when it has already
> determined the index to be constant.

Cool :) But I don't see how it can see find it out constant. The only
fn_handler[] caller is k_spec, using value as index. The only caller of
f_handler[] is kbd_keycode, using type as index, and keysym&0xff as
value.  That is definitely not constant :)  And it's only one caller, I
don't see how that can bloat.

Samuel