From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1751955AbdFPP7M (ORCPT <rfc822;w@1wt.eu>);
        Fri, 16 Jun 2017 11:59:12 -0400
Received: from hera.aquilenet.fr ([141.255.128.1]:42888 "EHLO
        hera.aquilenet.fr" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1750750AbdFPP7J (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Fri, 16 Jun 2017 11:59:09 -0400
Date: Fri, 16 Jun 2017 17:58:59 +0200
From: Samuel Thibault <samuel.thibault@ens-lyon.org>
To: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        kasan-dev <kasan-dev@googlegroups.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Alexander Potapenko <glider@google.com>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Networking <netdev@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Arend van Spriel <arend.vanspriel@broadcom.com>,
        Jiri Slaby <jslaby@suse.com>,
        Dmitry Torokhov <dmitry.torokhov@gmail.com>
Subject: Re: [PATCH v2 03/11] tty: kbd: reduce stack size with KASAN
Message-ID: <20170616155859.gn2einuermlncaku@var.youpi.perso.aquilenet.fr>
Mail-Followup-To: Samuel Thibault <samuel.thibault@ens-lyon.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        kasan-dev <kasan-dev@googlegroups.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Alexander Potapenko <glider@google.com>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Networking <netdev@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Arend van Spriel <arend.vanspriel@broadcom.com>,
        Jiri Slaby <jslaby@suse.com>,
        Dmitry Torokhov <dmitry.torokhov@gmail.com>
References: <20170614211556.2062728-1-arnd@arndb.de>
 <20170614211556.2062728-4-arnd@arndb.de>
 <20170615045221.GA26687@kroah.com>
 <20170615045347.GA26913@kroah.com>
 <CAK8P3a32abgm7H7s=dOTkLus3K9o79hRJ31s0eyEVZW30E9p7g@mail.gmail.com>
 <20170616130215.GC31057@kroah.com>
 <CAK8P3a2upDpcxGCLomyjvsXHfyeTyPmSj9qvymSsgf4pE+spDg@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAK8P3a2upDpcxGCLomyjvsXHfyeTyPmSj9qvymSsgf4pE+spDg@mail.gmail.com>
Organization: I am not organized
User-Agent: NeoMutt/20170113 (1.7.2)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

Arnd Bergmann, on ven. 16 juin 2017 17:41:47 +0200, wrote:
> The problem are the 'ch' and 'flag' variables that are passed into
> tty_insert_flip_char by value, and from there into
> tty_insert_flip_string_flags by reference.  In this case, kasan tries
> to detect whether tty_insert_flip_string_flags() does any out-of-bounds
> access on the pointers and adds 64 bytes redzone around each of
> the two variables.

Ouch.

> gcc-6.3.1 happens to inline 16 calls of tty_insert_flip_char() into
> kbd_keycode(), so the stack size grows from 168 bytes to
> 168+(16*2*64) = 2216 bytes. There are 10 calls to put_queue()
> in to_utf8(), 12 in emulate_raw() and another 4 in kbd_keycode()
> itself.

That's why I agreed for put_queue :)

I'm however afraid we'd have to mark a lot of static functions that way,
depending on the aggressivity of gcc... I'd indeed really argue that gcc
should consider stack usage when inlining.

static int f(int foo) {
	char c[256];
	g(c, foo);
}

is really not something that I'd want to see the compiler to inline.

> > And no, we shouldn't need to do this.  It sounds like ksan is the
> > problem here...
> 
> Of course kasan is the problem, but it really just does whatever we
> asked it to do, and cannot do any better as long as we inline many
> copies of tty_insert_flip_char() into kbd_keycode().

We didn't ask to inline put_queue into kbd_keycode.

Samuel