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

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Date:   Fri, 16 Jun 2017 17:58:59 +0200
From:   Samuel Thibault <samuel.thibault@...-lyon.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        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>,
        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 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