linux-kernel - Re: [PATCH] lkdtm: Mark lkdtm_rodata_do

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Date:	Wed, 3 Aug 2016 11:59:24 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Michael Ellerman <mpe@...erman.id.au>
Cc:	"linuxppc-dev@...abs.org" <linuxppc-dev@...abs.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>,
	Greg Kroah-Hartman <greg@...ah.com>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Anton Blanchard <anton@...ba.org>,
	Alan Modra <amodra@...il.com>, Nick Piggin <npiggin@...il.com>
Subject: Re: [PATCH] lkdtm: Mark lkdtm_rodata_do_nothing() notrace

On Tue, Aug 2, 2016 at 9:59 PM, Michael Ellerman <mpe@...erman.id.au> wrote:
> lkdtm_rodata_do_nothing() is an empty function which is generated in
> order to test the non-executability of rodata.
>
> Currently if function tracing is enabled then an mcount callsite will be
> generated for lkdtm_rodata_do_nothing(), and it will appear in the list
> of available functions for function tracing (available_filter_functions).
>
> Given it's purpose purely as a test function, it seems preferable for
> lkdtm_rodata_do_nothing() to be marked notrace, so it doesn't appear as
> traceable.
>
> This also avoids triggering a linker bug on powerpc:
>
>   https://sourceware.org/bugzilla/show_bug.cgi?id=20428
>
> When the linker sees code that needs to generate a call stub, eg. a
> branch to mcount(), it assumes the section is executable and
> dereferences a NULL pointer leading to a linker segfault. Marking
> lkdtm_rodata_do_nothing() notrace avoids triggering the bug because the
> function contains no other function calls.
>
> Signed-off-by: Michael Ellerman <mpe@...erman.id.au>

Awesome! Thanks for tracking this down. I've applied it to my tree, it
should get picked up by Greg on my next pull request.

-Kees

> ---
>  drivers/misc/lkdtm_rodata.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/misc/lkdtm_rodata.c b/drivers/misc/lkdtm_rodata.c
> index 166b1db3969f..3564477b8c2d 100644
> --- a/drivers/misc/lkdtm_rodata.c
> +++ b/drivers/misc/lkdtm_rodata.c
> @@ -4,7 +4,7 @@
>   */
>  #include "lkdtm.h"
>
> -void lkdtm_rodata_do_nothing(void)
> +void notrace lkdtm_rodata_do_nothing(void)
>  {
>         /* Does nothing. We just want an architecture agnostic "return". */
>  }
> --
> 2.7.4
>



-- 
Kees Cook
Brillo & Chrome OS Security