linux-kernel - Re: [kernel-hardening] Re: Linker segfault on powerpc when CONFIG_LKDTM=y (was Re: [kernel-hardening] [PATCH 3/5] lkdtm: add function for testing .rodata section)

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Date:	Wed, 03 Aug 2016 14:46:49 +1000
From:	Michael Ellerman <mpe@...erman.id.au>
To:	Kees Cook <keescook@...omium.org>
Cc:	"kernel-hardening\@lists.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Arnd Bergmann <arnd@...db.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"David S. Miller" <davem@...emloft.net>,
	Mauro Carvalho Chehab <mchehab@....samsung.com>,
	Jiri Slaby <jslaby@...e.cz>,
	Guenter Roeck <linux@...ck-us.net>,
	LKML <linux-kernel@...r.kernel.org>,
	"linuxppc-dev\@lists.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	Anton Blanchard <anton@...ba.org>,
	Alan Modra <amodra@...il.com>
Subject: Re: [kernel-hardening] Re: Linker segfault on powerpc when CONFIG_LKDTM=y (was Re: [kernel-hardening] [PATCH 3/5] lkdtm: add function for testing .rodata section)

Kees Cook <keescook@...omium.org> writes:
> On Mon, Aug 1, 2016 at 8:12 PM, Michael Ellerman <mpe@...erman.id.au> wrote:
>> Kees Cook <keescook@...omium.org> writes:
>>> On Mon, Aug 1, 2016 at 5:37 AM, Michael Ellerman <mpe@...erman.id.au> wrote:
>>>>
>>>>   scripts/link-vmlinux.sh: line 52: 36260 Segmentation fault      (core dumped) ${LD} ${LDFLAGS} ${LDFLAGS_vmlinux} -o ${2} -T ${lds} ${KBUILD_VMLINUX_INIT} --start-group ${KBUILD_VMLINUX_MAIN} --end-group ${1}
>>>>
>>>> Haven't had a chance to debug it further.
...
>> Interestingly I *can't* reproduce with the Ubuntu x86->ppc cross
>> (5.4.0-6ubuntu1~16.04.1).
>
> Oh, weird. Well, that does explains my lack of hitting the problem,
> though: that's the cross compiler I was using. :P

Actually that was a false negative.

The trick is you have to have LKDTM=y *and* FUNCTION_TRACER=y.

It is a linker bug:

  https://sourceware.org/bugzilla/show_bug.cgi?id=20428

Which Alan has already fixed.

But we need to workaround existing linkers that are out there.

We can do that by marking lkdtm_rodata_do_nothing() notrace, which I
think makes sense for all arches actually.

So I'll send you a patch to do that.

cheers