linux-kernel - Re: [PATCH] x86/boot: Refuse to build with data relocations

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Date:	Mon, 16 May 2016 12:30:51 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Kees Cook <keescook@...omium.org>, Borislav Petkov <bp@...e.de>,
	Baoquan He <bhe@...hat.com>, Yinghai Lu <yinghai@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"x86@...nel.org" <x86@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Andrey Ryabinin <aryabinin@...tuozzo.com>,
	"H.J. Lu" <hjl.tools@...il.com>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/boot: Refuse to build with data relocations


* H. Peter Anvin <hpa@...or.com> wrote:

> On 05/12/16 15:54, Kees Cook wrote:
> >>
> >> It would be far better to warn on the *type* of relocations rather than in which section they feel.
> > 
> > I'm open to specific changes. What's the best way to detect what you want here?
> > 
> 
> Use readelf -r and look for inappropriate relocation types (which are
> basically the same ones that we should have to muck with for the main
> kernel in relocs.c.)

I suspect initially we are good if we don't allow any relocations in 
arch/x86/boot/compressed/vmlinux:

 fomalhaut:~/linux/linux> readelf -r arch/x86/boot/compressed/vmlinux | grep -q  'There are no relocations in this file' ; echo $?
 0

versus a regular object file with lots of relocations:

 fomalhaut:~/linux/linux> readelf -r arch/x86/built-in.o | grep -q 'There are no relocations in this file' ; echo $?
 1

I.e. the relevant portion of Kees's patch would do something like:

quiet_cmd_check_data_rel = DATAREL $@
define cmd_check_data_rel
       for obj in $(filter %.o,$^); do \
               readelf -r $$obj | grep -qF 'There are no relocations in this file' && exit 0 || { \
                       echo "error: $$obj has data relocations!" >&2; \
                       exit 1; \
               } \
       done
endef

(totally untested)

Agreed?

Thanks,

	Ingo