linux-kernel - Re: [PATCH v15 13/25] x86/reboot: Add ljmp instructions to stacktool whitelist

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Date:	Tue, 12 Jan 2016 17:47:11 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
	Michal Marek <mmarek@...e.cz>,
	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Pedro Alves <palves@...hat.com>,
	Namhyung Kim <namhyung@...il.com>,
	Bernd Petrovitsch <bernd@...rovitsch.priv.at>,
	Chris J Arges <chris.j.arges@...onical.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jiri Slaby <jslaby@...e.cz>,
	Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: [PATCH v15 13/25] x86/reboot: Add ljmp instructions to stacktool
 whitelist

On Fri, Dec 18, 2015 at 06:39:27AM -0600, Josh Poimboeuf wrote:
> stacktool reports a false positive warning for the ljmp instruction in
> machine_real_restart().  Normally, ljmp isn't allowed in a function, but
> this is a special case where it's jumping into real mode.
> 
> Add the jumps to a whitelist which tells stacktool to ignore them.
> 
> Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> ---
>  arch/x86/kernel/reboot.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/reboot.c b/arch/x86/kernel/reboot.c
> index 02693dd..1ea1c5e 100644
> --- a/arch/x86/kernel/reboot.c
> +++ b/arch/x86/kernel/reboot.c
> @@ -9,6 +9,7 @@
>  #include <linux/sched.h>
>  #include <linux/tboot.h>
>  #include <linux/delay.h>
> +#include <linux/stacktool.h>
>  #include <acpi/reboot.h>
>  #include <asm/io.h>
>  #include <asm/apic.h>
> @@ -97,11 +98,13 @@ void __noreturn machine_real_restart(unsigned int type)
>  
>  	/* Jump to the identity-mapped low memory code */
>  #ifdef CONFIG_X86_32
> -	asm volatile("jmpl *%0" : :
> +	asm volatile(STACKTOOL_IGNORE_INSN
> +		     "jmpl *%0;" : :
>  		     "rm" (real_mode_header->machine_real_restart_asm),
>  		     "a" (type));
>  #else
> -	asm volatile("ljmpl *%0" : :
> +	asm volatile(STACKTOOL_IGNORE_INSN
> +		     "ljmpl *%0" : :
>  		     "m" (real_mode_header->machine_real_restart_asm),
>  		     "D" (type));
>  #endif

Well, I can't say that I'm crazy about all those new tools adding
markers to unrelated kernel code.

Can't you teach stacktool to ignore the whole machine_real_restart()
function simply?

-- 
Regards/Gruss,
    Boris.

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