linux-kernel - Re: [PATCH 03/19] x86/dumpstack: remove unnecessary stack pointer arguments

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Date:	Thu, 21 Jul 2016 14:56:52 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	"H . Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Brian Gerst <brgerst@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Byungchul Park <byungchul.park@....com>
Subject: Re: [PATCH 03/19] x86/dumpstack: remove unnecessary stack pointer arguments

On Thu, Jul 21, 2016 at 2:21 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> When calling show_stack_log_lvl() or dump_trace() with a regs argument,
> providing a stack pointer or frame pointer is redundant.
>

> diff --git a/arch/x86/kernel/dumpstack_32.c b/arch/x86/kernel/dumpstack_32.c
> index 358fe1c..c533b8b 100644
> --- a/arch/x86/kernel/dumpstack_32.c
> +++ b/arch/x86/kernel/dumpstack_32.c
> @@ -122,7 +122,7 @@ void show_regs(struct pt_regs *regs)
>                 u8 *ip;
>
>                 pr_emerg("Stack:\n");
> -               show_stack_log_lvl(NULL, regs, &regs->sp, 0, KERN_EMERG);
> +               show_stack_log_lvl(NULL, regs, NULL, 0, KERN_EMERG);

This is weird -- note the &.  You're at some risk of exposing a bug in
x86_32's kernel_stack_pointer() function, which is a mess.  (I don't
see why it's written the way it is -- the actual return stack pointer
given a pt_regs is quite well defined -- if regs->cs & 3 != 0, then
it's regs->sp, else it's &regs->sp.)

That being said, this isn't a big deal, so:

Reviewed-by: Andy Lutomirski <luto@...nel.org>

If you want to make this all a bit more reliably on x86_32, you could
fix kernel_stack_pointer().