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

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Date:	Thu, 21 Jul 2016 20:41:40 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
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 02:56:52PM -0700, Andy Lutomirski wrote:
> 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().

Ok.  The whole '&regs->sp' thing threw me for a loop.  I have no idea
what kernel_stack_pointer() is trying to do.  I just assumed it was
correct.  I'll take a look at it and try to fix it in another patch.

-- 
Josh