linux-kernel - Re: [PATCH v3 16/51] x86/32: put real return address on stack in entry code

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Message-ID: <20160815182522.6vjn6aheuiw65oca@treble>
Date:	Mon, 15 Aug 2016 13:25:22 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>, 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>,
	Nilay Vaish <nilayvaish@...il.com>
Subject: Re: [PATCH v3 16/51] x86/32: put real return address on stack in
 entry code

On Mon, Aug 15, 2016 at 11:04:42AM -0700, H. Peter Anvin wrote:
> On 08/15/16 08:09, Josh Poimboeuf wrote:
> > On Sun, Aug 14, 2016 at 12:31:47AM -0700, Andy Lutomirski wrote:
> >> On Fri, Aug 12, 2016 at 7:28 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> >>> This standardizes the stacks of idle tasks to be consistent with other
> >>> tasks on 32-bit.
> >>
> >> It might be nice to stick a ud2 or 1: hlt; jmp 1b or similar
> >> afterwards to make it clear that initial_code can't return.
> > 
> > Yeah, I'll do something like that.
> > 
> 
> "Standardizing the stack" how?  A zero on the stack terminates the stack
> trace.

Instead of zero, user tasks have a real return address at that spot.
This makes idle tasks consistent with that, so we have a well defined
"end of stack".  Also it makes the stack trace more useful since it
shows what entry code was involved in calling into C.

-- 
Josh