linux-kernel - Re: [PATCH 1/5] objtool: Change REG_SP

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Message-ID: <20210203145549.p2alqmtbbo3z75nl@treble>
Date:   Wed, 3 Feb 2021 08:55:49 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Miroslav Benes <mbenes@...e.cz>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Julien Thierry <jthierry@...hat.com>,
        Kees Cook <keescook@...omium.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] objtool: Change REG_SP_INDIRECT

On Wed, Feb 03, 2021 at 03:49:02PM +0100, Peter Zijlstra wrote:
> On Wed, Feb 03, 2021 at 08:42:15AM -0600, Josh Poimboeuf wrote:
> > On Wed, Feb 03, 2021 at 01:02:23PM +0100, Peter Zijlstra wrote:
> > > Currently REG_SP_INDIRECT is unused but means (%rsp + offset),
> > > change it to mean (%rsp) + offset.
> > > 
> > > This is somewhat unfortunate, since REG_BP_INDIRECT is used (by DRAP)
> > > and thus needs to retain the current (%rbp + offset).
> > 
> > Offset is going to be zero, should it not work either way?
> 
> For DRAP? I couldn't tell in a hurry. I'm ever quite clear on how DRAP
> works. Some day, when I figure it out, i'll write a comment.
> 
> Anyway, if it's always 0 for DRAP, then yes, I'll change both.

No, what I meant to say is that UNWIND_HINT_INDIRECT is used with no
arguments, which means that sp_offset is always zero.

-- 
Josh