linux-kernel - Re: [tip:x86/pti] x86/retpoline: Fill return stack buffer on vmexit

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Date:   Thu, 25 Jan 2018 16:03:18 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Borislav Petkov <bp@...en8.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     tim.c.chen@...ux.intel.com, pjt@...gle.com, jikos@...nel.org,
        gregkh@...ux-foundation.org, dave.hansen@...el.com,
        mingo@...nel.org, riel@...hat.com, luto@...capital.net,
        torvalds@...ux-foundation.org, ak@...ux.intel.com,
        keescook@...gle.com, peterz@...radead.org, tglx@...utronix.de,
        hpa@...or.com, linux-kernel@...r.kernel.org,
        linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/pti] x86/retpoline: Fill return stack buffer on vmexit

On Thu, 2018-01-25 at 16:51 +0100, Borislav Petkov wrote:
> 
> > And the seg fault is objtool's way of telling you you need a
> > ANNOTATE_NOSPEC_ALTERNATIVE above the alternative ;-)
> 
> Except that it blew up when I did this which doesn't have ALTERNATIVE
> (it's the diff I saved :-))

Yeah, ANNOTATE_NOSPEC_ALTERNATIVE just tells objtool "don't look at the
alternative; you're not going to like it".

If you start putting a __fill_rsb() function out of line somewhere and
only *calling* it from alternatives, then objtool is going to shit
itself when it sees that function, regardless.
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