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Message-ID: <1516896198.30244.133.camel@infradead.org>
Subject: Re: [tip:x86/pti] x86/retpoline: Fill return stack buffer on vmexit
From:   David Woodhouse <dwmw2@infradead.org>
To:     Borislav Petkov <bp@alien8.de>,
        Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     tim.c.chen@linux.intel.com, pjt@google.com, jikos@kernel.org,
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        mingo@kernel.org, riel@redhat.com, luto@amacapital.net,
        torvalds@linux-foundation.org, ak@linux.intel.com,
        keescook@google.com, peterz@infradead.org, tglx@linutronix.de,
        hpa@zytor.com, linux-kernel@vger.kernel.org,
        linux-tip-commits@vger.kernel.org
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         <20180125120743.ey32gvl5mjam4r2s@pd.tnic>
         <1516882849.30244.94.camel@infradead.org>
         <20180125124554.vdx7rrnfrxrzl2ng@pd.tnic>
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         <20180125155110.mw655b7fwgm7qqc7@pd.tnic>
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Date:   Thu, 25 Jan 2018 16:03:18 +0000
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On Thu, 2018-01-25 at 16:51 +0100, Borislav Petkov wrote:
>=20
> > And the seg fault is objtool's way of telling you you need a
> > ANNOTATE_NOSPEC_ALTERNATIVE above the alternative ;-)
>=20
> 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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