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Message-ID: <1516899639.30244.145.camel@infradead.org>
Subject: Re: [tip:x86/pti] x86/retpoline: Fill return stack buffer on vmexit
From:   David Woodhouse <dwmw2@infradead.org>
To:     Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     Borislav Petkov <bp@alien8.de>, tim.c.chen@linux.intel.com,
        pjt@google.com, jikos@kernel.org, gregkh@linux-foundation.org,
        dave.hansen@intel.com, 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
In-Reply-To: <20180125165646.ytc4upthpaqtwi45@treble>
References: <1515755487-8524-1-git-send-email-dwmw@amazon.co.uk>
         <tip-117cc7a908c83697b0b737d15ae1eb5943afe35b@git.kernel.org>
         <20180125120743.ey32gvl5mjam4r2s@pd.tnic>
         <1516882849.30244.94.camel@infradead.org>
         <20180125124554.vdx7rrnfrxrzl2ng@pd.tnic>
         <20180125151024.bidjr26r667vs7h5@treble>
         <20180125155110.mw655b7fwgm7qqc7@pd.tnic>
         <1516896198.30244.133.camel@infradead.org>
         <20180125165646.ytc4upthpaqtwi45@treble>
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Date:   Thu, 25 Jan 2018 17:00:39 +0000
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On Thu, 2018-01-25 at 10:56 -0600, Josh Poimboeuf wrote:
> On Thu, Jan 25, 2018 at 04:03:18PM +0000, David Woodhouse wrote:
> > On Thu, 2018-01-25 at 16:51 +0100, Borislav Petkov wrote:
> > >=C2=A0
> > > > And the seg fault is objtool's way of telling you you need a
> > > > ANNOTATE_NOSPEC_ALTERNATIVE above the alternative ;-)
> > >=C2=A0
> > > Except that it blew up when I did this which doesn't have ALTERNATIVE
> > > (it's the diff I saved :-))
> >=C2=A0
> > Yeah, ANNOTATE_NOSPEC_ALTERNATIVE just tells objtool "don't look at the
> > alternative; you're not going to like it".
> >=C2=A0
> > 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.
>=20
> Right, if you *really* want it always inline, the short term solution is
> to just patch it in with X86_FEATURE_ALWAYS.

And the whole problem here is that patching it in with alternatives is
painful on kernels < 4.1 because back then, we didn't cope with
oldinstr and altinstr being different lengths.

And they don't want to fix *that* because kABI...

I just stopped caring.
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