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Date:   Tue, 19 Apr 2022 15:10:29 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andrew Cooper <amc96@srcf.net>
Cc:     Andrew Cooper <Andrew.Cooper3@citrix.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        "x86@kernel.org" <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Miroslav Benes <mbenes@suse.cz>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <jgross@suse.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Subject: Re: [PATCH v2 06/25] x86/xen: Add ANNOTATE_ENDBR to startup_xen()
Message-ID: <20220419131029.GV2731@worktop.programming.kicks-ass.net>
References: <cover.1650300597.git.jpoimboe@redhat.com>
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On Tue, Apr 19, 2022 at 01:12:14PM +0100, Andrew Cooper wrote:

> > Subject: x86/xen: Add ANNOTATE_ENDBR to startup_xen()
> > From: Josh Poimboeuf <jpoimboe@redhat.com>
> > Date: Mon, 18 Apr 2022 09:50:25 -0700
> >
> > From: Josh Poimboeuf <jpoimboe@redhat.com>
> >
> > The startup_xen() kernel entry point is referenced by the ".note.Xen"
> > section, and is the real entry point of the VM. It *will* be
> > indirectly branched to, *however* currently Xen doesn't support PV VM
> > with CET active.
> 
> Technically it's always IRET'd to, but the point is that it's never
> "branched to" by the execution context of the VM.
> 
> So it would be better to say that it's never indirectly branched to.?
> That's what the IBT checks care about.

Right, so I was thinking the IRET could set the NEED_ENDBR bit, but
yeah, that might be stretching the definition of an indirect-branch a
wee bit.

How about so then?

---
Subject: x86/xen: Add ANNOTATE_NOENDBR to startup_xen()
From: Josh Poimboeuf <jpoimboe@redhat.com>
Date: Mon, 18 Apr 2022 09:50:25 -0700

From: Josh Poimboeuf <jpoimboe@redhat.com>

The startup_xen() kernel entry point is referenced by the ".note.Xen"
section, and is the real entry point of the VM. Control transfer is
through IRET, which *could* set NEED_ENDBR, however Xen currently does
no such thing.

Add ANNOTATE_NOENDBR to silence future objtool warnings.

Fixes: ed53a0d97192 ("x86/alternative: Use .ibt_endbr_seal to seal indirect calls")
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Link: https://lkml.kernel.org/r/a87bd48b06d11ec4b98122a429e71e489b4e48c3.1650300597.git.jpoimboe@redhat.com