Re: [PATCH v2 06/25] x86/xen: Add ANNOTATE_ENDBR to startup_xen()

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

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@xxxxxxxxxx>
> > Date: Mon, 18 Apr 2022 09:50:25 -0700
> >
> > From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxx>
Date: Mon, 18 Apr 2022 09:50:25 -0700

From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Link: 
https://lkml.kernel.org/r/a87bd48b06d11ec4b98122a429e71e489b4e48c3.1650300597.git.jpoimboe@xxxxxxxxxx