linux-kernel - Re: [PATCH v2 28/39] x86/ibt,xen: Sprinkle the ENDBR

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Date:   Fri, 25 Feb 2022 14:16:39 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     x86@...nel.org, joao@...rdrivepizza.com, hjl.tools@...il.com,
        andrew.cooper3@...rix.com, linux-kernel@...r.kernel.org,
        ndesaulniers@...gle.com, keescook@...omium.org,
        samitolvanen@...gle.com, mark.rutland@....com,
        alyssa.milburn@...el.com, mbenes@...e.cz, rostedt@...dmis.org,
        mhiramat@...nel.org, alexei.starovoitov@...il.com
Subject: Re: [PATCH v2 28/39] x86/ibt,xen: Sprinkle the ENDBR

On Thu, Feb 24, 2022 at 04:54:40PM -0800, Josh Poimboeuf wrote:
> On Thu, Feb 24, 2022 at 03:52:06PM +0100, Peter Zijlstra wrote:
> > +++ b/arch/x86/xen/xen-head.S
> > @@ -25,8 +25,11 @@
> >  SYM_CODE_START(hypercall_page)
> >  	.rept (PAGE_SIZE / 32)
> >  		UNWIND_HINT_FUNC
> > -		.skip 31, 0x90
> > -		RET
> > +		ANNOTATE_NOENDBR
> > +		/*
> > +		 * Xen will write the hypercall page, and sort out ENDBR.
> > +		 */
> > +		.skip 32, 0xcc
> 
> I seem to remember this UNWIND_HINT_FUNC was only there to silence
> warnings because of the ret.  With the ret gone, maybe the hint can be
> dropped as well.

vmlinux.o: warning: objtool: xen_hypercall_iret()+0x0: stack state mismatch: cfa1=4+8 cfa2=-1+0

and back it goes ;-)