linux-kernel - Re: x86 entry perf unwinding failure (missing IRET

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Message-ID: <20200428155413.b5xzef4s2kyzg5ed@treble>
Date:   Tue, 28 Apr 2020 10:54:13 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Jann Horn <jannh@...gle.com>, Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        alexandre.chartre@...cle.com
Subject: Re: x86 entry perf unwinding failure (missing IRET_REGS annotation
 on stack switch?)

On Tue, Apr 28, 2020 at 10:49:09AM -0500, Josh Poimboeuf wrote:
> On Tue, Apr 28, 2020 at 05:25:52PM +0200, Peter Zijlstra wrote:
> > On Tue, Apr 28, 2020 at 09:31:57AM -0500, Josh Poimboeuf wrote:
> > > That's quite the monstrosity, and I still don't see the point.  I
> > > thought we decided to just disallow CFI changes in alternatives anyway?
> > > That can be done much simpler.
> > 
> > Something like so then ?
> > 
> > ---
> > diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> > index 8443ec690051..d14d83e6edb0 100644
> > --- a/tools/objtool/check.c
> > +++ b/tools/objtool/check.c
> > @@ -940,6 +940,7 @@ static int handle_group_alt(struct objtool_file *file,
> >  
> >  		last_new_insn = insn;
> >  
> > +		insn->alt_group = true;
> >  		insn->ignore = orig_insn->ignore_alts;
> >  		insn->func = orig_insn->func;
> >  
> > @@ -2242,6 +2243,11 @@ static int handle_insn_ops(struct instruction *insn, struct insn_state *state)
> >  	list_for_each_entry(op, &insn->stack_ops, list) {
> >  		int res;
> >  
> > +		if (insn->alt_group) {
> > +			WARN_FUNC("alternative has CFI", insn->sec, insn->offset);
> > +			return 1;
> > +		}
> > +
> 
> ACK (separate patch)

BTW, since most people don't know what CFI is, how about something like

	"unsupported stack change in alternatives code"

-- 
Josh