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

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Date:   Tue, 28 Apr 2020 09:25:51 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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 10:54:13AM -0500, Josh Poimboeuf wrote:
> 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"

Would it be accurate to print

	"unsupported CFI stack change in alternatives code"?

to give the developer something more explicit to plug into their search
engine?