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

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Date:   Mon, 2 Mar 2020 09:52:40 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     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>
Subject: Re: x86 entry perf unwinding failure (missing IRET_REGS annotation
 on stack switch?)

On Mon, Mar 02, 2020 at 09:18:29AM -0600, Josh Poimboeuf wrote:
> > So I think on machines without X86_FEATURE_SMAP, trying to unwind from
> > the two NOPs at f41 and f42 will cause the unwinder to report an
> > error? Looking at unwind_next_frame(), "sp:(und)" without the "end:1"
> > marker seems to be reserved for errors.

I think we can blame this one on Peter ;-)

  764eef4b109a ("objtool: Rewrite alt->skip_orig")

With X86_FEATURE_SMAP, alt->skip_orig gets set, which tells objtool to
skip validation of the NOPs.  That has the side effect of not
propagating the ORC state to the NOPs as well.

-- 
Josh