linux-kernel - Re: [PATCH] x86/mm: annotate no_context with UNWIND

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Message-ID: <20181015162022.jia673dbkruo7k6f@treble>
Date:   Mon, 15 Oct 2018 11:20:22 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     natechancellor@...il.com, Andrew Lutomirski <luto@...nel.org>,
        nick.desaulniers@...il.com,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/mm: annotate no_context with UNWIND_HINTS

On Mon, Oct 15, 2018 at 09:03:40AM -0700, Andy Lutomirski wrote:
> That being said, the generic macro is:
> 
> # define unreachable() do { annotate_reachable(); do { } while (1); } while (0)
> 
> I'm probably missing some subtlety here, but shouldn't that be
> annotate_*un*reachable()?

That code should have had a comment, but that subtlety was intentional.
As I mentioned earlier, that was a hack for old versions of GCC which
didn't have __builtin_unreachable().  In those cases, GCC doesn't treat
"ud2" as fatal, so this was a way of telling objtool that.  Luckily we
can get rid of this hack now that the minimum supported GCC version has
gone up to 4.6.

-- 
Josh