linux-kernel - Re: [clang] stack protector and f1f029c7bf

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Date:   Thu, 24 May 2018 14:12:54 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     hpa@...or.com
Cc:     Alistair Strachan <astrachan@...gle.com>,
        Manoj Gupta <manojgupta@...gle.com>,
        Matthias Kaehlcke <mka@...gle.com>,
        Greg Hackmann <ghackmann@...gle.com>, sedat.dilek@...il.com,
        tstellar@...hat.com, LKML <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...gle.com>
Subject: Re: [clang] stack protector and f1f029c7bf

On Thu, May 24, 2018 at 1:52 PM Nick Desaulniers <ndesaulniers@...gle.com>
wrote:
> On Thu, May 24, 2018 at 1:26 PM Nick Desaulniers <ndesaulniers@...gle.com>
> wrote:
> > On Thu, May 24, 2018 at 11:59 AM <hpa@...or.com> wrote:
> > > Issue 3: Let's face it, reading and writing the flags should be
> builtins,
> > exactly because it has to do stack operations, which really means the
> > compiler should be involved.

> > I'm happy to propose that as a feature request to llvm+gcc.

> Oh, looks like both clang and gcc have:
> __builtin_ia32_readeflags_u64()

> https://godbolt.org/g/SwPjhq

> Maybe native_save_fl() and native_restore_fl() should be replaced in the
> kernel with
> __builtin_ia32_readeflags_u64() and __builtin_ia32_writeeflags_u64()?
> --
> Thanks,
> ~Nick Desaulniers

Looks like those builtins got added to GCC around the 4.9 timeframe:
https://godbolt.org/g/9VS2E9

Problematically, it seems that GCC does not have __has_builtin to do
feature detection:
https://clang.llvm.org/docs/LanguageExtensions.html#feature-checking-macros
https://godbolt.org/g/oku8ux
-- 
Thanks,
~Nick Desaulniers