linux-kernel - Re: [PATCH v2] compiler.h: give up __compiletime_assert

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Message-ID: <20180926184259.GB14797@kroah.com>
Date:   Wed, 26 Sep 2018 20:42:59 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Matthias Kaehlcke <mka@...omium.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Christopher Li <sparse@...isli.org>,
        Sparse Mailing-list <linux-sparse@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Daniel Santos <daniel.santos@...ox.com>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Jani Nikula <jani.nikula@...el.com>
Subject: Re: [PATCH v2] compiler.h: give up __compiletime_assert_fallback()

On Wed, Sep 26, 2018 at 11:26:46AM -0700, Kees Cook wrote:
> On Wed, Sep 26, 2018 at 11:03 AM, Nick Desaulniers
> <ndesaulniers@...gle.com> wrote:
> > On Wed, Sep 26, 2018 at 11:00 AM Matthias Kaehlcke <mka@...omium.org> wrote:
> >>
> >> On Fri, Aug 31, 2018 at 09:46:02AM -0700, Nick Desaulniers wrote:
> >> > On Tue, Aug 28, 2018 at 4:00 PM Nick Desaulniers
> >> > <ndesaulniers@...gle.com> wrote:
> >> > >
> >> > > On Mon, Aug 27, 2018 at 1:42 PM Daniel Santos <daniel.santos@...ox.com> wrote:
> >> > > >
> >> > > > Hello Nick,
> >> > > >
> >> > > > On 08/27/2018 03:09 PM, Nick Desaulniers wrote:
> >> > > > >>> Let's give up __compiletime_assert_fallback().  This commit does not
> >> > > > >>> change the current behavior since it just rips off the useless code.
> >> > > > >> Clang is not the only target audience of
> >> > > > >> __compiletime_assert_fallback().  Instead of ripping out something that
> >> > > > >> may benefit builds with gcc 4.2 and earlier, why not override its
> >> > > > > Note that with commit cafa0010cd51 ("Raise the minimum required gcc
> >> > > > > version to 4.6") that gcc < 4.6 is irrelevant.
> >> > > >
> >> > > > Ah, I guess I'm not keeping up, that's wonderful news!  Considering that
> >> > > > I guess I would be OK with its removal, but I still think it would be
> >> > > > better if a similar mechanism to break the Clang build could be found.
> >> > >
> >> > > I'm consulting with our best language lawyers to see what combinations
> >> > > of _Static_assert and __builtin_constant_p would do the trick.
> >> >
> >> > Linus,
> >> > Can this patch be merged in the meantime?
> >>
> >> friendly ping :)
> >>
> >> With c5c2b11894f4 ("drm/i915: Warn against variable length arrays")
> >> clang raises plenty of vla warnings about
> >> __compiletime_error_fallback() in the i915 driver. Would be great to
> >> get rid of those without having to revert that commit.
> >
> > I've been meaning to follow up on this, thanks Matthias.  I too would
> > really like this patch.
> 
> Adding Greg to the thread. Between Masahiro's detailed commit log and
> the Clang-familiar reviewers, I think this should land for 4.19 (as
> part of the other Clang-sanity patches that are already in 4.19). This
> has no impact on gcc now that we're requiring 4.6+.
> 
> https://lore.kernel.org/patchwork/patch/977668/

I'm not digging up a compiler.h patch from a web site and adding it to
the tree this late in the release cycle.  Especially given that it
hasn't had any testing anywhere...

nice try though :)

greg k-h