Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 22 Oct 2019 03:57:09 -0500
From:   Segher Boessenkool <segher@kernel.crashing.org>
To:     Nathan Chancellor <natechancellor@gmail.com>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v4 3/3] powerpc/prom_init: Use -ffreestanding to avoid a reference to bcmp
Message-ID: <20191022085709.GI28442@gate.crashing.org>
References: <20190911182049.77853-1-natechancellor@gmail.com> <20191014025101.18567-1-natechancellor@gmail.com> <20191014025101.18567-4-natechancellor@gmail.com> <20191014093501.GE28442@gate.crashing.org> <CAKwvOdmcUT2A9FG0JD9jd0s=gAavRc_h+RLG6O3mBz4P1FfF8w@mail.gmail.com> <20191014191141.GK28442@gate.crashing.org> <20191018190022.GA1292@ubuntu-m2-xlarge-x86> <20191018200210.GR28442@gate.crashing.org> <20191022051529.GA44041@ubuntu-m2-xlarge-x86>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20191022051529.GA44041@ubuntu-m2-xlarge-x86>
User-Agent: Mutt/1.4.2.3i
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon, Oct 21, 2019 at 10:15:29PM -0700, Nathan Chancellor wrote:
> On Fri, Oct 18, 2019 at 03:02:10PM -0500, Segher Boessenkool wrote:
> > I think the proper solution is for the kernel to *do* use -ffreestanding,
> > and then somehow tell the kernel that memcpy etc. are the standard
> > functions.  A freestanding GCC already requires memcpy, memmove, memset,
> > memcmp, and sometimes abort to exist and do the standard thing; why cannot
> > programs then also rely on it to be the standard functions.
> > 
> > What exact functions are the reason the kernel does not use -ffreestanding?
> > Is it just memcpy?  Is more wanted?
> 
> I think Linus summarized it pretty well here:
> 
> https://lore.kernel.org/lkml/CAHk-=wi-epJZfBHDbKKDZ64us7WkF=LpUfhvYBmZSteO8Q0RAg@mail.gmail.com/

GCC recognises __builtin_memcpy (or any other __builtin) just fine even
with -ffreestanding.

So the kernel wants a warning (or error) whenever a call to one of these
library functions is generated by the compiler without the user asking
for it directly (via a __builtin)?  And that is all that is needed for
the kernel to use -ffreestanding?

That shouldn't be hard.  Anything missing here?


Segher