linux-kernel - Re: [PATCH v4 3/3] powerpc/prom_init: Use -ffreestanding to avoid a reference to bcmp

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Message-ID: <20191022051529.GA44041@ubuntu-m2-xlarge-x86>
Date:   Mon, 21 Oct 2019 22:15:29 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 3/3] powerpc/prom_init: Use -ffreestanding to avoid a
 reference to bcmp

On Fri, Oct 18, 2019 at 03:02:10PM -0500, Segher Boessenkool wrote:
> On Fri, Oct 18, 2019 at 12:00:22PM -0700, Nathan Chancellor wrote:
> > Just as an FYI, there was some more discussion around the availablity
> > and use of bcmp in this LLVM bug which spawned
> > commit 5f074f3e192f ("lib/string.c: implement a basic bcmp").
> > 
> > https://bugs.llvm.org/show_bug.cgi?id=41035#c13
> > 
> > I believe this is the proper solution but I am fine with whatever works,
> > I just want our CI to be green without any out of tree patches again...
> 
> 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?
> 
> 
> Segher

I think Linus summarized it pretty well here:

https://lore.kernel.org/lkml/CAHk-=wi-epJZfBHDbKKDZ64us7WkF=LpUfhvYBmZSteO8Q0RAg@mail.gmail.com/

Cheers,
Nathan