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From:   Linus Torvalds <torvalds@linuxfoundation.org>
Date:   Mon, 6 Sep 2021 15:24:41 -0700
Message-ID: <CAHk-=wiKy45R2vMHhjr16_Q+iYea-70byuj=mHLvp1GURqdYPA@mail.gmail.com>
Subject: Re: [GIT PULL v2] Kbuild updates for v5.15-rc1
To:     Jakub Jelinek <jakub@redhat.com>
Cc:     Segher Boessenkool <segher@kernel.crashing.org>,
        Florian Weimer <fweimer@redhat.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        llvm@lists.linux.dev, linux-toolchains@vger.kernel.org
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On Mon, Sep 6, 2021 at 2:52 PM Jakub Jelinek <jakub@redhat.com> wrote:
>
> It is actually not that bad, stdlib.h is indeed included there because of 2
> intrinsics out of more than 5000 and when one doesn't need those, just
> #define _MM_MALLOC_H_INCLUDED

.. and on clang?

In other words, your suggestion is unworkable, and actively works
against the whole point of "use standard headers". It's a joke. And
not in a good way.

There, I think you have to undefine __STDC_HOSTED__. Maybe by using
-ffreestanding?

Except if you use -ffreestanding, you lose some very bvasic
functionality (*), so we don't do that except for very special code
(our 16-bit realmode code does it, for example, and some very
low-level library implementations).

Just face the facts. Those header files weren't designed for the
kernel, and we've done all the special x86 FPU stuff using inline
asms.

Which is fine. But it all just proves your arguments about "you guys
have to use our header files" to be just a pipe dream, and not true.

The compiler header files may in some very limited situations be
usable. But in the general case? No.

And they should very much *not*at*all* be considered some kind of
sacred "this is the only way to do things". Because that is clearly
not true, and has *never* been true.

The usable header files are the odd special case, not the general case.

Really.

Is it really so hard to just admit that the kernel shouldn't use those
headers? When we have 30 years of experience in doing exactly that?

                  Linus

(*) iirc, with -ffreestanding gcc doesn't do all the basic memcpy()
optimizations. But I forget the exact details.