Date:   Tue, 25 Feb 2020 07:33:27 +0100
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>, corbet@....net,
        masahiroy@...nel.org, Nathan Chancellor <natechancellor@...il.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-kbuild@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Clang-Built-Linux ML <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] Documentation/llvm: add documentation on building w/ Clang/LLVM

On Tue, Feb 25, 2020 at 5:08 AM Kees Cook <keescook@...omium.org> wrote:
>
> On Mon, Feb 24, 2020 at 09:41:28AM -0800, Nick Desaulniers wrote:
> > Added to kbuild documentation. Provides more official info on building
> > kernels with Clang and LLVM than our wiki.
> >
> > Suggested-by: Kees Cook <keescook@...omium.org>
> > Reviewed-by: Nathan Chancellor <natechancellor@...il.com>
> > Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> > ---
> >  Documentation/kbuild/index.rst |  1 +
> >  Documentation/kbuild/llvm.rst  | 80 ++++++++++++++++++++++++++++++++++
> >  2 files changed, 81 insertions(+)
> >  create mode 100644 Documentation/kbuild/llvm.rst
> >
> > diff --git a/Documentation/kbuild/index.rst b/Documentation/kbuild/index.rst
> > index 0f144fad99a6..3882bd5f7728 100644
> > --- a/Documentation/kbuild/index.rst
> > +++ b/Documentation/kbuild/index.rst
> > @@ -19,6 +19,7 @@ Kernel Build System
> >
> >      issues
> >      reproducible-builds
> > +    llvm
> >
> >  .. only::  subproject and html
> >
> > diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
> > new file mode 100644
> > index 000000000000..68ae022aebc0
> > --- /dev/null
> > +++ b/Documentation/kbuild/llvm.rst
> > @@ -0,0 +1,80 @@
> > +==============================
> > +Building Linux with Clang/LLVM
> > +==============================
> > +
> > +This document covers how to build the Linux kernel with Clang and LLVM
> > +utilities.
> > +
> > +About
> > +-----
> > +
> > +The Linux kernel has always traditionally been compiled with GNU toolchains
> > +such as GCC and binutils. On going work has allowed for `Clang
> > +<https://clang.llvm.org/>`_ and `LLVM <https://llvm.org/>`_ utilities to be
> > +used as viable substitutes. Distributions such as `Android
> > +<https://www.android.com/>`_, `ChromeOS
> > +<https://www.chromium.org/chromium-os>`_, and `OpenMandriva
> > +<https://www.openmandriva.org/>`_ use Clang built kernels.  `LLVM is a
> > +collection of toolchain components implemented in terms of C++ objects
> > +<https://www.aosabook.org/en/llvm.html>`_. Clang is a front-end to LLVM that
> > +supports C and the GNU C extensions required by the kernel, and is pronounced
> > +"klang," not "see-lang."
> > +
> > +Clang
> > +-----
> > +
> > +The compiler used can be swapped out via `CC=` command line argument to `make`.
> > +`CC=` should be set when selecting a config and during a build.
> > +
> > +     make CC=clang defconfig
> > +
> > +     make CC=clang
> > +
> > +Cross Compiling
> > +---------------
> > +
> > +A single Clang compiler binary will typically contain all supported backends,
> > +which can help simplify cross compiling.
> > +
> > +     ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make CC=clang
> > +
> > +`CROSS_COMPILE` is not used to suffix the Clang compiler binary, instead
>
> s/suffix/prefix/
>
> > +`CROSS_COMPILE` is used to set a command line flag: `--target <triple>`. For
> > +example:
> > +
> > +     clang --target aarch64-linux-gnu foo.c
> > +
> > +LLVM Utilities
> > +--------------
> > +
> > +LLVM has substitutes for GNU binutils utilities. These can be invoked as
> > +additional parameters to `make`.
> > +
> > +     make CC=clang AS=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip \\
> > +       OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump OBJSIZE=llvm-objsize \\
> > +       READELF=llvm-readelf HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar \\
> > +       HOSTLD=ld.lld
> > +
> > +Getting Help
> > +------------
> > +
> > +- `Website <https://clangbuiltlinux.github.io/>`_
> > +- `Mailing List <https://groups.google.com/forum/#!forum/clang-built-linux>`_: <clang-built-linux@...glegroups.com>
> > +- `Issue Tracker <https://github.com/ClangBuiltLinux/linux/issues>`_
> > +- IRC: #clangbuiltlinux on chat.freenode.net
> > +- `Telegram <https://t.me/ClangBuiltLinux>`_: @ClangBuiltLinux
> > +- `Wiki <https://github.com/ClangBuiltLinux/linux/wiki>`_
> > +- `Beginner Bugs <https://github.com/ClangBuiltLinux/linux/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22>`_
> > +
> > +Getting LLVM
> > +-------------
> > +
> > +- http://releases.llvm.org/download.html
> > +- https://github.com/llvm/llvm-project
> > +- https://llvm.org/docs/GettingStarted.html
> > +- https://llvm.org/docs/CMake.html
> > +- https://apt.llvm.org/
> > +- https://www.archlinux.org/packages/extra/x86_64/llvm/
> > +- https://github.com/ClangBuiltLinux/tc-build
> > +- https://github.com/ClangBuiltLinux/linux/wiki/Building-Clang-from-source
> > +- https://android.googlesource.com/platform/prebuilts/clang/host/linux-x86/
>
> Should this also include an update to Documentation/process/changes.rst
> with the minimum version required? (I would expect this to be "9" for Clang,
> and "11" for ld.lld.)
>
> Otherwise, yes, with Randy and Masahiro's suggestions, please consider it:
>
> Reviewed-by: Kees Cook <keescook@...omium.org>
>

Hi,

that update for documentation purposes was overdue.

My last experiments were with Linux v5.3 and llvm-toolchain 9.0 means
Clang compiler v9.0 and LLD linker v9.0 on x86-64.
With Debian's kernel-config I was able to build OOTB (out-of-the-box)
with no extra patches.
I cannot speak for higher Linux and/or llvm-toolchain versions/combinations.

I would prefer such an information also for the *supported* Linux
versions, so people have a good orientation.

So for the above scenario, you can add:

Reviewed-by: Sedat Dilek <sedat.dilek@...il.com>.(Clang and LLD v9.0,
Linux v5.3, x86-64)

Regards,
- Sedat -




> --
> Kees Cook
>
> --
> You received this message because you are subscribed to the Google Groups "Clang Built Linux" group.
> To unsubscribe from this group and stop receiving emails from it, send an email to clang-built-linux+unsubscribe@...glegroups.com.
> To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/202002242003.870E5F80%40keescook.

Powered by blists - more mailing lists