linux-kernel - Re: [PATCH v2] Makefile: Fix GCC_TOOLCHAIN_DIR prefix for Clang cross compilation

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Message-ID: <20200721104035.GC1676612@kroah.com>
Date:   Tue, 21 Jul 2020 12:40:35 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Fangrui Song <maskray@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com, stable@...r.kernel.org,
        Jian Cai <jiancai@...gle.com>,
        Bill Wendling <morbo@...gle.com>,
        Manoj Gupta <manojgupta@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH v2] Makefile: Fix GCC_TOOLCHAIN_DIR prefix for Clang
 cross compilation

On Mon, Jul 20, 2020 at 09:19:38PM -0700, Fangrui Song wrote:
> When CROSS_COMPILE is set (e.g. aarch64-linux-gnu-), if
> $(CROSS_COMPILE)elfedit is found at /usr/bin/aarch64-linux-gnu-elfedit,
> GCC_TOOLCHAIN_DIR will be set to /usr/bin/.  --prefix= will be set to
> /usr/bin/ and Clang as of 11 will search for both
> $(prefix)aarch64-linux-gnu-$needle and $(prefix)$needle.
> 
> GCC searchs for $(prefix)aarch64-linux-gnu/$version/$needle,
> $(prefix)aarch64-linux-gnu/$needle and $(prefix)$needle. In practice,
> $(prefix)aarch64-linux-gnu/$needle rarely contains executables.
> 
> To better model how GCC's -B/--prefix takes in effect in practice, newer
> Clang (since
> https://github.com/llvm/llvm-project/commit/3452a0d8c17f7166f479706b293caf6ac76ffd90)
> only searches for $(prefix)$needle. Currently it will find /usr/bin/as
> instead of /usr/bin/aarch64-linux-gnu-as.
> 
> Set --prefix= to $(GCC_TOOLCHAIN_DIR)$(CROSS_COMPILE)
> (/usr/bin/aarch64-linux-gnu-) so that newer Clang can find the
> appropriate cross compiling GNU as (when -no-integrated-as is in
> effect).
> 
> Reported-by: Nathan Chancellor <natechancellor@...il.com>
> Signed-off-by: Fangrui Song <maskray@...gle.com>
> Reviewed-by: Nathan Chancellor <natechancellor@...il.com>
> Tested-by: Nathan Chancellor <natechancellor@...il.com>
> Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1099
> ---
> Changes in v2:
> * Updated description to add tags and the llvm-project commit link.
> * Fixed a typo.


<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read:
    https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
for how to do this properly.

</formletter>