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Date:   Mon, 20 Jul 2020 17:27:44 -0700
From:   Fangrui Song <maskray@google.com>
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Masahiro Yamada <masahiroy@kernel.org>,
        Michal Marek <michal.lkml@markovi.net>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Manoj Gupta <manojgupta@google.com>,
        Jian Cai <jiancai@google.com>, Bill Wendling <morbo@google.com>
Subject: Re: [PATCH] Makefile: Fix GCC_TOOLCHAIN_DIR prefix for Clang cross
 compilation
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On 2020-07-20, Nick Desaulniers wrote:
>On Mon, Jul 20, 2020 at 11:16 AM Nathan Chancellor
><natechancellor@gmail.com> wrote:
>>
>> On Mon, Jul 20, 2020 at 11:12:22AM -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-,
>> > 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 only searches for $(prefix)$needle and for example it will find
>
>"newer Clang" requires the reader to recall that "Clang as of 11" was
>the previous frame of reference. I think it would be clearer to:
>1. call of clang-12 as having a difference in behavior.
>2. explicitly link to the commit, ie:
>Link: https://github.com/llvm/llvm-project/commit/3452a0d8c17f7166f479706b293caf6ac76ffd90
>
>> > /usr/bin/as instead of /usr/bin/aarch64-linux-gnu-as.
>
>That's a common source of pain (for example, when cross compiling
>without having the proper cross binutils installed, it's common to get
>spooky errors about unsupported configs or host binutils not
>recognizing flags specific to cross building).
>
>/usr/bin/as: unrecognized option '-EL'
>
>being the most common.  So in that case, I'm actually very happy with
>the llvm change if it solves that particularly common pain point.
>
>> >
>> > 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).
>> >
>> > Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
>> > Signed-off-by: Fangrui Song <maskray@google.com>
>> > Link: https://github.com/ClangBuiltLinux/linux/issues/1099
>>
>> Sorry that I did not pay attention before but this needs
>>
>> Cc: stable@vger.kernel.org
>
>Agreed.  This change to llvm will blow up all of our CI jobs that
>cross compile if not backported to stable.

Thanks. I did not know this.

>>
>> in the body so that it gets automatically backported into all of our
>> stable branches. I am not sure if Masahiro is okay with adding that
>> after the fact or if he will want a v2.
>>
>> I am fine with having my signed-off-by on the patch but I did not really
>> do much :) I am fine with having that downgraded to
>
>Not a big deal, but there's only really two cases I can think of where
>it's appropriate to attach someone else's "SOB" to a patch:
>1. It's their patch that you're resending on their behalf, possibly as
>part of a larger series.
>2. You're a maintainer, and...well I guess that's also case 1 above.
>
>Reported-by is more appropriate, and you can include the tags
>collected from this thread.  Please ping me internally for help
>sending a v2, if needed.

Nathan's draft patch on
https://github.com/ClangBuiltLinux/linux/issues/1099 actually works.
I removed the slash. The words are my own. Since Nathan explicitly
requested a downgrade of his tag, I'll do that for V2.

I'll do that anyway because I need to fix a typo in the description:

$(CROSS_COMPILE)elfedit is found at /usr/bin/aarch64-linux-gnu-
$(CROSS_COMPILE)elfedit is found at /usr/bin/aarch64-linux-gnu-elfedit

>>
>> Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
>> Tested-by: Nathan Chancellor <natechancellor@gmail.com>
>
>I tested with this llvm pre- and post-
>https://github.com/llvm/llvm-project/commit/3452a0d8c17f7166f479706b293caf6ac76ffd90.
>I also tested downstream Android kernel builds with
>3452a0d8c17f7166f479706b293caf6ac76ffd90. Builds that don't make use
>of CROSS_COMPILE (native host targets) are obviously unaffected.  We
>might see this issue pop up a few more times internally if the patch
>isn't picked up by stable, or if those downstream kernel trees don't
>rebase on stable kernel trees as often as they refresh their
>toolchain.
>
>Tested-by: Nick Desaulniers <ndesaulniers@google.com>

Thanks for offerring proofreading service! I'm working on the
description...

>>
>> if people find it odd.
>>
>> Thanks for sending this along!
>>
>> Cheers,
>> Nathan
>>
>> > ---
>> >  Makefile | 2 +-
>> >  1 file changed, 1 insertion(+), 1 deletion(-)
>> >
>> > diff --git a/Makefile b/Makefile
>> > index 0b5f8538bde5..3ac83e375b61 100644
>> > --- a/Makefile
>> > +++ b/Makefile
>> > @@ -567,7 +567,7 @@ ifneq ($(shell $(CC) --version 2>&1 | head -n 1 | grep clang),)
>> >  ifneq ($(CROSS_COMPILE),)
>> >  CLANG_FLAGS  += --target=$(notdir $(CROSS_COMPILE:%-=%))
>> >  GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE)elfedit))
>> > -CLANG_FLAGS  += --prefix=$(GCC_TOOLCHAIN_DIR)
>> > +CLANG_FLAGS  += --prefix=$(GCC_TOOLCHAIN_DIR)$(CROSS_COMPILE)
>> >  GCC_TOOLCHAIN        := $(realpath $(GCC_TOOLCHAIN_DIR)/..)
>> >  endif
>> >  ifneq ($(GCC_TOOLCHAIN),)
>> > --
>> > 2.28.0.rc0.105.gf9edc3c819-goog
>> >
>>
>> --
>
>-- 
>Thanks,
>~Nick Desaulniers