linux-kernel - Re: [PATCH bpf-next] selftests/bpf: Fix cross compilation with CLANG_CROSS

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Date:   Fri, 17 Feb 2023 16:27:36 +0100
From:   Florent Revest <revest@...omium.org>
To:     bpf@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        linux-kbuild@...r.kernel.org
Cc:     andrii@...nel.org, mykolal@...com, ast@...nel.org,
        daniel@...earbox.net, kpsingh@...nel.org, masahiroy@...nel.org,
        nathan@...nel.org, ndesaulniers@...gle.com
Subject: Re: [PATCH bpf-next] selftests/bpf: Fix cross compilation with CLANG_CROSS_FLAGS

On Fri, Feb 17, 2023 at 4:18 PM Florent Revest <revest@...omium.org> wrote:
>
> I cross-compile my BPF selftests with the following command:
>
> CLANG_CROSS_FLAGS="--target=aarch64-linux-gnu --sysroot=/sysroot/" \
>   make LLVM=1 CC=clang CROSS_COMPILE=aarch64-linux-gnu- SRCARCH=arm64
>
> (Note the use of CLANG_CROSS_FLAGS to specify a custom sysroot instead
> of letting clang use gcc's default sysroot)
>
> However, CLANG_CROSS_FLAGS gets propagated to host tools builds (libbpf
> and bpftool) and because they reference it directly in their Makefiles,
> they end up cross-compiling host objects which results in linking
> errors.
>
> This patch ensures that CLANG_CROSS_FLAGS is reset if CROSS_COMPILE
> isn't set (for example when reaching a BPF host tool build).

Note: I'm not entirely sure which tree should take that patch. I
tagged this patch as "bpf-next" because 1- that's the tree I know best
2- as far as I can tell, only BPF tools Makefiles reference
CLANG_CROSS_FLAGS directly and are currently broken in this way 3- I
figured this would be a simple enough patch that it's not too hard for
another tree to take it.

Anyway, I tried to CC other relevant folks, I figured there could be
different opinions on how this should get solved, for example the bpf
Makefiles using CLANG_CROSS_FLAGS could check whether CROSS_COMPILE is
set first. I'd be happy to adapt a v2 to any suggestion.