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).

Signed-off-by: Florent Revest <revest@chromium.org>
---
 tools/scripts/Makefile.include | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/tools/scripts/Makefile.include b/tools/scripts/Makefile.include
index 0efb8f2b33ce..ff527ac065cf 100644
--- a/tools/scripts/Makefile.include
+++ b/tools/scripts/Makefile.include
@@ -108,6 +108,8 @@ endif # GCC_TOOLCHAIN_DIR
 endif # CLANG_CROSS_FLAGS
 CFLAGS += $(CLANG_CROSS_FLAGS)
 AFLAGS += $(CLANG_CROSS_FLAGS)
+else
+CLANG_CROSS_FLAGS :=
 endif # CROSS_COMPILE
 
 # Hack to avoid type-punned warnings on old systems such as RHEL5:
-- 
2.39.2.637.g21b0678d19-goog

Hello:

This patch was applied to bpf/bpf-next.git (master)
by Alexei Starovoitov <ast@kernel.org>:

On Fri, 17 Feb 2023 16:18:32 +0100 you 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)
> 
> [...]

Here is the summary with links:
  - [bpf-next] selftests/bpf: Fix cross compilation with CLANG_CROSS_FLAGS
    https://git.kernel.org/bpf/bpf-next/c/b539a287baaa

You are awesome, thank you!
-- 
Deet-doot-dot, I am a bot.
https://korg.docs.kernel.org/patchwork/pwbot.html

On Fri, Feb 17, 2023 at 4:18 PM Florent Revest <revest@chromium.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.