linux-kernel - Re: [bpf-next v1 5/5] selftests/bpf: propagate LLVM toolchain to runqslower build

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Message-ID: <fa987c2b-b806-4aa7-a318-812fc7d0f414@linux.dev>
Date: Sun, 16 Nov 2025 22:04:15 -0800
From: Yonghong Song <yonghong.song@...ux.dev>
To: Hoyeon Lee <hoyeon.lee@...e.com>, bpf@...r.kernel.org
Cc: Andrii Nakryiko <andrii@...nel.org>, Eduard Zingerman
 <eddyz87@...il.com>, Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>,
 Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>, Shuah Khan <shuah@...nel.org>,
 Nathan Chancellor <nathan@...nel.org>,
 Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>,
 linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
 llvm@...ts.linux.dev
Subject: Re: [bpf-next v1 5/5] selftests/bpf: propagate LLVM toolchain to
 runqslower build



On 11/15/25 2:55 PM, Hoyeon Lee wrote:
> The selftests/bpf invokes a nested make when building runqslower, but
> LLVM toolchain version (clang/llvm-strip) is not propagated. As a
> result, runqslower is built with system default clang, not respecting
> specified LLVM version.
>
>      # LLVM=-21 make -C tools/testing/selftests/bpf
>      ...
>      make feature_display=0 -C /bpf/tools/bpf/runqslower                        \
>          OUTPUT=/bpf/tools/testing/selftests/bpf/tools/build/runqslower/        \
>          BPFOBJ_OUTPUT=/bpf/tools/testing/selftests/bpf/tools/build/libbpf/     \
>          BPFOBJ=/bpf/tools/testing/selftests/bpf/tools/build/libbpf/libbpf.a    \
>          BPF_INCLUDE=/bpf/tools/testing/selftests/bpf/tools/include             \
>          BPFTOOL_OUTPUT=/bpf/tools/testing/selftests/bpf/tools/build/bpftool/   \
>          VMLINUX_BTF=/sys/kernel/btf/vmlinux BPF_TARGET_ENDIAN=--target=bpfel   \
>          EXTRA_CFLAGS='-g -O0  ' EXTRA_LDFLAGS=' ' &&                           \
>          cp  /bpf/tools/testing/selftests/bpf/tools/build/runqslower/runqslower \
>              /bpf/tools/testing/selftests/bpf/runqslower
>      clang -g -O2 --target=bpfel -I/bpf/tools/testing/selftests/bpf/tools/build/runqslower/ \
>            -I/bpf/tools/testing/selftests/bpf/tools/include -I/bpf/tools/include/uapi       \
>            -c runqslower.bpf.c -o /bpf/tools/testing/selftests/bpf/tools/build/runqslower/runqslower.bpf.o && \
>            llvm-strip -g /bpf/tools/testing/selftests/bpf/tools/build/runqslower//runqslower.bpf.o
>      /bin/sh: 1: clang: not found

I tried with LLVM=-20 make -C tools/testing/selftests/bpf in my system and
there is no build error.

Also could you try with command line
    make -C tools/testing/selftests/bpf LLVM=1
for clang build kernel or selftests, LLVM=1 is recommended as it
encodes a bunch of clang command lines:
   CC              = $(LLVM_PREFIX)clang$(LLVM_SUFFIX)
   LD              = $(LLVM_PREFIX)ld.lld$(LLVM_SUFFIX)
   AR              = $(LLVM_PREFIX)llvm-ar$(LLVM_SUFFIX)
   NM              = $(LLVM_PREFIX)llvm-nm$(LLVM_SUFFIX)
   OBJCOPY         = $(LLVM_PREFIX)llvm-objcopy$(LLVM_SUFFIX)
   OBJDUMP         = $(LLVM_PREFIX)llvm-objdump$(LLVM_SUFFIX)
   READELF         = $(LLVM_PREFIX)llvm-readelf$(LLVM_SUFFIX)
   STRIP           = $(LLVM_PREFIX)llvm-strip$(LLVM_SUFFIX)




>
> Explicitly propagate CLANG and LLVM_STRIP to the runqslower sub-make so
> that the LLVM toolchain selection from lib.mk is preserved.
>
> Signed-off-by: Hoyeon Lee <hoyeon.lee@...e.com>
> ---
>   tools/testing/selftests/bpf/Makefile | 1 +
>   tools/testing/selftests/lib.mk       | 1 +
>   2 files changed, 2 insertions(+)
>
> diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
> index 34ea23c63bd5..79ab69920dca 100644
> --- a/tools/testing/selftests/bpf/Makefile
> +++ b/tools/testing/selftests/bpf/Makefile
> @@ -306,6 +306,7 @@ endif
>   
>   $(OUTPUT)/runqslower: $(BPFOBJ) | $(DEFAULT_BPFTOOL) $(RUNQSLOWER_OUTPUT)
>   	$(Q)$(MAKE) $(submake_extras) -C $(TOOLSDIR)/bpf/runqslower	       \
> +		    CLANG=$(CLANG) LLVM_STRIP=$(LLVM_STRIP)		       \
>   		    OUTPUT=$(RUNQSLOWER_OUTPUT) VMLINUX_BTF=$(VMLINUX_BTF)     \
>   		    BPFTOOL_OUTPUT=$(HOST_BUILD_DIR)/bpftool/		       \
>   		    BPFOBJ_OUTPUT=$(BUILD_DIR)/libbpf/			       \
> diff --git a/tools/testing/selftests/lib.mk b/tools/testing/selftests/lib.mk
> index a448fae57831..f14255b2afbd 100644
> --- a/tools/testing/selftests/lib.mk
> +++ b/tools/testing/selftests/lib.mk
> @@ -8,6 +8,7 @@ LLVM_SUFFIX := $(LLVM)
>   endif
>   
>   CLANG := $(LLVM_PREFIX)clang$(LLVM_SUFFIX)
> +LLVM_STRIP := $(LLVM_PREFIX)llvm-strip$(LLVM_SUFFIX)
>   
>   CLANG_TARGET_FLAGS_arm          := arm-linux-gnueabi
>   CLANG_TARGET_FLAGS_arm64        := aarch64-linux-gnu