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

[Yonghong Song]

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....@suse.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