samples/bpf: Fix broken vmlinux path for VMLINUX_BTF

Commit 13b25489b6f8 ("kbuild: change working directory to external
module directory with M=") changed kbuild working directory of bpf
sample programs to samples/bpf, which broke the vmlinux path for
VMLINUX_BTF, as the Makefiles assume the current work directory to be
the kernel output directory and use a relative path (i.e., ./vmlinux):

  Makefile:316: *** Cannot find a vmlinux for VMLINUX_BTF at any of "  /path/to/linux/samples/bpf/vmlinux", build the kernel or set VMLINUX_BTF like "VMLINUX_BTF=/sys/kernel/btf/vmlinux" or VMLINUX_H variable.  Stop.

Correctly refer to the kernel output directory using $(objtree).

Fixes: 13b25489b6f8 ("kbuild: change working directory to external module directory with M=")
Signed-off-by: Jinghao Jia <jinghao7@illinois.edu>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Ruowen Qin <ruqin@redhat.com>
Link: https://lore.kernel.org/bpf/20250203085506.220297-3-jinghao7@illinois.edu

diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
index dd9944a97b7e68cb560c90aff35fe5a0e91a1dd3..5b632635e00dde8cea9ab2d3e7151ce3d39532d5 100644 (file)
--- a/samples/bpf/Makefile
+++ b/samples/bpf/Makefile
@@ -307,7 +307,7 @@ $(obj)/$(TRACE_HELPERS): TPROGS_CFLAGS := $(TPROGS_CFLAGS) -D__must_check=
 
 VMLINUX_BTF_PATHS ?= $(abspath $(if $(O),$(O)/vmlinux))                                \
                     $(abspath $(if $(KBUILD_OUTPUT),$(KBUILD_OUTPUT)/vmlinux)) \
-                    $(abspath ./vmlinux)
+                    $(abspath $(objtree)/vmlinux)
 VMLINUX_BTF ?= $(abspath $(firstword $(wildcard $(VMLINUX_BTF_PATHS))))
 
 $(obj)/vmlinux.h: $(VMLINUX_BTF) $(BPFTOOL)