Fix vmlinux.h creation when not specified by user

In case where BPFOBJECT_VMLINUX_H is not set and BPFOBJECT_BPFTOOL_EXE is set but not compiled yet, we need to make sure cmake compiles bpftool before attempting to generate vmlinux.h using it Signed-off-by: Oren Cohen <orcohen@akamai.com>

Author: orenc17

tools/cmake/FindBpfObject.cmake

@@ -105,15 +105,12 @@ elseif(BPFOBJECT_BPFTOOL_EXE)
  # Generate vmlinux.h
  set(GENERATED_VMLINUX_DIR ${CMAKE_CURRENT_BINARY_DIR})
  set(BPFOBJECT_VMLINUX_H ${GENERATED_VMLINUX_DIR}/vmlinux.h)
- execute_process(COMMAND ${BPFOBJECT_BPFTOOL_EXE} btf dump file /sys/kernel/btf/vmlinux format c
- OUTPUT_FILE ${BPFOBJECT_VMLINUX_H}
- ERROR_VARIABLE VMLINUX_error
- RESULT_VARIABLE VMLINUX_result)
- if(${VMLINUX_result} EQUAL 0)
- set(VMLINUX ${BPFOBJECT_VMLINUX_H})
- else()
- message(FATAL_ERROR "Failed to dump vmlinux.h from BTF: ${VMLINUX_error}")
- endif()
+ add_custom_command(OUTPUT ${BPFOBJECT_VMLINUX_H}
+ COMMAND ${BPFOBJECT_BPFTOOL_EXE} btf dump file /sys/kernel/btf/vmlinux format c > ${BPFOBJECT_VMLINUX_H}
+ DEPENDS ${BPFOBJECT_BPFTOOL_EXE}
+ VERBATIM
+ COMMENT "[vmlinux] Generating header: ${BPFOBJECT_VMLINUX_H}"
+ )
 endif()
 
 include(FindPackageHandleStandardArgs)
@@ -123,6 +120,7 @@ find_package_handle_standard_args(BpfObject
  BPFOBJECT_CLANG_EXE
  LIBBPF_INCLUDE_DIRS
  LIBBPF_LIBRARIES
+ BPFOBJECT_VMLINUX_H
  GENERATED_VMLINUX_DIR)
 
 # Get clang bpf system includes
@@ -171,7 +169,7 @@ macro(bpf_object name input)
  -isystem ${LIBBPF_INCLUDE_DIRS} -c ${BPF_C_FILE} -o ${BPF_O_FILE}
  COMMAND_EXPAND_LISTS
  VERBATIM
- DEPENDS ${BPF_C_FILE} ${BPF_H_FILES}
+ DEPENDS ${BPF_C_FILE} ${BPF_H_FILES} ${BPFOBJECT_VMLINUX_H}
  COMMENT "[clang] Building BPF object: ${name}")
 
  # Build BPF skeleton header