tools: bpftool: improve and check builds for different make invocations

There are a number of alternative "make" invocations that can be used to
compile bpftool. The following invocations are expected to work:

  - through the kbuild system, from the top of the repository
    (make tools/bpf)
  - by telling make to change to the bpftool directory
    (make -C tools/bpf/bpftool)
  - by building the BPF tools from tools/
    (cd tools && make bpf)
  - by running make from bpftool directory
    (cd tools/bpf/bpftool && make)

Additionally, setting the O or OUTPUT variables should tell the build
system to use a custom output path, for each of these alternatives.

The following patch fixes the following invocations:

  $ make tools/bpf
  $ make tools/bpf O=<dir>
  $ make -C tools/bpf/bpftool OUTPUT=<dir>
  $ make -C tools/bpf/bpftool O=<dir>
  $ cd tools/ && make bpf O=<dir>
  $ cd tools/bpf/bpftool && make OUTPUT=<dir>
  $ cd tools/bpf/bpftool && make O=<dir>

After this commit, the build still fails for two variants when passing
the OUTPUT variable:

  $ make tools/bpf OUTPUT=<dir>
  $ cd tools/ && make bpf OUTPUT=<dir>

In order to remember and check what make invocations are supposed to
work, and to document the ones which do not, a new script is added to
the BPF selftests. Note that some invocations require the kernel to be
configured, so the script skips them if no .config file is found.

v2:
- In make_and_clean(), set $ERROR to 1 when "make" returns non-zero,
  even if the binary was produced.
- Run "make clean" from the correct directory (bpf/ instead of bpftool/,
  when relevant).

Reported-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

1 files changed, 7 insertions, 5 deletions

diff --git a/tools/bpf/bpftool/Makefile b/tools/bpf/bpftool/Makefile
index cd0fc05464e7..3fc82ff9b52c 100644
--- a/tools/bpf/bpftool/Makefile
+++ b/tools/bpf/bpftool/Makefile
@@ -17,21 +17,23 @@ endif
 BPF_DIR = $(srctree)/tools/lib/bpf/
 
 ifneq ($(OUTPUT),)
-  BPF_PATH = $(OUTPUT)
+  LIBBPF_OUTPUT = $(OUTPUT)/libbpf/
+  LIBBPF_PATH = $(LIBBPF_OUTPUT)
 else
-  BPF_PATH = $(BPF_DIR)
+  LIBBPF_PATH = $(BPF_DIR)
 endif
 
-LIBBPF = $(BPF_PATH)libbpf.a
+LIBBPF = $(LIBBPF_PATH)libbpf.a
 
 BPFTOOL_VERSION := $(shell make -rR --no-print-directory -sC ../../.. kernelversion)
 
 $(LIBBPF): FORCE
-	$(Q)$(MAKE) -C $(BPF_DIR) OUTPUT=$(OUTPUT) $(OUTPUT)libbpf.a
+	$(if $(LIBBPF_OUTPUT),@mkdir -p $(LIBBPF_OUTPUT))
+	$(Q)$(MAKE) -C $(BPF_DIR) OUTPUT=$(LIBBPF_OUTPUT) $(LIBBPF_OUTPUT)libbpf.a
 
 $(LIBBPF)-clean:
 	$(call QUIET_CLEAN, libbpf)
-	$(Q)$(MAKE) -C $(BPF_DIR) OUTPUT=$(OUTPUT) clean >/dev/null
+	$(Q)$(MAKE) -C $(BPF_DIR) OUTPUT=$(LIBBPF_OUTPUT) clean >/dev/null
 
 prefix ?= /usr/local
 bash_compdir ?= /usr/share/bash-completion/completions